Alves, M.E.D.S.,
Tinto, M., 2011, "Pulsar timing sensitivities to gravitational waves from
relativistic metric theories of gravity," Physical Review D, 83,
123529.
Pulsar timing experiments aimed at the
detection of gravitational radiation have been performed for decades now. With
the forthcoming construction of large arrays capable of tracking multiple
millisecond pulsars, it is very likely we will be able to make the first detection
of gravitational radiation in the nano-Hertz band, and test Einstein’s theory
of relativity by measuring the polarization components of the detected signals.
Since a gravitational wave predicted by the most general relativistic metric
theory of gravity accounts for six polarization modes (the usual two Einstein’s
tensor polarizations as well as two vector and two scalar wave components), we
have estimated the single-antenna sensitivities to these six polarizations. We
find pulsar timing experiments to be significantly more sensitive, over their
entire observational frequency band (≈10-9–10-6Hz),
to scalar-longitudinal and vector waves than to scalar-transverse and tensor
waves. At 10-7Hz and with pulsars at a distance of 1 kpc, for
instance, we estimate an average sensitivity to scalar-longitudinal waves that
is more than two orders of magnitude better than the sensitivity to tensor
waves. Our results imply that a direct detection of gravitational radiation by
pulsar timing will result into a test of the theory of general relativity that
is more stringent than that based on monitoring the decay of the orbital period
of a binary system.
Baland, R.-M., van Hoolst, T.,
Yseboodt, M., Karatekin, Ö., 2011, "Titan's
obliquity as evidence of a subsurface ocean?," Astronomy and
Astrophysics, 530, 141.
On the basis of gravity and radar
observations with the Cassini spacecraft, the moment of inertia of Titan and
the orientation of Titan's rotation axis have been estimated in recent studies.
According to the observed orientation, Titan is close to the Cassini state.
However, the observed obliquity is inconsistent with the estimate of the moment
of inertia for an entirely solid Titan occupying the Cassini state. We propose
a new Cassini state model for Titan in which we assume the presence of a liquid
water ocean beneath an ice shell and consider the gravitational and pressure
torques arising between the different layers of the satellite. With the new
model, we find a closer agreement between the moment of inertia and the
rotation state than for the solid case, strengthening the possibility that
Titan has a subsurface ocean.
Barker, A.J., 2011,
"Three-dimensional simulations of internal wave breaking and the fate of
planets around solar-type stars," Monthly Notices of the Royal
Astronomical Society, 414, 1365-1378.
We study the fate of internal gravity
waves approaching the centre of an initially non-rotating solar-type star, by
performing three-dimensional numerical simulations using a Boussinesq-type model.
These waves are excited at the top of the radiation zone by the tidal forcing
of a short-period planet on a circular, coplanar orbit. This extends previous
work done in two dimensions by Barker & Ogilvie. We first derive a linear
wave solution, which is not exact in three dimensions; however, the reflection
of ingoing waves from the centre is close to perfect for moderate amplitude
waves. Waves with sufficient amplitude to cause isentropic
overturning break, and deposit their angular momentum near the centre.
This forms a critical layer, at which the angular velocity of the flow matches
the orbital angular frequency of the planet. This efficiently absorbs ingoing
waves, and spins up the star from the inside out, while the planet spirals into
the star.
We also perform numerical integrations to determine the
linearized adiabatic tidal response throughout the star, in a wide range of
solar-type stellar models with masses in the range 0.5 ≤m★/Msun≤ 1.1,
throughout their main-sequence lifetimes. The aim is to study the influence of
the launching region for these waves at the top of the radiation zone in more
detail, and to determine the accuracy of a semi-analytic approximation for the
tidal torque on the star, which was derived under the assumption that all ingoing
wave angular momentum is absorbed in a critical layer.
The main conclusion of this work is that this non-linear
mechanism of tidal dissipation could provide an explanation for the survival of
all short-period extrasolar planets observed around FGK stars, while it
predicts the destruction of more massive planets. This work provides further
support for the model outlined in a previous paper by Barker & Ogilvie, and
makes predictions that will be tested by ongoing observational studies, such as
WASP and Kepler.
Barker, A.J.,
Ogilvie, G.I., 2011, "Stability analysis of a tidally excited internal
gravity wave near the centre of a solar-type star," Monthly Notices of
the Royal Astronomical Society, 1181.
We perform a stability analysis of a
tidally excited non-linear internal gravity wave near the centre of a
solar-type star in two-dimensional cylindrical geometry. The motivation is to
understand the tidal interaction between short-period planets and their slowly
rotating solar-type host stars, which involves the launching of internal
gravity waves at the top of the radiation zone that propagate towards the
centre of the star. Studying the instabilities of these waves near the centre,
where non-linearities are most important, is essential, since it may have
implications for the survival of short-period planets orbiting solar-type
stars. When these waves have sufficient amplitude to overturn the
stratification, they break and form a critical layer, which efficiently absorbs
subsequent ingoing wave angular momentum, and can result in the planet
spiralling into the star. However, in previous simulations the waves have not
been observed to undergo instability for smaller amplitudes. Here we perform a
stability analysis of a non-linear standing internal gravity wave in the
central regions of a solar-type star. This work has two aims: to determine any
instabilities that set in for small-amplitude waves, and to further understand
the breaking process for large-amplitude waves that overturn the stratification.
Our results are compared with the stability of a plane internal gravity wave in
a uniform stratification, and with previous work by Kumar & Goodman on a
similar problem to our own. Our main result is that the waves undergo
parametric instabilities for any amplitude (in the absence of viscosity and
thermal conduction). However, because the non-linearity is spatially localized
in the innermost wavelengths, the growth rates of these instabilities tend to
be sufficiently small that they do not result in astrophysically important
tidal dissipation. Indeed, we estimate that the modified tidal quality factors
of the star that result are Q'<
Bear, E., Soker, N., Harpaz, A.,
2011, "Possible Implications of the Planet Orbiting the Red Horizontal
Branch Star HIP 13044," The Astrophysical Journal Letters, 733,
L44.
We propose a scenario to account for the
surprising orbital properties of the planet orbiting the metal-poor red horizontal
branch star HIP 13044. The orbital period of 16.2 days implies that the planet
went through a common envelope phase inside the red giant branch (
Béky, B., Bakos, G.Á., Hartman, J.,
Torres, G., Latham, D.W., Jordán, A., Arriagada, P., Bayliss, D., Kiss, L.L.,
Kovács, G., Quinn, S.N., Marcy, G.W., Howard, A.W., Fischer, D.A., Johnson,
J.A., Esquerdo, G.A., Noyes, R.W., Buchhave, L.A., Sasselov, D.D., Stefanik,
R.P., Perumpilly, G., Lázár, J., Papp, I., Sári, P., 2011, "HAT-P-27b: A
Hot Jupiter Transiting a G Star on a 3 Day Orbit," The Astrophysical
Journal, 734, 109.
We report the discovery of HAT-P-27b, an
exoplanet transiting the moderately bright G8 dwarf star GSC 0333-00351 (V =
12.214). The orbital period is 3.039586 ± 0.000012 days, the reference epoch of
transit is 2455186.01879 ± 0.00054 (BJD), and the transit duration is 0.0705 ±
0.0019 days. The host star with its effective temperature 5300 ± 90 K is
somewhat cooler than the Sun and is more metal-rich with a metallicity of +0.29
± 0.10. Its mass is 0.94 ± 0.04 M sun and radius is 0.90+0.05 -
0.04 R sun. For the planetary companion we determine
a mass of 0.660 ± 0.033 M J and radius of 1.038+0.077 -
0.058 R J. For the 30 known transiting exoplanets
between 0.3 M J and 0.8 M J, a negative
correlation between host star metallicity and planetary radius and an
additional dependence of planetary radius on equilibrium temperature are
confirmed at a high level of statistical significance.
Benisty, M., Renard, S., Natta, A.,
Berger, J.P., Massi, F., Malbet, F., Garcia, P.J.V., Isella, A., Mérand, A.,
Monin, J.L., Testi, L., Thiébaut, E., Vannier, M., Weigelt, G., 2011, "A
low optical depth region in the inner disk of the Herbig Ae star HR 5999,"
Astronomy and Astrophysics, 531, 84.
Context.
Circumstellar disks surrounding young stars are known to be the birthplaces of
planetary systems, and the innermost astronomical unit is of particular
interest. Near-infrared interferometric studies have revealed a complex
morphology for the close environment surrounding Herbig Ae stars. <BR />
Aims: We present new long-baseline spectro-interferometric observations of the
Herbig Ae star, HR 5999, obtained in the H and K bands with the AMBER
instrument at the VLTI, and aim to produce near-infrared images at the sub-AU
spatial scale.
Methods: We spatially resolve the circumstellar material and
reconstruct images in the H and K bands using the MiRA algorithm. In addition,
we interpret the interferometric observations using models that assume that the
near-infrared excess is dominated by the emission of a circumstellar disk. We
compare the images reconstructed from the VLTI measurements to images obtained
using simulated model data.
Results: The K-band image reveals three main elements: a
ring-like feature located at ~0.65 AU, a low surface brightness region inside
0.65 AU, and a central spot. At the maximum angular resolution of our observations
(B/? ~ 1.3 mas), the ring is resolved while the central spot is only marginally
resolved, preventing us from revealing the exact morphology of the
circumstellar environment. We suggest that the ring traces silicate
condensation, i.e., an opacity change, in a circumstellar disk around HR 5999.
We build a model that includes a ring at the silicate sublimation radius and an
inner disk of low surface brightness responsible for a large amount of the
near-infrared continuum emission. The model successfully fits the SED,
visibilities, and closure phases in the H and K bands, and provides evidence of
a low surface brightness region inside the silicate sublimation radius.
Conclusions: This study provides milli-arcsecond resolution
images of the environment of HR 5999 and additional evidence that in Herbig Ae
stars, there is material in a low surface brightness region, probably a low
optical depth region, located inside the silicate sublimation radius and of
unknown nature. The possibility that the formation of such a region in a thick
disk is related to disk evolution should be investigated.
Beust, H.,
Bonneau, D., Mourard, D., Lafrasse, S., Mella, G., Duvert, G., Chelli, A.,
2011, "On the use of the Virtual Observatory to select calibrators for
phase-referenced astrometry of exoplanet-host stars," Monthly Notices
of the Royal Astronomical Society, 414, 108-115.
Phase-referenced interferometric
astrometry offers the possibility to look for exoplanets around bright stars.
Instruments like PRIMA (Phase-Referenced Imaging and Micro-arcsecond
Astrometry) will measure the astrometric wobble of a candidate star due to an
exoplanet relative to a close-by 'calibrator' star, located within the
instrument's observing field (1 arcmin in the PRIMA case). Stars with already
known exoplanets will constitute the first targets for this technique, as it
will provide a way to further specify the characteristics of the known
exoplanets, such as the inclinations. The main requirement is to have a
calibrator in the vicinity of the star. We provide here a list of calibrators
for all stars with known exoplanets obtained using data mining and Virtual
Observatory techniques. This list is available online and revised regularly.
The calibrators are found from catalogues available at Centre de Données
astronomiques de Strasbourg (CDS) using the SEARCHCAL software developed at
Jean-Marie Mariotti Center (JMMC). In our test case, the calibrators are found
within 1 arcmin angular distance for approximately 50 per cent of the stars tested,
and often closer. They are all faint objects from the Two Micron All Sky Survey
(2MASS) with K magnitudes between 13 and 15. A list of the most promising
targets is also given.
Bhattacharyya, S.,
2011, "Ways to constrain neutron star equation of state models using
relativistic disc lines," Monthly Notices of the Royal Astronomical
Society, 844.
Relativistic spectral lines from the
accretion disc of a neutron star low-mass X-ray binary can be modelled to infer
the disc inner edge radius. A small value of this radius tentatively implies
that the disc terminates either at the neutron star hard surface, or at the
innermost stable circular orbit (
Borucki, W.J., Koch, D.G., Basri,
G., Batalha, N., Brown, T.M., Bryson, S.T., Caldwell, D.,
Christensen-Dalsgaard, J., Cochran, W.D., DeVore, E., Dunham, E.W., Gautier,
T.N.,
On 2011 February 1 the Kepler
mission released data for 156,453 stars observed from the beginning of the
science observations on 2009 May 2 through September 16. There are 1235 planetary candidates
with transit-like signatures detected in this period. These are associated with
997 host stars. Distributions of the characteristics of the planetary
candidates are separated into five class sizes: 68 candidates of approximately
Earth-size (R p < 1.25 R ⊕), 288 super-Earth-size (1.25
R ⊕ <= R p <
2 R ⊕), 662 Neptune-size (2 R ⊕ <= R p<
6 R ⊕), 165 Jupiter-size (6 R ⊕ <= R p <
15 R ⊕), and 19 up to twice the size of
Jupiter (15 R ⊕ <= R p <
22 R ⊕). In the temperature range
appropriate for the habitable zone, 54 candidates are found with sizes ranging
from Earth-size to larger than that of Jupiter. Six are less than twice the
size of the Earth. Over 74% of the planetary candidates are smaller than
Bromley, B.C., Kenyon, S.J., 2011,
"Migration of Planets Embedded in a Circumstellar Disk," The
Astrophysical Journal, 735, 29.
Planetary migration poses a serious
challenge to theories of planet formation. In gaseous and planetesimal disks,
migration can remove planets as quickly as they form. To explore migration in a
planetesimal disk, we combine analytic and numerical approaches. After deriving
general analytic migration rates for isolated planets, we use N-body
simulations to confirm these results for fast and slow migration modes.
Migration rates scale as m -1(for massive planets) and (1 +
(e H/3)3)-1, where m is the mass of a
planet and e H is the eccentricity of the background
planetesimals in Hill units. When multiple planets stir the disk, our
simulations yield the new result that large-scale migration ceases. Thus,
growing planets do not migrate through planetesimal disks. To extend these
results to migration in gaseous disks, we compare physical interactions and
rates. Although migration through a gaseous disk is an important issue for the
formation of gas giants, we conclude that migration has little impact on the
formation of terrestrial planets.
Brown, D.J.A., Collier Cameron, A.,
Hall, C., Hebb, L., Smalley, B., 2011, "Are falling planets spinning up
their host stars?," Monthly Notices of the
Royal Astronomical Society, 415, 605-618.
We investigate the effects of tidal
interactions on the planetary orbits and stellar spin rates of the WASP-18 and
WASP-19 planetary systems using a forward integration scheme. By fitting the
resulting evolutionary tracks to the observed eccentricity, semimajor axis and
stellar rotation rate, and to the stellar age derived from isochronal fitting,
we are able to place constraints on the stellar and planetary reduced tidal
quality factors, Q's and Q'p. We find that for
WASP-18, log (Q's) = 8.21+0.90-0.52 and
log (Q'p) = 7.77+1.54-1.25, implying a system
age of 0.579+0.305-0.250 Gyr. For WASP-19 we obtain
values of log (Q's) = 6.47+2.19-0.95 and
log (Q'p) = 6.75+1.86-1.77, suggesting a
system age of 1.60+2.84-0.79 Gyr and a remaining
lifetime of 0.0067+1.1073-0.0061 Gyr. We investigate
a range of evolutionary histories consistent with these results and the
observed parameters for both systems, and find that the majority imply that the
stars have been spun up through tidal interactions as the planets spiral
towards their Roche limits. We examine a variety of evidence for WASP-19 A's
age, both for the value above and for a younger age consistent with
gyrochronology, and conclude that the older estimate is more likely to be
correct. This suggests that WASP-19 b might be in the final stages of the
spiral-in process, although we are unable to rule out the possibility that it
has a substantial remaining lifetime.
Brož, M., Rozehnal, J., 2011,
"Eurybates - the only asteroid family among Trojans?,"
Monthly Notices of the Royal Astronomical Society, 414, 565-574.
We study the orbital and physical
properties of Trojan asteroids of Jupiter. We try to discern all the families
previously discussed in the literature, but we conclude that there is only one
significant family among the Trojans, namely the cluster around the asteroid
(3548) Eurybates. This is the only cluster that has all of the following
characteristics: (i) it is clearly concentrated in the proper-element space;
(ii) the size-frequency distribution is different from that of background
asteroids; (iii) we have a reasonable collisional/dynamical model of the
family. Henceforth, we can consider it as a real collisional family. We also
report the discovery of a possible family around the asteroid (4709) Ennomos,
composed mostly of small asteroids. The asteroid (4709) Ennomos is known to
have a very high albedo pV ≃ 0.15, which may be related to the
hypothetical cratering event that exposed ice. The relation between the
collisional family and the exposed surface of the parent body offers a unique
means to study the physics of cratering events. However, more data are needed
to confirm the existence of this family and its relationship with Ennomos.
Brož, M., Vokrouhlický, D.,
Morbidelli, A., Nesvorný, D., Bottke, W.F., 2011, "Did the Hilda
collisional family form during the late heavy bombardment?,"
Monthly Notices of the Royal Astronomical Society, 414,
2716-2727.
We model the long-term evolution of the
Hilda collisional family located in the 3/2 mean-motion resonance with Jupiter.
Its eccentricity distribution evolves mostly due to the Yarkovsky/YORP effect
and assuming that (i) impact disruption was isotropic and (ii) albedo
distribution of small asteroids is the same as for large ones, we can estimate
the age of the Hilda family to be 4+0-1 Gyr. We also
calculate collisional activity in the J3/2 region. Our results indicate that
current collisional rates are very low for a 200-km parent body such that the
number of expected events over gigayears is much smaller than 1.
The large age and the low probability of the collisional
disruption lead us to the conclusion that the Hilda family might have been
created during the late heavy bombardment (LHB) when the collisions were much
more frequent. The Hilda family may thus serve as a test of orbital behaviour
of planets during the LHB. We have tested the influence of the giant-planet
migration on the distribution of the family members. The scenarios that are
consistent with the observed Hilda family are those with fast migration
time-scales ≃0.3-3 Myr, because longer time-scales produce a family that
is depleted and too much spread in eccentricity. Moreover, there is an
indication that Jupiter and Saturn were no longer in a compact configuration
(with period ratio PS/PJ > 2.09) at the time when
the Hilda family was created.
Buchhave, L.A., Bakos, G.Á.,
Hartman, J.D., Torres, G., Latham, D.W., Andersen, J., Kovács, G., Noyes, R.W.,
Shporer, A., Esquerdo, G.A., Fischer, D.A., Johnson, J.A., Marcy, G.W., Howard,
A.W., Béky, B., Sasselov, D.D., F?rész, G., Quinn, S.N., Stefanik, R.P.,
Szklenár, T., Berlind, P., Calkins, M.L., Lázár, J., Papp, I., Sári, P., 2011,
"Hat-P-28b and Hat-P-29b: Two Sub-Jupiter Mass Transiting Planets," The
Astrophysical Journal, 733, 116.
We present the discovery of two
transiting exoplanets. HAT-P-28b orbits a V = 13.03 G3 dwarf star with a period
P = 3.2572 days and has a mass of 0.63 ± 0.04 M J and a
radius of 1.21+0.11 - 0.08 R J yielding
a mean density of 0.44 ± 0.09 g cm-3. HAT-P-29b orbits a V = 11.90
F8 dwarf star with a period P = 5.7232 days and has a mass of 0.78+0.08 -
0.04 M J and a radius of 1.11+0.14-
0.08 R J yielding a mean density of 0.71 ± 0.18
g cm-3. We discuss the properties of these planets in the context of
other known transiting planets.
Chan, T., Ingemyr,
M., Winn, J.N., Holman, M.J., Sanchis-Ojeda, R., Esquerdo, G.,
We present transit photometry of three
exoplanets, TrES-4b, HAT-P-3b, and WASP-12b, allowing for refined estimates of
the systems' parameters. TrES-4b and WASP-12b were confirmed to be
"bloated" planets, with radii of 1.706 ± 0.056R Jup and
1.736 ± 0.092R Jup, respectively. These planets are too large
to be explained with standard models of gas giant planets. In contrast,
HAT-P-3b has a radius of 0.827 ± 0.055R Jup, smaller than a
pure hydrogen-helium planet and indicative of a highly metal-enriched
composition. Analyses of the transit timings revealed no significant departures
from strict periodicity. For TrES-4, our relatively recent observations allow
for improvement in the orbital ephemerides, which is useful for planning future
observations.
Chicone, C., Mashhoon, B., Rosquist,
K., 2011, "Double-Kasner spacetime: Peculiar velocities and cosmic
jets," Physical Review D, 83, 124029.
In dynamic spacetimes in which
asymmetric gravitational collapse/expansion is taking place, the timelike
geodesic equation appears to exhibit an interesting property: Relative to the
collapsing configuration, free test particles undergo gravitational
“acceleration” and form a double-jet configuration parallel to the axis of
collapse. We illustrate this aspect of peculiar motion in simple spatially
homogeneous cosmological models such as the Kasner spacetime. To estimate the
effect of spatial inhomogeneities on cosmic jets, timelike geodesics in the
Ricci-flat double-Kasner spacetime are studied in detail. While spatial
inhomogeneities can significantly modify the structure of cosmic jets, we find
that under favorable conditions the double-jet pattern can initially persist
over a finite period of time for sufficiently small inhomogeneities.
Christou, A.A.,
Asher, D.J., 2011, "A long-lived horseshoe companion to the Earth," Monthly
Notices of the Royal Astronomical Society, 414, 2965-2969.
We present a dynamical investigation of
a newly found asteroid, 2010 SO16, and the discovery that it is a horseshoe
companion of the Earth. The object's absolute magnitude (H= 20.7) makes this
the largest object of its type known to-date. By carrying out numerical
integrations of dynamical clones, we find that (a) its status as a horseshoe is
secure given the current accuracy of its ephemeris, and (b) the time spent in
horseshoe libration with the Earth is several times 105 yr, two
orders of magnitude longer than determined for other horseshoe asteroids of the
Earth. Further, using a model based on Hill's approximation to the three-body
problem, we show that, apart from the low eccentricity which prevents close
encounters with other planets or the Earth itself, its stability can be
attributed to the value of its Jacobi constant far from the regime that allows
transitions into other co-orbital modes or escape from the resonance
altogether. We provide evidence that the eventual escape of the asteroid from
horseshoe libration is caused by the action of planetary secular perturbations
and the stochastic evolution of the eccentricity. The questions of its origin
and the existence of as-yet-undiscovered co-orbital companions of the Earth are
discussed.
Cottereau, L.,
Rambaux, N., Lebonnois, S., Souchay, J., 2011, "The various contributions
in Venus rotation rate and
Context. Thanks
to the Venus Express Mission, new data on the properties of Venus could be
obtained, in particular concerning its rotation. <BR /> Aims: In view of these upcoming results, the purpose of this
paper is to determine and compare the major physical processes influencing the
rotation of Venus and, more particularly, the angular rotation rate.
Methods: Applying models already used for Earth, the effect
of the triaxiality of a rigid Venus on its period of rotation are computed.
Then the variations of Venus rotation caused by the elasticity, the atmosphere,
and the core of the planet are evaluated. <BR /> Results: Although the
largest irregularities in the rotation rate of the Earth on short time scales
are caused by its atmosphere and elastic deformations, we show that the
irregularities for Venus are dominated by the tidal torque exerted by the Sun
on its solid body. Indeed, as Venus has a slow rotation, these effects have a large amplitude of two minutes of time (mn). These
variations in the rotation rate are greater than the one induced by atmospheric
wind variations that can reach 25-50 s of time (s), depending on the simulation
used. The variations due to the core effects that vary with its size between 3
and 20 s are smaller. Compared to these effects, the influence of the elastic
deformation caused by the zonal tidal potential is negligible.
Conclusions: As the variations in the rotation of Venus
reported here are close to 3 mn peak to peak, they should influence past,
present, and future observations, thereby providing further constraints on the
planet's internal structure and atmosphere.
Crepp, J.R., Johnson, J.A., 2011,
"Estimates of the Planet Yield from Ground-based High-contrast Imaging
Observations as a Function of Stellar Mass," The Astrophysical Journal,
733, 126.
We use
Damljanovic, G.,
Milic, I.S., 2011, "Corrected mu_delta for Stars of Hipparcos Catalogue
from Independent Latitude Observations over Many Decades," Serbian
Astronomical Journal, 182, 35-41.
During the last century, there were many
so-called independent latitude (IL) stations with the observations which were
included into data of a few international organizations (like Bureau
International de l'Heure - BIH, International Polar Motion Service - IPMS) and
the Earth rotation programmes for determining the Earth Orientation Parameters
-
Deng, X., Finn, L.S., 2011,
"Pulsar timing array observations of gravitational wave source timing
parallax," Monthly Notices of the Royal Astronomical Society, 414,
50-58.
Pulsar timing arrays (PTAs) act to
detect gravitational waves by observing the small, correlated effect the waves
have on pulse arrival times at the Earth. This effect has conventionally been
evaluated assuming the gravitational wave phase fronts are planar across the
array, an assumption that is valid only for sources at distances R≫ 2πL2/λ, where
L is physical extent of the array and λ is the radiation wavelength. In
the case of PTAs, the array size is of the order of the pulsar-Earth distance
(kpc) and λ is of the order of parsec. Correspondingly, for point
gravitational wave sources closer than ˜100 Mpc, the
Donnison, J.R.,
2011, "The Hill stability of binary asteroid and binary Kuiper Belt
systems," Monthly Notices of the Royal Astronomical Society, 415,
470-486.
The dynamical stability of a bound
triple system composed of a binary asteroid system or Kuiper Belt binary system
moving on an orbit inclined to a central third body, the Sun, is discussed in
terms of Hill stability for the full three-body problem. The regions of Hill
stability of these triple systems, where the binary mass is very small compared
with that of the third body, can be determined against
the possibility of disruption, component exchange and capture. The critical
Hill stability curves for the binary mass range of these types of systems are
determined for different secondary-to-primary mass ratios as a function of
their orbital eccentricity. The regions of stability are found to increase with
increasing binary mass. The regions, however, decrease in size substantially
with increasing orbital eccentricity and also decrease slightly as the
secondary/primary mass ratio of the binary is decreased. The currently observed
binary and multiple asteroid systems are discussed generally. In the majority
of systems, the primary component is very much larger than the secondary
component, forming an asteroid-satellite system. It was found that those
systems where the binary mass is well determined would lie in stable regions if
they moved on circular orbits, but when their eccentricity is taken into
account, it is less clear that the systems are stable. The same is likely to be
true for the systems where the masses are not well established. Upper mass
limits could be placed on these systems that would ensure they are Hill stable.
The currently observed Kuiper Belt binaries were also discussed generally. The
majority of these binary systems have secondary components which are often
comparable to the diameter of the primary component forming a true binary system.
Similar to the asteroid binaries, it was found that binary systems where the
mass was well determined were stable if they moved on circular orbits relative
to the Sun. When the eccentricity is taken into account, it is less clear that
the systems are stable. The same conclusions are also likely to be true for the
systems with unknown masses. Upper mass limits again can be placed on these
systems that would ensure they are Hill stable.
Dybczyński, P.A., Królikowska,
M.g., 2011, "Where do long-period comets come from? Moving through the
Jupiter-Saturn barrier," Monthly Notices of the Royal Astronomical
Society, 1183.
The past and future dynamical evolution
of all 64 long-period comets having 1/aori < 1 × 10-4 au-1,
qosc > 3.0 au and discovered after 1970 is studied. For this
sample of Oort-spike comets we have obtained a new, homogeneous set of
osculating orbits, including 15 orbits with detected non-gravitational
parameters. The non-gravitational effects for 11 comets have been determined
for the first time. This means that more than 50 per cent of all comets with
perihelion distances between 3 and 4 au and discovered after 1970 show
detectable deviations from purely gravitational motion. Each comet was then
replaced with a swarm of 5001 virtual comets representing the observations
well. These swarms were propagated numerically back and forth up to a
heliocentric distance of 250 au, constituting sets of original and future
orbits together with their uncertainties. This allowed us to show that the 1/aori distribution
is significantly different in shape as well as in maximum position when
non-gravitational orbits are included. Next, we followed the dynamical
evolution under Galactic tides for one orbital revolution to the past and
future, obtaining orbital elements at the previous and next perihelion
passages. We obtained a clear dependence of the last revolution change in
perihelion distance on 1/aori, which confirmed theoretical
expectations.
Based
on these results, we discuss the possibility of discriminating between
dynamically new and old comets with the aid of their previous perihelion
distances. We have shown that about 50 per cent of all comets investigated have
their previous perihelion distance below the 15-au limit. This resulted in
classifying 31 comets as dynamically new, 26 as dynamically old and 7 as having
unclear status. We showed that this classification seems to be immune to
perturbations from all known stars. However, discoveries of new, strong stellar
perturbers, while rather improbable, may change the situation.
We also present several examples of cometary motion through
the Jupiter-Saturn barrier, some of them with a previous perihelion distance
smaller than the observed one. New interpretations of the source pathways of
long-period comets are also discussed in the light of the suggestions of Kaib
& Quinn[DDM1] .
Emelyanov, N.V., Andreev, M.V.,
Berezhnoi, A.A., Bekhteva, A.S., Vashkovyak, S.N., Velikodskii, Y.I.,
Vereshchagina, I.A., Gorshanov, D.L., Devyatkin, A.V., Izmailov, I.S., Ivanov,
A.V., Irsmambetova, T.R., Kozlov, V.A., Karashevich, S.V., Kurenya, A.N.,
Naiden, Y.V., Naumov, K.N., Parakhin, N.A., Raskhozhev, V.N., Selyaev, S.A.,
Sergeev, A.V., Sokov, E.N., Khovrichev, M.Y., Khrutskaya, E.V., Chernikov,
M.M., 2011, "Astrometric results of observations at Russian observatories
of mutual occultations and eclipses of Jupiter's Galilean satellites in
2009," Solar System Research, 45, 264-277.
In 2009, in five Russian observatories
photometric observations of Jupiter’s Galilean satellites during their mutual
occultations and eclipses were carried out. Based on these observations, an
original method was used to ascertain astrometric results such as the
difference between the coordinates of pairs of satellites. Fifty-three
phenomena were successfully observed. A total of 94 light curves of satellites
were measured. The error in the coordinates of satellites due to random errors
in photometry, calculated on all data obtained, was 0.041″ in right
ascension and 0.046″ in declination. The discrepancies
between the theory and observations in these coordinates was found to be
0.060″ and 0.057″, respectively. The results were uploaded to the
common database for all observations of natural satellites of planets at the
Natural Satellites Data Center (NSDC), which is available online
at http://www.sai.msu.ru/neb/nss/index.htm. For the first time in the
practice of photometric observations of satellites in epochs of mutual occultations
and eclipses a new method of observation was tested, which eliminates from
astrometric results the major systematic errors caused by an inaccurate account
of the background level. The tests were conducted in the Terskol Observatory
and the observatory of the Crimean laboratory of the Sternberg State
Astronomical Institute of the
Escapa, A., 2011, "Corrections
stemming from the non-osculating character of the Andoyer variables used in the
description of rotation of the elastic Earth," Celestial Mechanics and
Dynamical Astronomy, 110, 99-142.
We explore the evolution of the angular velocity
of an elastic Earth model, within the Hamiltonian formalism. The evolution of
the rotation state of the Earth is caused by the tidal deformation exerted by
the Moon and the Sun. It can be demonstrated that the tidal perturbation to
spin depends not only upon the instantaneous orientation of the Earth, but also
upon its instantaneous angular velocity. Parameterizing the orientation of the
Earth figure axis with the three Euler angles, and introducing the canonical
momenta conjugated to these, one can then show that the tidal perturbation
depends both upon the angles and the momenta. This circumstance complicates the
integration of the rotational motion. Specifically, when the integration is
carried out in terms of the canonical Andoyer variables (which are the
rotational analogues to the orbital Delaunay variables), one should keep in
mind the following subtlety: under the said kind of perturbations, the
functional dependence of the angular velocity upon the Andoyer elements differs
from the unperturbed dependence (Efroimsky in Proceedings of Journées 2004:
Systèmes de référence spatio-temporels. l’Observatoire de Paris, pp
74–81, 2005; Efroimsky and Escapa in Celest. Mech. Dyn.
Astron. 98:251–283, 2007). This happens because, under
angular velocity dependent perturbations, the requirement for the Andoyer
elements to be canonical comes into a contradiction with the requirement for
these elements to be osculating, a situation that parallels a similar antinomy
in orbital dynamics. Under the said perturbations, the expression for the
angular velocity acquires an additional contribution, the so called convective
term. Hence, the time variation induced on the angular velocity by the tidal
deformation contains two parts. The first one comes from the direct terms,
caused by the action of the elastic perturbation on the torque-free expressions
of the angular velocity. The second one arises from the convective terms. We
compute the variations of the angular velocity through the approach developed
in Getino and Ferrándiz (Celest. Mech. Dyn. Astron. 61:117–180,1995), but considering the contribution of the
convective terms. Specifically, we derive analytical formulas that determine
the elastic perturbations of the directional angles of the angular velocity
with respect to a non-rotating reference system, and also of its Cartesian
components relative to the Tisserand reference system of the Earth. The
perturbation of the directional angles of the angular velocity turns out to be
different from the evolution law found in Kubo (Celest. Mech. Dyn. Astron. 105:261–274, 2009), where it was
stated that the evolution of the angular velocity vector mimics that of the
figure axis. We investigate comprehensively the source of this
discrepancy, concluding that the difference between our results and those
obtained in Ibid. stems from an oversimplification made by Kubo
when computing the direct terms. Namely, in his computations Kubo disregarded
the motion of the tide raising bodies with respect to a non-rotating reference
system when compared with the Earth rotational motion. We demonstrate that,
from a numerical perspective, the convective part provides the principal
contribution to the variation of the directional angles and of length of day.
In the case of the x and y components in the
Tisserand system, the convective contribution is of the same order of magnitude
as the direct one. Finally, we show that the approximation employed in Kubo (Ibid.)
leads to significant numerical differences at the level of a
hundred micro-arcsecond.
Fedorov, P.N., Akhmetov, V.S.,
Bobylev, V.V., 2011, "Residual rotation of the Hipparcos/Tycho-2 system as
determined from the data of the XPM catalogue," Monthly Notices of the
Royal Astronomical Society, 1176.
From a comparison of absolute proper
motions of stars from the XPM catalogue with those of the same stars from the
PPMXL, UCAC3, Tycho-2 and XC1 catalogues, the equatorial components of the
rotation vector of these coordinate systems are determined with respect to the
coordinate system specified by positions and proper motions of XPM. These
parameters are calculated with the use of about 90 million stars from the UCAC3
catalogue and about 300 million stars from the PPMXL catalogue. It is shown
that the Hipparcos Celestial Reference Frame represented by the Tycho-2, PPMXL,
UCAC3 and XC1 catalogues has a significant rotation component ωz=
(-1.8 ± 0.16) mas yr-1 about the equatorial axis directed to
the celestial pole. The result is confirmed by an analysis of the formal proper
motions of the extragalactic sources contained in the considered catalogues.
Finkelstein, A.M., Kaidanovskii,
M.N., Sal'Nikov, A.I., Mikhailov, A.G., Bezrukov, I.A., Skurikhina, E.A.,
Surkis, I.F., 2011, "Prompt determination of universal time corrections in
e-VLBI mode," Astronomy Letters, 37, 431-439.
The results of a prompt determination of
the Universal Time corrections in the e-VLBI mode on the Quasar VLBI network in
2009-2011 are presented. For this purpose, the hardware-software tools for
observational data transfer from the Svetloe, Zelenchukskaya, and Badary
Observatories to the
The computational load inside a do loop
is equalized by folding the loop appropriately if the amount of load is a
linear function of the loop index. Using this idea, we develop an efficient
parallel computation scheme of Newtonian all-pairs N-body acceleration vectors
with help from OpenMP architecture. Using a consumer PC with a quad-core
eight-thread processor, the new parallel scheme runs 4.2-4.9 times faster than
a serial computation when the number of particles exceeds a few hundred.
Gelman, S.E., Elkins-Tanton, L.T.,
Seager, S., 2011, "Effects of Stellar Flux on Tidally Locked Terrestrial
Planets: Degree-1 Mantle Convection and Local Magma Ponds," The
Astrophysical Journal, 735, 72.
We model the geodynamical evolution of
super-Earth exoplanets in synchronous rotation about their star. While
neglecting the effects of a potential atmosphere, we explore the parameter
spaces of both the Rayleigh number and intensity of incoming stellar flux, and
identify two main stages of mantle convection evolution. The first is a transient
stage in which a lithospheric temperature and thickness dichotomy emerges
between the substellar and the antistellar hemispheres, while the style of
mantle convection is dictated by the Rayleigh number. The second stage is the
development of degree-1 mantle convection. Depending on mantle properties, the
timescale of onset of this second stage of mantle evolution varies from order 1
to 100 billion years of simulated planetary evolution. Planets with higher
Rayleigh numbers (due to, for instance, larger planetary radii than the Earth)
and planets whose incoming stellar flux is high (likely for most detectable
exoplanets) will develop degree-1 mantle convection most quickly, on the order
of 1 billion years, which is within the age of many planetary systems. Surface
temperatures range from 220 K to 830 K, implying the possibility of liquid
water in some regions near the surface. These results are discussed in the
context of stable molten magma ponds on hotter planets, and the habitability of
super-Earths which may lie outside the Habitable Zone.
Girard, T.M., van Altena, W.F.,
Zacharias, N., Vieira, K., Casetti-Dinescu, D.I., Castillo, D., Herrera, D.,
Lee, Y.S., Beers, T.C., Monet, D.G., López, C.E., 2011, "The Southern
Proper Motion Program. IV. The SPM4 Catalog," The
Astronomical Journal, 142, 15.
We present the fourth installment of the
Yale/San Juan Southern Proper Motion Catalog, SPM4. The SPM4 contains absolute
proper motions, celestial coordinates, and B, V photometry for over 103 million
stars and galaxies between the south celestial pole and -20° declination. The
catalog is roughly complete to V = 17.5 and is based on photographic and CCD
observations taken with the Yale Southern Observatory's double astrograph at
Cesco Observatory in El Leoncito, Argentina. The proper-motion precision, for
well-measured stars, is estimated to be 2-3 mas yr-1, depending on
the type of second-epoch material. At the bright end, proper motions are on the
International Celestial Reference System by way of Hipparcos Catalog stars,
while the faint end is anchored to the inertial system using external galaxies.
Systematic uncertainties in the absolute proper motions are on the order of 1 mas yr-1.
Golimowski, D.A., Krist, J.E.,
Stapelfeldt, K.R., Chen, C.H., Ardila, D.R., Bryden, G., Clampin, M., Ford,
H.C., Illingworth, G.D., Plavchan, P., Rieke, G.H., Su, K.Y.L., 2011,
"Hubble and Spitzer Space Telescope Observations of the Debris Disk around
the nearby K Dwarf HD 92945," The Astronomical Journal, 142,
30.
We present the first resolved images of
the debris disk around the nearby K dwarf HD 92945, obtained with the Hubble
Space Telescope's (HST 's) Advanced Camera for
Surveys. Our F606W (Broad V) and F814W (Broad I) coronagraphic images reveal an
inclined, axisymmetric disk consisting of an inner ring about 2farcs0-3farcs0
(43-65 AU) from the star and an extended outer disk whose surface brightness
declines slowly with increasing radius approximately 3farcs0-5farcs1 (65-110
AU) from the star. A precipitous drop in the surface brightness beyond 110 AU
suggests that the outer disk is truncated at that distance. The radial
surface-density profile is peaked at both the inner ring and the outer edge of
the disk. The dust in the outer disk scatters neutrally but isotropically, and
it has a low V-band albedo of 0.1. This combination of axisymmetry, ringed and
extended morphology, and isotropic neutral scattering is unique among the 16
debris disks currently resolved in scattered light. We also present new
infrared photometry and spectra of
Hess, S.L.G.,
Zarka, P., 2011, "Modeling the radio signature of the orbital parameters,
rotation, and magnetic field of exoplanets," Astronomy and
Astrophysics, 531, 29.
Context.
Since the first extra-solar planet discovery in 1995, several hundreds of these
planets have been discovered. Most are hot Jupiters, i.e. massive planets
orbiting close to their star. These planets may be powerful radio emitters.
<BR /> Aims: We simulate the radio dynamic spectra resulting from various
interaction models between an exoplanet and its parent star, i.e.
exoplanet-induced stellar emission and three variants of the exoplanet's
magnetospheric auroral radio emission (full auroral oval, active sector fixed
in longitude, and active sector fixed in local time).
Methods: We show the physical information about the system
that can be drawn from radio observations, and how this can be achieved. This
information includes the magnetic field strength and the rotation period of the
emitting body (planet or star), the orbital period, the orbit's inclination,
and the magnetic field tilt relative to the rotation axis or offset relative to
the center of the planet. For most of these parameters, radio observations
provide a unique means of measuring them.
Results: Our results should provide the proper framework of
analysis and interpretation for future detections of radio emissions from
exoplanetary systems - or from magnetic white dwarf-planet or white dwarf-brown
dwarf systems -, that are expected to commence soon as part of extensive
programs at large radiotelescopes such as LOFAR, UTR2 or the GMRT. Our
methodology can be easily adapted to simulate specific observations, once
effective detection is achieved.
Hou, X.Y., Liu,
L., 2011, "On motions around the collinear libration points in the
elliptic restricted three-body problem," Monthly Notices of the Royal
Astronomical Society, 1008.
Motions around the collinear libration
points in the elliptic restricted three-body problem are studied. Literal
expansions of the Lissajous orbits and the halo orbits are obtained. These
expansions depend on two amplitude parameters and the orbital eccentricity of
the two primaries. Numerical simulations are done to check the validity of
these literal series and to compare them with the results in the circular
restricted three-body problem. According to the properties of these literal
expansions, three kinds of symmetric periodic orbits around the collinear libration
points are discussed. Finally, applications of these results in the Earth-Moon
system and the Sun-Earth+Moon system are discussed.
Synchronous binary asteroids may exist
in a long-term stable equilibrium, where the opposing torques from mutual body
tides and the binary YORP (BYORP) effect cancel. Interior of this equilibrium,
mutual body tides are stronger than the BYORP effect and the mutual orbit
semimajor axis expands to the equilibrium; outside of the equilibrium, the
BYORP effect dominates the evolution and the system semimajor axis will
contract to the equilibrium. If the observed population of
small (0.1-10 km diameter) synchronous binaries are in static
configurations that are no longer evolving, then this would be confirmed by a
null result in the observational tests for the BYORP effect. The confirmed
existence of this equilibrium combined with a shape model of the secondary of
the system enables the direct study of asteroid geophysics through the tidal
theory. The observed synchronous asteroid population cannot exist in this
equilibrium if described by the canonical "monolithic" geophysical
model. The "rubble pile" geophysical model proposed by Goldreich
& Sari is sufficient, however it predicts a tidal
Love number directly proportional to the radius of the asteroid, while the best
fit to the data predicts a tidal Love number inversely proportional to the
radius. This deviation from the canonical and Goldreich & Sari models
motivates future study of asteroid geophysics. Ongoing BYORP detection
campaigns will determine whether these systems are in an
equilibrium, and future determination of secondary shapes will allow
direct determination of asteroid geophysical parameters.
Kane, S.R., Henry, G.W., Dragomir,
D., Fischer, D.A., Howard, A.W., Wang, X., Wright, J.T., 2011, "Revised
Orbit and Transit Exclusion for HD 114762b," The Astrophysical Journal
Letters, 735, L41.
Transiting planets around bright stars
have allowed the detailed follow-up and characterization of exoplanets, such as
the study of exoplanetary atmospheres. The Transit Ephemeris Refinement and
Monitoring Survey is refining the orbits of the known
exoplanets to confirm or rule out both transit signatures and the presence of
additional companions. Here we present results for the companion orbiting HD
114762 in an eccentric 84 day orbit. Radial velocity analysis performed on 19
years of Lick Observatory data constrain the uncertainty in the predicted time
of mid-transit to ~5 hr, which is less than the predicted one-half day transit
duration. We find no evidence of additional companions in this system. New
photometric observations with one of our Automated Photoelectric Telescopes at
Fairborn Observatory taken during a revised transit time for companion b, along
with 23 years of nightly automated observations, allow us to rule out on-time
central transits to a limit of ~0.001 mag. Early or late central transits are
ruled out to a limit of ~0.002 mag, and transits with half the duration of a
central transit are ruled out to a limit of ~0.003 mag.
Karastergiou, A.,
Roberts, S.J., Johnston, S., Lee, H., Weltevrede, P., Kramer, M., 2011, "A
transient component in the pulse profile of PSR J0738-4042," Monthly
Notices of the Royal Astronomical Society, 415, 251-256.
One of the tenets of the radio pulsar
observational picture is that the integrated pulse profiles are constant with
time. This assumption underpins much of the fantastic science made possible via
pulsar timing. Over the past few years, however, this assumption has come under
question with a number of pulsars showing pulse shape changes on a range of
time-scales. Here, we show the dramatic appearance of a bright component in the
pulse profile of PSR J0738-4042 (B0736-40). The component arises on the leading
edge of the profile. It was not present in 2004 but strongly present in 2006
and all observations thereafter. A subsequent search through the literature
shows that the additional component varies in flux density over time-scales of
decades. We show that the polarization properties of the transient component
are consistent with the picture of competing orthogonal polarization modes.
Faced with the general problem of identifying and characterizing average
profile changes, we outline and apply a statistical technique based on a hidden
Markov model. The value of this technique is established through simulations
and is shown to work successfully in the case of low signal-to-noise ratio
profiles.
Kipping, D.M., 2011, "LUNA: an
algorithm for generating dynamic planet-moon transits," Monthly Notices
of the Royal Astronomical Society, 1132.
t has been
previously shown that moons of extrasolar planets may be detectable with the
Kepler Mission, for moon masses above ~0.2 M⊕. Transit timing effects have been
formerly identified as a potent tool to this end, exploiting the dynamics of
the system. In this work, we explore the simulation of transit light curves of
a planet plus a single moon including not only the transit timing effects, but
also the light-curve signal of the moon itself. We introduce our new algorithm,
LUNA, which produces transit light curves for both bodies, analytically
accounting for shadow overlaps, stellar limb darkening and planet-moon
dynamical motion. By building the dynamics into the core of LUNA, the routine
automatically accounts for transit-timing/duration variations and
ingress/egress asymmetries for not only the planet, but also the moon.
We then generate some artificial data for two feasibly
detectable hypothetical systems of interest: (i) prograde and (ii) retrograde
Earth-like moons around a habitable-zone
Koch, F.E.,
Hansen, B.M.S., 2011, "On collisional capture rates of irregular
satellites around the gas-giant planets and the minimum mass of the solar
nebula," Monthly Notices of the Royal Astronomical Society,
1069.
We investigate the probability that an inelastic
collision of planetesimals within the Hill sphere of the Jovian planets could
explain the presence and orbits of observed irregular satellites. Capture of
satellites via this mechanism is highly dependent on not only the mass of the
protoplanetary disc, but also the shape of the planetesimal size distribution.
We performed 2000 simulations for integrated time intervals ~2 Myr and found
that, given the currently accepted value for the minimum mass solar nebula and
planetesimal number density based upon the Nesvorný et al. and Charnoz &
Morbidelli size distribution dN~D-3.5dD, the collision rates for the
different Jovian planets range between ~0.6 and ≳170 Myr-1 for
objects with radii 1 km ≤r≤ 10 km. Additionally, we found that the
probability that these collisions remove enough orbital energy to yield a bound
orbit was ≲10-5 and had very little dependence on the
relative size of the planetesimals. Of these collisions, the collision energy
between two objects was ≳103 times the gravitational binding energy
for objects with radii ~100 km. We find that capturing irregular satellites via
collisions between unbound objects can only account for ~0.1 per cent of the
observed population, hence this cannot be the sole method of producing
irregular satellites.
Köhler, R., 2011,
"The orbit of GG Tauri A," Astronomy and Astrophysics, 530,
126.
Aims: We present a study of the orbit of
the pre-main-sequence binary system GG Tau A and its relation to its
circumbinary disk, in order to find an explanation for the sharp inner edge of
the disk.
Methods: Three new relative astrometric positions of the
binary were obtained with NACO at the VLT. We combine them with data from the
literature and fit orbit models to the dataset.
Results: We find that an orbit coplanar with the disk and
compatible with the astrometric data is too small to explain the inner gap of
the disk. On the other hand, orbits large enough to cause the gap are tilted
with respect to the disk. If the disk gap is indeed caused by the stellar
companion, then the most likely explanation is a combination of underestimated
astrometric errors and a misalignment between the planes of the disk and the
orbit.
Koot, L., de
Viron, O., 2011, "Atmospheric contributions to nutations and implications
for the estimation of deep Earth's properties from nutation observations,"
Geophysical Journal International, 185, 1255-1265.
We propose a new estimation of the
atmospheric contributions to Earth's nutations based on three reanalyses of
atmospheric global circulation models (GCM), namely the two reanalyses of the
Kovács, T., Bene,
G., Tél, T., 2011, "Relativistic effects in the chaotic Sitnikov
problem," Monthly Notices of the Royal Astronomical Society, 414,
2275-2281.
We investigate the phase-space structure
of the relativistic Sitnikov problem in the first post-Newtonian approximation.
The phase-space portraits show a strong dependence on the gravitational radius
which describes the strength of the relativistic pericentre advance.
Bifurcations appearing at various gravitational radii are presented. Transient
chaotic behaviour related to escapes from the primaries is also studied.
Finally, the numerically determined chaotic saddle is investigated in the
context of hyperbolic and non-hyperbolic dynamics as a function of the
gravitational radius.
Krijt, S., Dominik, C., 2011,
"Stirring up the dust: a dynamical model for halo-like dust clouds in
transitional disks," Astronomy and Astrophysics, 531, 80.
Context. A
small number of young stellar objects show signs of a halo-like structure of
optically thin dust, in addition to a circumstellar disk. This halo or torus is
located within a few AU of the star, but its origin has not yet been
understood.
Aims: A dynamically excited cloud of planetesimals colliding
to eventually form dust could produce such a structure. The cause of the
dynamical excitation could be one or more planets, perhaps on eccentric orbits,
or a migrating planet. This work investigates an inwardly migrating planet that
is dynamically scattering planetesimals as a possible cause for the observed
structures. If this mechanism is responsible, the observed halo-like structure
could be used to infer the existence of planets in these systems.
Methods: We present analytical estimates on the maximum
inclination reached owing to dynamical interactions between planetesimals and a
migrating planet. In addition, a symplectic integrator is used to simulate the
effect of a migrating planet on a population of planetesimals. Collision time
scales are estimated for the resulting population of planetesimals and the size
distribution of the dust created in catastrophic collisions is determined.
Results: It is found that an inwardly migrating planet is
only able to scatter the material it encounters to highly-inclined orbits if
the material is on an eccentric orbit. Such eccentric orbits can be the result
of resonance trapping and eccentricity pumping. Simulations show that for a
certain range of migration rates and planet masses, resonance capture combined
with planetary migration indeed causes the planetesimals to reach eccentric
orbits and subsequently get scattered to highly-inclined orbits. The size
distribution of the resulting dust is calculated determined to find the total
mass and optical depth, which are found to compare
reasonably well with the observed structures.
Conclusions: Dynamical scattering of planetesimals caused by
a planet migrating in, followed by the grinding down of these planetesimals to
dust grains, appears to be a promising explanation for the inferred
circumstellar dust clouds. Further study is needed to see if the haloes can be
used to infer the presence of planets.
Kubo, Y., 2011, "Kinematical
modeling of the Earth rotation, focusing on the Oppolzer terms in a rigid Earth
and the Oppolzer-like terms in an elastic Earth," Celestial Mechanics
and Dynamical Astronomy, 110, 143-168.
Under perturbations from outer bodies,
the Earth experiences changes of its angular momentum axis, figure axis and
rotational axis. In the theory of the rigid Earth, in addition to the
precession and nutation of the angular momentum axis given by the Poisson
terms, both the figure axis and the rotational axis suffer forced deviation
from the angular momentum axis. This deviation is expressed by the so-called
Oppolzer terms describing separation of the averaged figure axis, called CIP
(Celestial Intermediate Pole) or CEP (Celestial Ephemeris Pole), and the
mathematically defined rotational axis, from the angular momentum axis. The CIP
is the rotational axis in a frame subject to both precession and nutation,
while the mathematical rotational axis is that in the inertial (non-rotating)
frame. We investigate, kinematically, the origin of the separation between
these two axes - both for the rigid Earth and an elastic Earth. In the case of
an elastic Earth perturbed by the same outer bodies, there appear further
deviations of the figure and rotational axes from the angular momentum axis.
These deviations, though similar to the Oppolzer terms in the rigid Earth, are
produced by quite a different physical mechanism. Analysing this mechanism, we
derive an expression for the Oppolzer-like terms in an elastic Earth. From this
expression we demonstrate that, under a certain approximation (in neglect of
the motion of the perturbing outer bodies), the sum of the direct and convective perturbations of the spin axis coincides with the
direct perturbation of the figure axis. This equality, which is approximate,
gets violated when the motion of the outer bodies is taken into account.
Labadie, L., Martín, G.,
Context.
Observations of milliarcsecond-resolution scales and high dynamic range hold a
central place in the exploration of distant planetary systems in order to achieve,
for instance, the spectroscopic characterization of exo-Earths or the detailed
mapping of their protoplanetary disc birthplace. Multi-aperture infrared
interferometry, either from the ground or from space, is a very powerful
technique to tackle these goals. However, significant technical efforts still
need to be undertaken to achieve a simplification of these instruments if we
wish to recombine the light from a large number of telescopes.
Integrated-optics concepts appear to be a suitable alternative to the current
conventional designs, especially if their use can be extended to a higher
number of astronomical bands.
Aims: This article reports, for the first time to our
knowledge, the experimental demonstration of the feasibility of an integrated-optics
approach to mid-infrared beam combination for single-mode stellar
interferometry.
Methods: We fabricated a two-telescope beam combiner
prototype integrated on a substrate of chalcogenide glass, a material
transparent from ~1 ?m to ~14 ?m. We developed
laboratory tools to characterize in the mid-infrared the modal properties and
the interferometric capabilities of our device.
Results: We obtain interferometric fringes at 10 mm and measure a mean contrast V =
0.981 ± 0.001 with high repeatability over one week and high stability over a
time-period of ~5 h. We show experimentally - as well as on the basis of
modeling considerations - that the component has a single-mode behavior at this
wavelength, which is essential to achieve high-accuracy interferometry. From
previous studies, the propagation losses are estimated to be 0.5
dB/cm for this type of component. We also discuss possible issues that
may impact the interferometric contrast.
Conclusions: The IO beam combiner performs well at the
tested wavelength. We also anticipate the requirement of a closer matching
between the numerical apertures of the component and the (de)coupling optics to
optimize the total throughput. The next step foreseen is the achievement of wide-band
interferograms.
Lara, M.,
Available observations of the shape of
Ceres show it as a rotationally symmetric oblate spheroid. However, deviations
from axisymmetry even at the level of observational accuracy may show
significant effects on its rotational dynamics. These presumed deviations can
be accounted for in a purely analytical way by means of perturbation theory. In
our approach, the spherical rotor is taken as the unperturbed part of the
motion instead of the more common torque-free motion or uniaxial body
approaches. This alternative allows us to compute an analytical solution for
the rotation of Ceres under the gravitational pull of the Sun by proceeding
with a successive elimination of the different angles, which only involves
quadratures of straightforward computation.
Lazaridis, K., Verbiest, J.P.W.,
Tauris, T.M., Stappers, B.W., Kramer, M., Wex, N., Jessner, A., Cognard, I.,
Desvignes, G., Janssen, G.H., Purver, M.B., Theureau, G., Bassa, C.G., Smits,
R., 2011, "Evidence for gravitational quadrupole moment variations in the
companion of PSR J2051-0827," Monthly Notices of the Royal Astronomical
Society, 414, 3134-3144.
We have conducted radio timing
observations of the eclipsing millisecond binary pulsar PSR J2051-0827 with the
European Pulsar Timing Array network of telescopes and the Parkes radio
telescope, spanning over 13 yr. The increased data span allows significant
measurements of the orbital eccentricity, e= (6.2 ± 1.3) × 10-5, and
composite proper motion, μt= 7.3 ± 0.4 mas yr-1. Our
timing observations have revealed secular variations of the projected semimajor
axis of the pulsar orbit which are much more extreme than those previously
published and of the orbital period of the system. Investigations of the
physical mechanisms producing such variations confirm that the variations of
the semimajor axis are most probably caused by classical spin-orbit coupling in
the binary system, while the variations in orbital period are most likely
caused by tidal dissipation leading to changes in the gravitational quadrupole
moment of the companion.
Lee, K.J., Wex,
N., Kramer, M., Stappers, B.W., Bassa, C.G., Janssen, G.H., Karuppusamy, R.,
Smits, R., 2011, "Gravitational wave astronomy of single sources with a
pulsar timing array," Monthly Notices of the Royal Astronomical
Society, 414, 3251-3264.
The stability of radio millisecond pulsars as celestial
clocks allows for the possibility to detect and study the properties of
gravitational waves (GWs) when the received pulses are timed jointly in a
'Pulsar Timing Array' (
We find that, at low redshift, a
For our findings, we derive an analytical expression to
describe the accuracy of a pulsar distance measurement via timing parallax. We
consider 5 yr of bi-weekly observations at a precision of 15 ns for close-by
(˜0.5-1 kpc) pulsars. Timing 20 pulsars would allow us to detect a GW source
with an amplitude larger than 5 × 10-17. We
calculate the corresponding GW and binary orbital parameters and their
measurement precision. The accuracy of measuring the binary orbital inclination
angle, the sky position and the GW frequency is calculated as functions of the
GW amplitude. We note that the 'pulsar term', which is commonly regarded as
noise, is essential for obtaining an accurate measurement for the GW source
location.
We also show that utilizing the information encoded in the
GW signal passing the Earth also increases the accuracy of pulsar distance
measurements. If the GW is strong enough, one can achieve sub-parsec distance
measurements for nearby pulsars with distance less than ˜0.5-1 kpc.
Libert, A.-S., Hubaux, C., Carletti,
T., 2011, "The Global Symplectic Integrator: an efficient tool for
stability studies of dynamical systems. Application to the
Kozai resonance in the restricted three-body problem," Monthly Notices
of the Royal Astronomical Society, 414, 659-667.
Following the discovery of extrasolar
systems, the study of long-term evolution and stability of planetary systems is
enjoying a renewed interest. While non-symplectic integrators are very
time-consuming because of the very long time-scales and the small integration
steps required to have a good energy preservation,
symplectic integrators are well suited for the study of such orbits on long
time-spans. However, stability studies of dynamical systems generally rely on
non-symplectic integrations of deviation vectors. In this work we propose a
numerical approach to distinguish between regular and chaotic orbits in
Hamiltonian systems, hereby called Global Symplectic Integrator. It consists of
the simultaneous integration of the orbit and the deviation vectors using a
symplectic scheme of any order. In particular, due to its symplectic properties,
the proposed method allows us to recover the correct orbit characteristics
using very large integration time-steps, fluctuations of energy around a
constant value and short CPU times. It proves to be more efficient than
non-symplectic schemes to correctly identify the behaviour of a given orbit,
especially on dynamics acting on long time-scales. To illustrate the numerical
performances of the global symplectic integrator, we will apply it to the
well-known toy problem of Hénon-Heiles and the challenging problem of the Kozai
resonance in the restricted three-body problem, whose secular effects have
periods of the order of 104-105 yr.
Liu, X., Baoyin,
H., Ma, X., 2011, "Equilibria, periodic orbits around equilibria, and
heteroclinic connections in the gravity field of a rotating homogeneous
cube," Astrophysics and Space Science, 333, 409-418.
This paper investigates the dynamics of
a particle orbiting around a rotating homogeneous cube, and shows fruitful
results that have implications for examining the dynamics of orbits around
non-spherical celestial bodies. This study can be considered as an extension of
previous research work on the dynamics of orbits around simple shaped bodies,
including a straight segment, a circular ring, an annulus disk, and simple
planar plates with backgrounds in celestial mechanics. In the synodic reference
frame, the model of a rotating cube is established, the equilibria are
calculated, and their linear stabilities are determined. Periodic orbits around
the equilibria are computed using the traditional differential correction
method, and their stabilities are determined by the eigenvalues of the
monodromy matrix. The existence of homoclinic and heteroclinic orbits
connecting periodic orbits around the equilibria is examined and proved
numerically in order to understand the global orbit structure of the system.
This study contributes to the investigation of irregular shaped celestial
bodies that can be divided into a set of cubes.
Louchet-Chauvet,
A., Farah, T., Bodart, Q., Clairon, A., Landragin, A., Merlet, S., Pereira Dos
Santos, F., 2011, "The influence of transverse motion within an atomic
gravimeter," New Journal of Physics, 13, 5025.
Limits on the long-term stability and
accuracy of a second generation cold atom gravimeter are investigated. We
demonstrate a measurement protocol based on four interleaved measurement
configurations, which allows rejection of most of the systematic effects, but
not those related to Coriolis acceleration and wave-front distortions. Both are
related to the transverse motion of the atomic cloud. Carrying out measurements
with opposite orientations with respect to the Earth's rotation vector
direction allows us to separate the effects and correct for the Coriolis shift.
Finally, measurements at different atomic temperatures are presented and
analyzed. In particular, we show the difficulty of extrapolating these
measurements to zero temperature, which is required in order to correct for the
bias due to wave-front distortions.
Luzum, B., Capitaine, N., Fienga,
A., Folkner, W.,
In the 2006-2009 triennium,
the International Astronomical Union (IAU) Working Group on Numerical Standards
for Fundamental Astronomy determined a list of Current Best Estimates (CBEs).
The IAU 2009 Resolution B2 adopted these CBEs as the IAU (2009) System of
Astronomical Constants. Additional work continues to define the process of updating
the CBEs and creating a standard electronic document.
The formation and evolution of the
circumstellar disc in the collapsing molecular cloud with and without magnetic
field is investigated from the pre-stellar stage resolving both the molecular
cloud core and the protostar itself. In the collapsing cloud core, the first
(adiabatic) core appears prior to the protostar formation. Reflecting the
thermodynamics of the collapsing gas, the first core is much more massive than
the protostar. When the molecular cloud has no angular momentum, the first core
falls on to the protostar and disappears a few years after the protostar
formation. On the other hand, when the molecular cloud has an angular momentum,
the first core does not disappear even after the protostar formation, and
directly evolves into the circumstellar disc with a Keplerian rotation. There
are two paths for the formation of the circumstellar disc. When the initial
cloud has a considerably small rotational energy, two nested discs appear just
after the protostar formation. During the early main accretion phase, the inner
disc increases its size and merges with the outer disc (i.e. first core) to
form a single circumstellar disc with a Keplerian rotation. On the other hand,
when the molecular cloud has a rotational energy comparable to observations, a
single centrifugally supported disc that corresponds to the first core already
exists prior to the protostar formation. In such a cloud, the first core
density gradually increases, maintaining the Keplerian rotation and forms the
protostar inside it. The magnetic field rarely affects the early formation of
the circumstellar disc because the magnetic field dissipates in the
high-density gas region where the circumstellar disc forms. As a result, in any
case, the protostar at its formation is already surrounded by a massive
circumstellar disc. The circumstellar disc is about 10-100 times more massive
than the protostar in the main accretion phase. Such discs are favourable sites
for the formation of binary companions and gas-giant planets.
Marzari, F., Thébault,
P., 2011, "On how optical depth tunes the effects of the interstellar
medium on debris discs," Monthly Notices of the Royal Astronomical
Society, 1049.
The flux of neutral atoms of the
interstellar medium (ISM) surrounding stars and their environment affects the
motion of dust particles in debris discs, causing a significant dynamical
evolution. Large values of eccentricity and inclination can be excited, and
strong correlations settle in among the orbital angles. This dynamical
behaviour, in particular for bound dust grains, can potentially cause
significant asymmetries in dusty discs around solar-type stars, which might be
detected by observations. However, the amount of orbital change as a result of
this non-gravitational perturbation is strongly limited by the collisional
lifetime of dust particles. We show that, for large values of the disc's
optical depth, the influence of the ISM flow on the disc shape is almost
negligible. This is because the grains are collisionally destroyed before they
can accumulate enough orbital changes as a result of the ISM perturbations.
However, for values smaller than 10-3, peculiar asymmetric patterns
appear in the density profile of the disc when we consider grains of 1-10
μm grains, just above the blow-out threshold. The extent and relevance of
these asymmetries grow for lower values of the optical depth. An additional
sink mechanism, which might prevent the formation of large clumps and warping
in the discs, is related to the fast inward migration as a result of the drag
component of the forces. When a significant eccentricity is enlarged by the ISM
perturbations, the drag forces (Poynting-Robertson drag and, in particular, ISM
drag) drive the disc particles on fast migrating tracks, leading them into the star
on a short time-scale. It is then expected that discs with small optical depth
expand inside the parent body ring all the way towards the star, while discs
with large optical depth would not significantly extend inside.
Moór, A., Frey, S., Lambert, S.B.,
Titov, O.A., Bakos, J., 2011, "On the Connection of the Apparent Proper
Motion and the VLBI Structure of Compact Radio Sources," The
Astronomical Journal, 141, 178.
Many of the compact extragalactic radio
sources that are used as fiducial points to define the celestial reference
frame are known to have proper motions detectable with long-term
geodetic/astrometric very long baseline interferometry (VLBI) measurements.
These changes can be as high as several hundred microarcseconds per year for certain
objects. When imaged with VLBI at milliarcsecond (mas) angular resolution,
these sources (radio-loud active galactic nuclei) typically show structures
dominated by a compact, often unresolved "core" and a one-sided
"jet." The positional instability of compact radio sources is
believed to be connected with changes in their brightness distribution
structure. For the first time, we test this assumption in a statistical sense
on a large sample rather than on only individual objects. We investigate a sample
of 62 radio sources for which reliable long-term time series of astrometric
positions as well as detailed 8 GHz VLBI brightness distribution models are
available. We compare the characteristic direction of their extended jet
structure and the direction of their apparent proper motion. We present our
data and analysis method, and conclude that there is indeed a correlation
between the two characteristic directions. However, there are cases where the
~1-10 mas scale VLBI jet directions are significantly misaligned with respect
to the apparent proper motion direction.
Nichols, J.D., 2011,
"Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with
internal plasma sources: implications for detectability of auroral radio
emissions," Monthly Notices of the Royal Astronomical Society, 414,
2125-2138.
In this paper we provide the first
consideration of magnetosphere-ionosphere coupling at Jupiter-like exoplanets
with internal plasma sources such as volcanic moons. We estimate the radio
power emitted by such systems under the condition of near-rigid corotation
throughout the closed magnetosphere, in order to examine the behaviour of the
best candidates for detection with next generation radio telescopes. We thus
estimate for different stellar X-ray-UV (XUV) luminosity cases the orbital
distances within which the ionospheric Pedersen conductance would be high
enough to maintain near-rigid corotation, and we then consider the magnitudes
of the large-scale magnetosphere-ionosphere currents flowing within the
systems, and the resulting radio powers, at such distances. We also examine the
effects of two key system parameters, i.e. the planetary angular velocity and
the plasma mass outflow rate from sources internal to the magnetosphere. In all
XUV luminosity cases studied, a significant number of parameter combinations
within an order of magnitude of the jovian values are capable of producing
emissions observable beyond 1 pc, in most cases requiring exoplanets orbiting
at distances between ~1 and 50 au, and for the higher XUV luminosity cases
these observable distances can reach beyond ~50 pc for massive, rapidly
rotating planets. The implication of these results is that the best candidates
for detection of such internally generated radio emissions are rapidly rotating
Jupiter-like exoplanets orbiting stars with high XUV luminosity at orbital
distances beyond ~1 au, and searching for such emissions may offer a new method
of detection of more distant-orbiting exoplanets.
Patience, J., Bulger, J., King,
R.R., Ayliffe, B., Bate, M.R., Song, I., Pinte, C., Koda, J., Dowell, C.D.,
Kovács, A., 2011, "Spatially resolved submillimeter imaging of the HR 8799
debris disk," Astronomy and Astrophysics, 531, L17.
Dynamical interactions between planets
and debris disks may sculpt the disk structure and impact planetary orbits, but
only a few systems with both imaged planets and spatially resolved debris disks
are known. With the Caltech Submm Observatory (CSO), we have observed the HR
8799 debris disk at 350 ?m. The 350 mm map is the first spatially
resolved measurement of the debris disk encircling the HR 8799 planetary system
at this wavelength. Both the flux and size of the emission are consistent with
a Kuiper belt of dust extending from ~100-300 AU. Although the resolution of
the current map is limited, the map shows an indication of offset asymmetric
emission, and several scenarios for this possibility are explored with
radiative transfer calculations of a star-disk system and N-body numerical
simulations of planet-disk interactions with parameters representative of the
HR 8799 system. Based on observations obtained at the Caltech Submillimeter
Observatory.Figures 3 and 4 are available in electronic form at http://www.aanda.org
Pont, F., Husnoo, N., Mazeh, T.,
Fabrycky, D., 2011, "Determining eccentricities of transiting planets: a
divide in the mass-period plane," Monthly Notices of the Royal
Astronomical Society, 414, 1278-1284.
The two dominant features in the
distribution of orbital parameters for close-in exoplanets are the prevalence
of circular orbits for very short periods, and the observation that planets on
closer orbits tend to be heavier. The first feature is interpreted as a signature
of tidal evolution, while the origin of the second, a 'mass-period relation'
for hot Jupiters, is not understood. In this paper we reconsider the ensemble
properties of transiting exoplanets with well-measured parameters, focusing on
orbital eccentricity and the mass-period relation. We recalculate the
constraints on eccentricity in a homogeneous way, using new radial velocity
data, with particular attention to statistical biases. We find that planets on
circular orbits gather in a well-defined region of the mass-period plane, close
to the minimum period for any given mass. Exceptions to this pattern reported
in the literature can be attributed to statistical biases. The ensemble data is
compatible with classical tide theory with orbital circularization caused by
tides raised on the planet, and suggest that tidal
circularization and the stopping mechanisms for close-in planets are closely
related to each other. The position mass-period relation is compatible with a
relation between a planet's Hill radius and its present orbit.
Porter, S.B.,
Grundy, W.M., 2011, "Post-capture Evolution of Potentially Habitable
Exomoons," The Astrophysical Journal Letters, 736, L14.
The satellites of extrasolar planets
(exomoons) have been recently proposed as astrobiological targets. Since giant
planets in the habitable zone are thought to have migrated there, it is
possible that they may have captured a former terrestrial planet or
planetesimal. We therefore attempt to model the dynamical evolution of a
terrestrial planet captured into orbit around a giant planet in the habitable
zone of a star. We find that approximately half of loose elliptical orbits
result in stable circular orbits over timescales of less than a few million
years. We also find that those orbits are mostly at low inclination, but have
no prograde/retrograde preference. In addition, we calculate the transit timing
and duration variations for the resulting systems, and find that potentially
habitable Earth-mass exomoons should be detectable.
Qian, S.-B., Liu,
L., Liao, W.-P., Li, L.-J., Zhu, L.-Y., Dai, Z.-B., He, J.-J., Zhao, E.-G.,
Zhang, J., Li, K., 2011, "Detection of a planetary system orbiting the
eclipsing polar HU Aqr," Monthly Notices of the Royal Astronomical
Society, 414, L16-L20.
Using the precise times of mid-egress of
the eclipsing polar HU Aqr, we discovered that this polar is orbited by two or
more giant planets. The two planets detected so far have masses of at least 5.9
and 4.5MJup. Their respective distances from the polar are 3.6 and
5.4 au with periods of 6.54 and 11.96 yr, respectively. The observed rate of
decrease of period derived from the downward parabolic change in the observed -
calculated (O - C) curve is a factor of 15 larger than the value expected for gravitational
radiation. This indicates that it may be only a part of a long-period cyclic
variation, revealing the presence of one more planet. It is interesting to note
that the two detected circumbinary planets follow the Titus-Bode law of solar
planets with n= 5 and 6. We estimate that another 10 yr of observations will
reveal the presence of the predicted third planet.
Quillen, A., 2011, "Alessandra
Celletti: Stability and Chaos in Celestial Mechanics," Celestial
Mechanics and Dynamical Astronomy, 33.
Ramos-Caro, J.,
Pedraza, J.F., Letelier, P.S., 2011, "Motion around a monopole + ring
system - I. Stability of equatorial circular orbits versus regularity of
three-dimensional motion," Monthly Notices of the Royal Astronomical
Society, 414, 3105-3116.
We study the motion of test particles
around a centre of attraction represented by a monopole (with and without
spheroidal deformation) surrounded by a ring, given as a superposition of
Morgan and Morgan discs. We deal with two kinds of bounded orbits: (i)
equatorial circular orbits and (ii) general three-dimensional orbits. The first
case provides a method to perform a linear stability analysis of these
structures by studying the behaviour of vertical and epicyclic frequencies as
functions of the mass ratio, the size of the ring and/or the quadrupolar
deformation. In the second case, we study the influence of these parameters in
the regularity or chaoticity of motion. We find that there is a close
connection between linear stability (or instability)
of equatorial circular orbits and regularity (or chaoticity) of the
three-dimensional motion.
Rodin, A.E., Chen,
D., 2011, "Optimal filtration and a pulsar time scale," Astronomy
Reports, 55, 622-628.
An algorithm is proposed for
constructing a group (ensemble) pulsar time based on the application of optimal
Wiener filters. This algorithm makes it possible to separate the contributions
of variations of the atomic time scale and of the pulsar rotation to
barycentric residual deviations of the pulse arrival times. The method is
applied to observations of the pulsars PSR B1855+09 and PSR B1937+21, and is
used to obtain corrections to UTC relative to the group pulsar time PTens.
Direct comparison of the terrestial time TT(BIPM06)
and the group pulsar time PTens shows that they disagree by no
more than 0.4 ± 0.17 µs. Based on the fractional instability of the time
difference TT(BIPM06)-PTens, σ z = (0.5 ±
2) × 10-15, a new limit for the energy density of the
gravitational-wave background is established at the level Ω g h 2 ˜
10-9.
Roedig, C., Dotti,
M., Sesana, A., Cuadra, J., Colpi, M., 2011, "Limiting eccentricity of
subparsec massive black hole binaries surrounded by self-gravitating gas
discs," Monthly Notices of the Royal Astronomical Society,
979.
We study the dynamics of supermassive
black hole binaries embedded in circumbinary gaseous discs, with the smoothed
particle hydrodynamics code GADGET-2. The subparsec binary (of total mass M and
mass ratio q= 1/3) has excavated a gap and transfers its angular momentum to
the self-gravitating disc (Mdisc= 0.2M). We explore the changes of
the binary eccentricity, e, by simulating a sequence of binary models that
differ in the initial eccentricity e0 only. In initially
low-eccentric binaries, the eccentricity increases with time, while in
high-eccentric binaries e declines, indicating the existence of a limiting
eccentricity ecrit that is found to fall in the interval [0.6,
0.8]. We also present an analytical interpretation for this saturation limit.
An important consequence of the existence of ecrit is the
detectability of a significant residual eccentricity eLISA by
the proposed gravitational wave detector Laser Interferometer Space Antenna
(LISA). It is found that at the moment of entering the LISA frequency domain eLISA˜
10-3-10-2, a signature of its earlier coupling with the
massive circumbinary disc. We also observe large periodic inflows across the
gap, occurring on the binary and disc dynamical time-scales rather than on the
viscous time. These periodic changes in the accretion rate (with amplitudes up
to ˜100 per cent, depending on the binary eccentricity) can be considered a
fingerprint of eccentric subparsec binaries migrating inside a circumbinary
disc.
Rogers, P.D.,
Wadsley, J., 2011, "The importance of photosphere cooling in simulations
of gravitational instability in the inner regions of protostellar discs," Monthly
Notices of the Royal Astronomical Society, 414, 913-929.
The fragmentation of protostellar discs
via gravitational instability (GI) has been proposed as a mechanism for forming
giant planets. We present methods and tests for a new implementation of
radiative transfer in the Gasoline TreeSPH code with which we investigate the
viability of this mechanism in the inner tens of au of discs. Accurate
photosphere boundary treatments are of fundamental importance to the outcome of
GI. We demonstrate that previous photosphere treatments using edge-particles
overestimate the cooling rate, and present a new treatment that can accurately
determine photosphere cooling, as demonstrated by its ability to satisfy the
relaxation test of Boley et al. (2007b). This is the first time this test has
been carried out with smoothed particle hydrodynamics. We carry out simulations
of protostellar discs that are unstable in the inner tens of au and find that
they do not fragment because they do not cool fast enough. One of these cases
has been previously found to undergo fragmentation. The fact that we do not
observe fragmentation in this simulation emphasizes the use of accurate
photosphere boundary treatments and the difficulty of forming giant planets in
the inner disc (inside of 40 au) via the GI mechanism.
Sanchis-Ojeda, R.,
Winn, J.N., Holman, M.J., Carter, J.A., Osip, D.J., Fuentes, C.I., 2011,
"Starspots and Spin-orbit Alignment in the WASP-4 Exoplanetary
System," The Astrophysical Journal, 733, 127.
We present the analysis of 4 months of Kepler photometry of
the K4V star HAT-P-11, including 26 transits of its "super-Neptune"
planet. The transit data exhibit numerous anomalies that we interpret as
passages of the planet over dark starspots. These spot-crossing anomalies
preferentially occur at two specific phases of the transit. These phases can be
understood as the intersection points between the transit chord and the active
latitudes of the host star, where starspots are most abundant. Based on the
measured characteristics of spot-crossing anomalies, and previous observations
of the Rossiter-McLaughlin effect, we find two solutions for the stellar
obliquity (psi) and active latitude (l): either psi = 106 and l = 19.7, or psi = 97 and l = 67 (all in degrees). If the active
latitude changes with time in analogy with the "butterfly diagram" of
the Sun's activity cycle, future observations should reveal changes in the
preferred phases of spot-crossing anomalies.
Schwarz, R.,
Haghighipour, N., Eggl, S., Pilat-Lohinger, E., Funk, B., 2011, "Prospects
of the detection of circumbinary planets with Kepler and CoRoT using the
variations of eclipse timing," Monthly Notices of the Royal
Astronomical Society, 414, 2763-2770.
In close eclipsing binaries,
measurements of the variations in binary's eclipse timing may be used to infer
information about the existence of circumbinary objects. To determine the
possibility of the detection of such variations with CoRoT and Kepler space
telescopes, we have carried out an extensive study of the dynamics of a binary
star system with a circumbinary planet, and calculated its eclipse timing
variations (ETVs) for different values of the mass ratio and orbital elements
of the binary and the perturbing body. Here, we present the results of our
study and assess the detectability of the planet by comparing the resulting
values of ETVs with the temporal sensitivity of CoRoT and Kepler. Results point
to extended regions in the parameter space where the perturbation of a planet
may be large enough to create measurable variations in the eclipse timing of
the secondary star. Many of these variations point to potentially detectable
ETVs and the possible inference of Jovian-type planets.
Simpson, E.K., Pollacco, D.,
Cameron, A.C., Hébrard, G., Anderson, D.R., Barros, S.C.C., Boisse, I., Bouchy,
F., Faedi, F., Gillon, M., Hebb, L., Keenan, F.P., Miller, G.R.M., Moutou, C.,
Queloz, D., Skillen, I., Sorensen, P., Stempels, H.C., Triaud, A., Watson,
C.A., Wilson, P.A., 2011, "The spin-orbit angles of the transiting
exoplanets WASP-1b, WASP-24b, WASP-38b and HAT-P-8b from Rossiter-McLaughlin
observations," Monthly Notices of the Royal Astronomical Society, 414,
3023-3035.
We report on the discovery of WASP-37b,
a transiting hot Jupiter orbiting an m v =
12.7 G2-type dwarf, with a period of 3.577469 ± 0.000011 d, transit epoch T 0 =
2455338.6188 ± 0.0006 (HJD; dates throughout the paper are given in Coordinated
Universal Time (UTC)), and a transit duration 0.1304+0.0018 –0.0017 d.
The planetary companion has a mass M p = 1.80
± 0.17M J and radius R p =
1.16+0.07 –0.06 R J,
yielding a mean density of 1.15+0.12 –0.15 ρJ.
>From a spectral analysis, we find that the host star has M 
=
0.925 ± 0.120 M 
, R =
1.003 ± 0.053 R , T eff =
5800 ± 150 K, and [Fe/H] = –0.40 ± 0.12. WASP-37 is therefore one of the lowest
metallicity stars to host a transiting planet.
Tereno,
The observed acceleration of the
universe, explained through dark energy, could alternatively be explained
through a modification of gravity that would also induce modifications in the
evolution of cosmological perturbations. We use new weak lensing data from the
COSMOS survey to test for deviations from general relativity (GR). The
departure from GR is parametrized in a model-independent way that consistently
parametrizes the two-point cosmic shear amplitude and growth. Using
Todorovic, N.,
2011, "The Role of a Steepness Parameter in the Exponential Stability of a
Model Problem.
Numerical Aspects," Serbian Astronomical Journal, 182, 25-33.
The Nekhoroshev theorem considers quasi
integrable Hamiltonians providing stability of actions in exponentially long
times. One of the hypothesis required by the theorem
is a mathematical condition called steepness. Nekhoroshev conjectured that
different steepness properties should imply numerically observable differences
in the stability times. After a recent study on this problem (Guzzo et al.
2011, Todorovic et al. 2011) we show some additional numerical results on the
change of resonances and the diffusion laws produced by the increasing effect
of steepness. The experiments are performed on a 4-dimensional steep symplectic
map designed in a way that a parameter smoothly regulates the steepness
properties in the model.
Uozumi, T.,
Yumoto, K., Tokunaga, T., Solovyev, S.I., Shevtsov, B.M.,
In this report we present a temporal
relationship between ground Pi2 and auroral kilometric radiation (
Valeri, D., 2011,
"The Lyrid Fireball (2008). (Italian Title: Il bolide delle Lyridi
(2008))," Astronomia. La rivista dell' Unione Astrofili Italiani, 3, 30-33.
On
van Haasteren, R., Levin, Y.,
Janssen, G.H., Lazaridis, K., Kramer, M., Stappers, B.W., Desvignes, G.,
Purver, M.B., Lyne, A.G., Ferdman, R.D., Jessner, A., Cognard, I., Theureau,
G., D'Amico, N., Possenti, A., Burgay, M., Corongiu, A., Hessels, J.W.T.,
Smits, R., Verbiest, J.P.W., 2011, "Placing limits on the stochastic
gravitational-wave background using European Pulsar Timing Array data," Monthly
Notices of the Royal Astronomical Society, 414, 3117-3128.
Direct detection of low-frequency
gravitational waves (GWs, 
Hz)
is the main goal of pulsar timing array (
,
where f is the GW frequency. In this paper we use the current
data from the European
Wu, Y., Lithwick,
Y., 2011, "Secular Chaos and the Production of Hot Jupiters," The
Astrophysical Journal, 735, 109.
In a planetary system with two or more
well-spaced, eccentric, inclined planets, secular interactions may lead to
chaos. The innermost planet may gradually become very eccentric and/or inclined
as a result of the secular degrees of freedom drifting toward equipartition of
angular momentum deficit. Secular chaos is known to be responsible for the
eventual destabilization of Mercury in our own solar system. Here we focus on
systems with three giant planets. We characterize the secular chaos and
demonstrate the criterion for it to occur, but leave a detailed understanding
of secular chaos to a companion paper. After an extended period of eccentricity
diffusion, the inner planet's pericenter can approach the star to within a few
stellar radii. Strong tidal interactions and ensuing tidal dissipation extract
orbital energy from the planet and pull it inward, creating a hot Jupiter. In
contrast to other proposed channels for the production of hot Jupiters, such a
scenario (which we term "secular migration") explains a range of
observations: the pile-up of hot Jupiters at 3 day orbital periods, the fact
that hot Jupiters are in general less massive than other radial velocity
planets, that they may have misaligned inclinations with respect to stellar
spin, and that they have few easily detectable companions (but may have giant
companions in distant orbits). Secular migration can also explain close-in
planets as low in mass as
Xu, Y., Yang, Y.,
Zhang, Q., Xu, G., 2011, "Solar oblateness and Mercury's perihelion
precession," Monthly Notices of the Royal Astronomical Society,
1043.
The Keplerian laws of planetary motion
are solutions of the two-body gravitational problem. Solar oblateness resulting
from the rotation of the Sun distorts the gravitational force acting on a
planet and disturbs its Keplerian motion. An analytic solution of a planetary
orbit disturbed by the solar gravitational oblateness is derived. In addition
to short- and long-periodic disturbances there are secular disturbances, which
lead to a perihelion precession and a nodal regression as well as to a
mean-motion advance. The magnitude of the short-periodic perihelion precession
could disturb observations of the secular effect if the survey is shorter than
one Julian year. Transformation of formulae from the solar equatorial plane to
the ecliptic plane is discussed. Numerical estimates of the secular perihelion
precessions of Mercury, Venus and Mars are in good agreement with published
results, confirming our theory. Inversely, the solar oblateness could be
determined through observation of the perihelion precession of a planet. The
solution is also valid for satellite orbits in the solar gravitational field.
Yan, W.M., Manchester, R.N., Hobbs,
G., van Straten, W., Reynolds, J.E., Wang, N., Bailes, M., Bhat, N.D.R.,
Burke-Spolaor, S., Champion, D.J., Chaudhary, A., Coles, W.A., Hotan, A.W.,
Khoo, J., Oslowski, S., Sarkissian, J.M., Yardley, D.R.B., 2011, "Rotation
measure variations for 20 millisecond pulsars," Astrophysics and Space
Science, 214.
We report on variations in the mean
position angle of the 20 millisecond pulsars being observed as part of the
Parkes Pulsar Timing Array (PPTA) project. It is found that the observed
variations are dominated by changes in the Faraday rotation occurring in the
Earth's ionosphere. Two ionospheric models are used to correct for the
ionospheric contribution and it is found that one based on the International
Reference Ionosphere gave the best results. Little or no significant long-term
variation in interstellar RM was found with limits typically about 0.1 rad m-2 yr-1 in
absolute value. In a few cases, apparently significant RM variations over
timescales of a few 100 days or more were seen. These are unlikely to be due to
localised magnetised regions crossing the line of sight since the implied
magnetic fields are too high. Most probably they are statistical fluctuations
due to random spatial and temporal variations in the interstellar electron
density and magnetic field along the line of sight.
Yan, W.M., Manchester, R.N., van
Straten, W., Reynolds, J.E., Hobbs, G., Wang, N., Bailes, M., Bhat, N.D.R.,
Burke-Spolaor, S., Champion, D.J., Coles, W.A., Hotan, A.W., Khoo, J.,
Oslowski, S., Sarkissian, J.M., Verbiest, J.P.W., Yardley, D.R.B., 2011,
"Polarization observations of 20 millisecond pulsars," Monthly
Notices of the Royal Astronomical Society, 414, 2087-2100.
Polarization profiles are presented for
20 millisecond pulsars that are being observed as part of the Parkes Pulsar
Timing Array project. The observations used the Parkes multibeam receiver with
a central frequency of 1369 MHz and the Parkes digital filter bank pulsar
signal-processing system PDFB2. Because of the large total observing time, the
summed polarization profiles have very high signal-to-noise ratios and show
many previously undetected profile features. 13 of the 20 pulsars show emission
over more than half of the pulse period. Polarization variations across the
profiles are complex, and the observed position angle variations are generally
not in accord with the rotating vector model for pulsar polarization. Nevertheless,
the polarization properties are broadly similar to those of normal
(non-millisecond) pulsars, suggesting that the basic radio emission mechanism
is the same in both classes of pulsar. The results support the idea that radio
emission from millisecond pulsars originates high in the pulsar magnetosphere,
probably close to the emission regions for high-energy X-ray and gamma-ray
emission. Rotation measures were obtained for all 20 pulsars, eight of which
had no previously published measurements.
Yardley, D.R.B., Coles, W.A., Hobbs,
G.B., Verbiest, J.P.W., Manchester, R.N., van Straten, W., Jenet, F.A., Bailes,
M., Bhat, N.D.R., Burke-Spolaor, S., Champion, D.J., Hotan, A.W., Oslowski, S.,
Reynolds, J.E., Sarkissian, J.M., 2011, "On detection of the stochastic
gravitational-wave background using the Parkes pulsar timing array," Monthly
Notices of the Royal Astronomical Society, 414, 1777-1787.
We search for the signature of an
isotropic stochastic gravitational-wave background in pulsar timing
observations using a frequency-domain correlation technique. These
observations, which span roughly 12 yr, were obtained with the 64-m Parkes
radio telescope augmented by public domain observations from the Arecibo
Observatory. A wide range of signal processing issues unique to pulsar timing
and not previously presented in the literature are discussed. These include the
effects of quadratic removal, irregular sampling and variable errors which
exacerbate the spectral leakage inherent in estimating the steep red spectrum
of the gravitational-wave background. These observations are found to be
consistent with the null hypothesis that no gravitational-wave background is
present, with 76 per cent confidence. We show that the detection statistic is
dominated by the contributions of only a few pulsars because of the
inhomogeneity of this data set. The issues of detecting the signature of a
gravitational-wave background with future observations are discussed.