Artz, T., Tesmer NéBöann, S., Nothnagel, A., 2011, "Assessment of periodic sub-diurnal Earth rotation variations at tidal frequencies through transformation of VLBI normal equation systems," Journal of Geodesy, 18.
We present an empirical model for periodic variations of diurnal and sub-diurnal Earth rotation parameters (ERPs) that was derived based on the transformation of normal equation (NEQ) systems of Very Long Baseline Interferometry (VLBI) observing sessions. NEQ systems that contain highly resolved polar motion and UT1-TAI with a temporal resolution of 15 min were generated and then transformed to the coefficients of the tidal ERP model to be solved for. To investigate the quality of this model, comparisons with empirical models from the Global Positioning System (GPS), another VLBI model and the model adopted by the conventions of the International Earth Rotation and Reference Systems Service (IERS) were performed. The absolute coefficients of these models agree almost completely within 7.5 mas in polar motion and 0.5 ms in UT1-TAI. Several bigger differences exist, which are discussed in this paper. To be able to compare the model estimates with results of the continuous VLBI campaigns, where signals with periods of 8 and 6 h were detected, terms in the ter- and quarter-diurnal band were included in the tidal ERP model. Unfortunately, almost no common features with the results of continuous VLBI campaigns or ERP predictions in these tidal bands can be seen.
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, 399.
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★/M⊙≤ 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.
Batalha, N.M., Borucki, W.J., Bryson, S.T., Buchhave, L.A., Caldwell, D.A., Christensen-Dalsgaard, J., Ciardi, D., Dunham, E.W., Fressin, F., Gautier, T.N., Gilliland, R.L., Haas, M.R., Howell, S.B., Jenkins, J.M., Kjeldsen, H., Koch, D.G., Latham, D.W., Lissauer, J.J., Marcy, G.W., Rowe, J.F., Sasselov, D.D., Seager, S., Steffen, J.H., Torres, G., Basri, G.S., Brown, T.M., Charbonneau, D., Christiansen, J., Clarke, B., Cochran, W.D., Dupree, A., Fabrycky, D.C., Fischer, D., Ford, E.B., Fortney, J., Girouard, F.R., Holman, M.J., Johnson, J., Isaacson, H., Klaus, T.C., Machalek, P., Moorehead, A.V., Morehead, R.C., Ragozzine, D., Tenenbaum, P., Twicken, J., Quinn, S., VanCleve, J., Walkowicz, L.M., Welsh, W.F., Devore, E., Gould, A., 2011, "Kepler's First Rocky Planet: Kepler-10b," The Astrophysical Journal, 729, 27.
NASA's Kepler Mission uses transit photometry to determine the frequency of Earth-size planets in or near the habitable zone of Sun-like stars. The mission reached a milestone toward meeting that goal: the discovery of its first rocky planet, Kepler-10b. Two distinct sets of transit events were detected: (1) a 152 ± 4 ppm dimming lasting 1.811 ± 0.024 hr with ephemeris T [BJD] =2454964.57375+0.00060 .0.00082 + N*0.837495+0.000004 .0.000005 days and (2) a 376 ± 9 ppm dimming lasting 6.86 ± 0.07 hr with ephemeris T [BJD] =2454971.6761+0.0020 .0.0023 + N*45.29485+0.00065 .0.00076 days. Statistical tests on the photometric and pixel flux time series established the viability of the planet candidates triggering ground-based follow-up observations. Forty precision Doppler measurements were used to confirm that the short-period transit event is due to a planetary companion. The parent star is bright enough for asteroseismic analysis. Photometry was collected at 1 minute cadence for >4 months from which we detected 19 distinct pulsation frequencies. Modeling the frequencies resulted in precise knowledge of the fundamental stellar properties. Kepler-10 is a relatively old (11.9 ± 4.5 Gyr) but otherwise Sun-like main-sequence star with T eff = 5627 ± 44 K, M = 0.895 ± 0.060 M , and R = 1.056 ± 0.021 R . Physical models simultaneously fit to the transit light curves and the precision Doppler measurements yielded tight constraints on the properties of Kepler-10b that speak to its rocky composition: M P = 4.56+1.17 .1.29 M ⊕, R P = 1.416+0.033 .0.036 R ⊕, and ρP = 8.8+2.1 .2.9 g cm.3. Kepler-10b is the smallest transiting exoplanet discovered to date.
Bro., M., Rozehnal, J., 2011, "Eurybates - the only asteroid family among Trojans?," Monthly Notices of the Royal Astronomical Society, 389.
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.
Burt, B.J., Lommen, A.N., Finn, L.S., 2011, "Optimizing Pulsar Timing Arrays to Maximize Gravitational Wave Single-source Detection: A First Cut," The Astrophysical Journal, 730, 17.
Pulsar Timing Arrays (PTAs) use high accuracy timing of a collection of low timing noise pulsars to search for gravitational waves (GWs) in the microhertz to nanohertz frequency band. The sensitivity of such a PTA depends on (1) the direction of the GW source, (2) the timing accuracy of the pulsars in the array, and (3) how the available observing time is allocated among those pulsars. Here, we present a simple way to calculate the sensitivity of the PTA as a function of direction of a single GW source, based only on the location and root-mean-square residual of the pulsars in the array. We use this calculation to suggest future strategies for the current North American Nanohertz Observatory for Gravitational Waves PTA in its goal of detecting single GW sources. We also investigate the effects of an additional pulsar on the array sensitivity, with the goal of suggesting where PTA pulsar searches might be best directed. We demonstrate that, in the case of single GW sources, if we are interested in maximizing the volume of space to which PTAs are sensitive, there exists a slight advantage to finding a new pulsar near where the array is already most sensitive. Further, the study suggests that more observing time should be dedicated to the already low-noise pulsars in order to have the greatest positive effect on the PTA sensitivity. We have made a Web-based sensitivity mapping tool available.
Celikel, O., 2011, "Application of the vector modulation method to the north finder capability gyroscope as a directional sensor," Measurement Science and Technology, 22, 5203.
This paper presents the application of the vector modulation method (VMM) to an open-loop interferometric fiber optic gyroscope, called the north finder capability gyroscope (NFCG), designed and assembled in TUBITAK UME (National Metrology Institute of Turkey). The method contains a secondary modulation/demodulation circuit with an AD630 chip, depending on the periodic variation of the orientation of the sensing coil sensitive surface vector with respect to geographic north at a laboratory latitude and collection of dc voltage at the secondary demodulation circuit output in the time domain. The resultant dc voltage proportional to the first-kind Bessel function based on Sagnac phase shift for the first order is obtained as a result of vector modulation together with the Earth's rotation. A new model function is developed and introduced to evaluate the angular errors of the NFCG with VMM in finding geographic north.
Escapa, A., Fukushima, T., 2011, "Free Translational Oscillations of Icy Bodies with a Subsurface Ocean Using a Variational Approach," The Astronomical Journal, 141, 77.
We analyze the influence of the interior structure of an icy body with an internal ocean on the relative translational motions of its solid constituents. We consider an isolated body differentiated into three homogeneous layers with spherical symmetry: an external ice-I layer, a subsurface ammonia-water ocean, and a rocky inner core. This composition represents icy bodies such as Europa, Titania, Oberon, and Triton, as well as Pluto, Eris, Sedna, and 2004 DW. We construct the equations of motion by assuming that the solid constituents are rigid and that the ocean is an ideal fluid, the internal motion being characterized by the relative translations of the solids and the induced flow in the fluid. Then we determine the dynamics of the icy body using the methods of analytical mechanics, that is, we compute the kinetic energy and the gravitational potential energy, and obtain the Lagrangian function. The resulting solution of the Lagrange equations shows that the solid layers perform translational oscillations of different amplitudes with respect to the barycenter of the body. We derive the dependence of the frequency of the free oscillations of the system on the characteristics of each layer, expressing the period of the oscillations as a function of the densities and masses of the ocean and the rocky inner core, and the mass of the icy body. We apply these results to previously developed subsurface models and obtain numerical values for the period and the ratio between the amplitudes of the translational oscillations of the solid components. The features obtained are quite different from the cases of Earth and Mercury. Our analytical formulas satisfactorily explain the source of these differences. When models of the same icy body, compatible with the existence of an internal ocean, differ in the thickness of the ice-I layer, their associated periods experience a relative variation of at least 10%. In particular, the different models for Titania and Oberon exhibit a larger variation of about 37% and 30%. This indicates an absolute difference of the order of three and two hours, respectively. This suggests that the free period of the internal oscillations might provide a new procedure to constrain the internal structure of icy bodies with a subsurface ocean.
Fomalont, E., Johnston, K., Fey, A., Boboltz, D., Oyama, T., Honma, M., 2011, "The Position/Structure Stability of Four ICRF2 Sources," The Astronomical Journal, 141, 91.
Four close radio sources in the International Celestial Reference Frame (ICRF) catalog were observed using phase referencing with the VLBA at 43, 23, and 8.6 GHz, and with VERA at 23 GHz over a one-year period. The goal was to determine the stability of the radio cores and to assess structure effects associated with positions in the ICRF. Although the four sources were compact at 8.6 GHz, the VLBA images at 43 GHz with 0.3 mas resolution showed that all were composed of several components. A component in each source was identified as the radio core using some or all of the following emission properties: compactness, spectral index, location at the end of the extended emission region, and stationary in the sky. Over the observing period, the relative positions between the four radio cores were constant to 0.02 mas, the phase-referencing positional accuracy obtained at 23 and 43 GHz among the sources, suggesting that once a radio core is identified, it remains stationary in the sky to this accuracy. Other radio components in two of the four sources had detectable motion in the radio jet direction. Comparison of the 23 and 43 GHz VLBA images with the VLBA 8.6 GHz images and the ICRF positions suggests that some ICRF positions are dominated by a moving jet component; hence, they can be displaced up to 0.5 mas from the radio core and may also reflect the motion of the jet component. Future astrometric efforts to determine a more accurate quasar reference frame at 23 and 43 GHz and from the VLBI2010 project are discussed, and supporting VLBA or European VLBI Network observations of ICRF sources at 43 GHz are recommended in order to determine the internal structure of the sources. A future collaboration between the radio (ICRF) and the optical frame of GAIA is discussed.
Ghosh, P., Sen, S., Riaz, S.S., Ray, D.S., 2011, "Controlling birhythmicity in a self-sustained oscillator by time-delayed feedback," Physical Review E, 83, 36205.
Time-delayed feedback is a practical method for controlling various nonlinear dynamical systems. We consider its influence on the dynamics of a multicycle van der Pol oscillator that is birhythmic in nature. It has been shown that depending on the strength of delay the bifurcation space can be divided into two subspaces for which the dynamical response of the system is generically distinct. We observe an interesting collapse and revival of birhythmicity with the variation of the delay time. Depending on the parameter space the system also exhibits a transition between birhythmicity and monorhythmic behavior. Our analysis of amplitude equation corroborates with the results obtained by numerical simulation of the dynamics.
Haranas, I., Ragos, O., Mioc, V., 2011, "Yukawa-type potential effects in the anomalistic period of celestial bodies," Astrophysics and Space Science, 332, 107-113.
Several contemporary modified models of gravity predict the existence of a non-Newtonian Yukawa-type correction to the classical gravitational potential. We study the motion of a secondary celestial body under the influence of the corrected gravitational force of a primary. We derive two equations to approximate the periastron time rate of change and its total variation over one revolution (i.e., the difference between the anomalistic period and the Keplerian period) under the influence of the non-Newtonian radial acceleration. Kinematically, this influence produces apsidal motion. We performed numerical estimations for Mercury, for the companion star of the pulsar PSR 1913+16, and for the extrasolar Planet b of the star HD 80606. We also considered the case of the artificial Earth satellite GRACE-A, but the results present a low degree of reliability from a practical standpoint.
Harbison, R.A., Thomas, P.C., Nicholson, P.C., 2011, "Rotational modeling of Hyperion," Celestial Mechanics and Dynamical Astronomy, 12.
Saturn.s moon, Hyperion, is subject to strongly-varying solid body torques from its primary and lacks a stable spin state resonant with its orbital frequency. In fact, its rotation is chaotic, with a Lyapunov timescale on the order of 100 days. In 2005, Cassini made three close passes of Hyperion at intervals of 40 and 67 days, when the moon was imaged extensively and the spin state could be measured. Curiously, the spin axis was observed at the same location within the body, within errors, during all three fly-bys.~ 30° from the long axis of the moon and rotating between 4.2 and 4.5 times faster than the synchronous rate. Our dynamical modeling predicts that the rotation axis should be precessing within the body, with a period of ~ 16 days. If the spin axis retains its orientation during all three fly-bys, then this puts a strong constraint on the in-body precessional period, and thus the moments of inertia. However, the location of the principal axes in our model are derived from the shape model of Hyperion, assuming a uniform composition. This may not be a valid assumption, as Hyperion has significant void space, as shown by its density of 544± 50 kg m−3 (Thomas et al. in Nature 448:50, 2007). This paper will examine both a rotation model with principal axes fixed by the shape model, and one with offsets from the shape model. We favor the latter interpretation, which produces a best-fit with principal axes offset of ~ 30° from the shape model, placing the A axis at the spin axis in 2005, but returns a lower reduced χ 2 than the best-fit fixed-axes model.
Harris, A.W., Mommert, M., Hora, J.L., Mueller, M., Trilling, D.E., Bhattacharya, B., Bottke, W.F., Chesley, S., Delbo, M., Emery, J.P., Fazio, G., Mainzer, A., Penprase, B., Smith, H.A., Spahr, T.B., Stansberry, J.A., Thomas, C.A., 2011, "ExploreNEOs. II. The Accuracy of the Warm Spitzer Near-Earth Object Survey," The Astronomical Journal, 141, 75.
We report on results of observations of near-Earth objects (NEOs) performed with the NASA Spitzer Space Telescope as part of our ongoing (2009-2011) Warm Spitzer NEO survey ("ExploreNEOs"), the primary aim of which is to provide sizes and albedos of some 700 NEOs. The emphasis of the work described here is an assessment of the overall accuracy of our survey results, which are based on a semi-empirical generalized model of asteroid thermal emission. The NASA Spitzer Space Telescope has been operated in the so-called Warm Spitzer mission phase since the cryogen was depleted in 2009 May, with the two shortest-wavelength channels, centered at 3.6 ?m and 4.5 ?m, of the Infrared Array Camera continuing to provide valuable data. The set of some 170 NEOs in our current Warm Spitzer results catalog contains 28 for which published taxonomic classifications are available, and 14 for which relatively reliable published diameters and albedos are available. A comparison of the Warm Spitzer results with previously published results ("ground truth"), complemented by a Monte Carlo error analysis, indicates that the rms Warm Spitzer diameter and albedo errors are ±20% and ±50%, respectively. Cases in which agreement with results from the literature is worse than expected are highlighted and discussed; these include the potential spacecraft target 138911 2001 AE2. We confirm that 1.4 appears to be an appropriate overall default value for the relative reflectance between the V band and the Warm Spitzer wavelengths, for use in correction of the Warm Spitzer fluxes for reflected solar radiation.
Héard, G., Ehrenreich, D., Bouchy, F., Delfosse, X., Moutou, C., Arnold, L., Boisse, I., Bonfils, X., Dí, R.F., Eggenberger, A., Forveille, T., Lagrange, A.-M., Lovis, C., Pepe, F., Perrier, C., Queloz, D., Santerne, A., Santos, N.C., Séansan, D., Udry, S., Vidal-Madjar, A., 2011, "The retrograde orbit of the HAT-P-6b exoplanet," Astronomy and Astrophysics, 527, L11.
We observed the transit of the HAT-P-6b exoplanet across its host star with the SOPHIE spectrograph (OHP, France). The resulting stellar radial velocities display the Rossiter-McLaughlin anomaly and reveal a retrograde orbit: the planetary orbital spin and the stellar rotational spin point in approximately opposite directions. A fit to the anomaly measures a sky-projected angle λ = 166° ± 10° between these two spin axes. All seven known retrograde planets are hot Jupiters with masses Mp < 3 MJup. About two thirds of the planets in this mass range, however, are prograde and aligned (λ ≃ 0°). In contrast, most of the more massive planets (Mp > 4 MJup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below ~3.5 MJup.
Hernáez-Mena, C., Benet, L., 2011, "Statistics and universality in simplified models of planetary formation," Monthly Notices of the Royal Astronomical Society, 412, 95-106.
In this paper, we modify Laskar's simplified model of planetary evolution and accretion to account for the full conservation of the total angular momentum of the system, and extend it to incorporate an accretion probability that depends on the mass and relative velocity of the colliding particles. We present statistical results for the mass and eccentricity of the planets formed, in terms of their semimajor axes, for a large number of realizations of different versions of the model. In particular, we find that by combining the mass-dependent accretion probability and the velocity-selection mechanism, the planets formed display a systematic occurrence at specific locations. By introducing properly scaled variables, our results are universal with respect to the total angular momentum of the system, the mass of the planetesimal disc and the mass of the central star.
Howard, A.W., Johnson, J.A., Marcy, G.W., Fischer, D.A., Wright, J.T., Henry, G.W., Isaacson, H., Valenti, J.A., Anderson, J., Piskunov, N.E., 2011, "The NASA-UC Eta-Earth Program. III. A Super-Earth Orbiting HD 97658 and a Neptune-mass Planet Orbiting Gl 785," The Astrophysical Journal, 730, 10.
We report the discovery of planets orbiting two bright, nearby early K dwarf stars, HD 97658 and Gl 785. These planets were detected by Keplerian modeling of radial velocities measured with Keck-HIRES for the NASA-UC Eta-Earth Survey. HD 97658 b is a close-in super-Earth with minimum mass Msin i = 8.2 ± 1.2 M ⊕, orbital period P = 9.494 ± 0.005 days, and an orbit that is consistent with circular. Gl 785 b is a Neptune-mass planet with Msin i = 21.6 ± 2.0 M ⊕, P = 74.39 ± 0.12 days, and orbital eccentricity e = 0.30 ± 0.09. Photometric observations with the T12 0.8 m automatic photometric telescope at Fairborn Observatory show that HD 97658 is photometrically constant at the radial velocity period to 0.09 mmag, supporting the existence of the planet.
Huang, C.-L., Dehant, V., Liao, X.-H., Van Hoolst, T., Rochester, M.G., 2011, "On the coupling between magnetic field and nutation in a numerical integration approach," Journal of Geophysical Research (Solid Earth), 116, 03403.
Nutation amplitudes are computed in a displacement field approach that incorporates the influence of a prescribed magnetic field inside the Earth's core. The existence of relative nutational motions between the liquid core and its surrounding solid parts induces a shearing of the magnetic field. An incremental magnetic field is then created, which in return perturbs the nutations themselves. This problem has already been addressed within a nutation model computed from an angular momentum budget approach. Here we incorporate the magnetic field influence directly in the motion equation and in the boundary conditions used in precise nutation theory, and a new strategy to compute nutations is established. As in previous studies, we assume that the root-mean-square of the radial magnetic field amplitude at the core-mantle boundary is 6.9 Gauss, that the magnetic diffusivity at the bottom of the mantle and in the fluid outer core side is 1.6 m2, and that the thickness of the conductive layer at the bottom of the mantle is 200 m. The Coriolis force is included in this work. The results show that the free core nutation period decreases by 0.38 days, and that the out-of-phase (in-phase) amplitudes of the retrograde 18.6 year and the retrograde annual nutations increase (decrease) by 20 and 39 mas, respectively. Comparisons of these results with previous studies are made, and discussions are also presented on the contribution of Coriolis force and the prescribed magnetic field on the coupling constants.
Husnoo, N., Pont, F., Héard, G., Simpson, E., Mazeh, T., Bouchy, F., Moutou, C., Arnold, L., Boisse, I., Dí, R.F., Eggenberger, A., Shporer, A., 2011, "Orbital eccentricity of WASP-12 and WASP-14 from new radial velocity monitoring with SOPHIE," Monthly Notices of the Royal Astronomical Society, 424.
As part of the long-term radial velocity monitoring of known transiting planets, we have acquired new radial velocity data for the two transiting systems WASP-12 and WASP-14, each harbouring a gas giant on a close orbit (orbital period of 1.09 and 2.24 d, respectively). In both cases, the initial orbital solution suggested a significant orbital eccentricity, 0.049 ± 0.015 for WASP-12b and 0.091 ± 0.003 for WASP-14b. Since then, measurements of the occultation of WASP-12 in the infrared have indicated that one projection of the eccentricity (e cos ω) was close to zero, casting doubt on the eccentricity from the initial radial velocity orbit. Our measurements show that the radial velocity data are compatible with a circular orbit. A MCMC analysis taking into account the presence of correlated systematic noise in both the radial velocity and photometric data gives e= 0.017+0.015−0.010. In contrast, we confirm the orbital eccentricity of WASP-14b, and refine its value to e= 0.0877 ± 0.0030, a 10σ detection. WASP-14b is thus the closest presently known planet with a confirmed eccentric orbit.
Kane, S.R., Gelino, D.M., 2011, "On the Inclination Dependence of Exoplanet Phase Signatures," The Astrophysical Journal, 729, 74.
Improved photometric sensitivity from space-based telescopes has enabled the detection of phase variations for a small sample of hot Jupiters. However, exoplanets in highly eccentric orbits present unique opportunities to study the effects of drastically changing incident flux on the upper atmospheres of giant planets. Here we expand upon previous studies of phase functions for these planets at optical wavelengths by investigating the effects of orbital inclination on the flux ratio as it interacts with the other effects induced by orbital eccentricity. We determine optimal orbital inclinations for maximum flux ratios and combine these calculations with those of projected separation for application to coronagraphic observations. These are applied to several of the known exoplanets which may serve as potential targets in current and future coronagraph experiments.
Kocsis, B., Sesana, A., 2011, "Gas-driven massive black hole binaries: signatures in the nHz gravitational wave background," Monthly Notices of the Royal Astronomical Society, 411, 1467-1479.
Pulsar timing arrays (PTAs) measure nHz frequency gravitational waves (GWs) generated by orbiting massive black hole binaries (MBHBs) with periods between 0.1 and 10 yr. Previous studies on the nHz GW background assumed that the inspiral is purely driven by GWs. However, torques generated by a gaseous disc can shrink the binary much more efficiently than GW emission, reducing the number of binaries at these separations. We use simple disc models for the circumbinary gas and for the binary.disc interaction to follow the orbital decay of MBHBs through physically distinct regions of the disc, until GWs take over their evolution. We extract MBHB cosmological merger rates from the Millennium simulation, generate Monte Carlo realizations of a population of gas-driven binaries and calculate the corresponding GW amplitudes of the most luminous individual binaries and the stochastic GW background. For steady state α-discs with α > 0.1, we find that the nHz GW background can be significantly modified. The number of resolvable binaries is however not changed by the presence of gas; we predict 1.10 individually resolvable sources to stand above the noise for a 1.50 ns timing precision. Gas-driven migration reduces predominantly the number of small total mass or unequal mass ratio binaries, which leads to the attenuation of the mean stochastic GW background, but increases the detection significance of individually resolvable binaries. The results are sensitive to the model of binary.disc interaction. The GW background is not attenuated significantly for time-dependent models of Ivanov, Papaloizou & Polnarev.
Kocsis, B., Tremaine, S., 2011, "Resonant relaxation and the warp of the stellar disc in the Galactic Centre," Monthly Notices of the Royal Astronomical Society, 412, 187-207.
Observations of the spatial distribution and kinematics of young stars in the Galactic Centre can be interpreted as showing that the stars occupy one, or possibly two, discs of radii ~0.05-0.5 pc. The most prominent ('clockwise') disc exhibits a strong warp: the normals to the mean orbital planes in the inner and the outer third of the disc differ by ~60°. Using an analytical model based on Laplace-Lagrange theory, we show that such warps arise naturally and inevitably through vector resonant relaxation between the disc and the surrounding old stellar cluster.
Kushvah, B.S., 2011, "Trajectory and stability of Lagrangian point L2 in the Sun-Earth system," Astrophysics and Space Science, 332, 99-106.
This paper describes design of the trajectory and analysis of the stability of collinear point L2 in the Sun-Earth system. The modified restricted three body problem with additional gravitational potential from the belt is used as the model for the Sun-Earth system. The effect of radiation pressure of the Sun and oblate shape of the Earth are considered. The point L2 is asymptotically stable up to a specific value of time t correspond to each set of values of parameters and initial conditions. The results obtained from this study would be applicable to locate a satellite, a telescope or a space station around the point L2.
Lafreniè, D., Jayawardhana, R., Janson, M., Helling, C., Witte, S., Hauschildt, P., 2011, "Discovery of an ~23 M Jup Brown Dwarf Orbiting ~700 AU from the Massive Star HIP 78530 in Upper Scorpius," The Astrophysical Journal, 730, 42.
We present the discovery of a substellar companion on a wide orbit around the ~ 2.5 M star HIP 78530, which is a member of the 5 Myr old Upper Scorpius association. We have obtained follow-up imaging over two years and show that the companion and primary share common proper motion. We have also obtained JHK spectroscopy of the companion and confirm its low surface gravity, in accordance with the young age of the system. A comparison with DRIFT-PHOENIX synthetic spectra indicates an effective temperature of 2800 ± 200 K and a comparison with template spectra of young and old dwarfs indicates a spectral type of M8 ± 1. The mass of the companion is estimated to be 19-26 M Jup based on its bolometric luminosity and the predictions of evolutionary models. The angular separation of the companion is 45, which at the distance of the primary star, 156.7 pc, corresponds to a projected separation of ~710 AU. This companion features one of the lowest mass ratios (~0.009) of any known companion at separations greater than 100 AU..
Lazorenko, P.F., Sahlmann, J., Séansan, D., Figueira, P., Lovis, C., Martin, E., Mayor, M., Pepe, F., Queloz, D., Rodler, F., Santos, N., Udry, S., 2011, "Astrometric search for a planet around VB 10," Astronomy and Astrophysics, 527, 25.
We observed VB 10 in August and September 2009 using the FORS2 camera of the VLT with the aim of measuring its astrometric motion and of probing for the presence of the announced planet VB 10b. We used the published STEPS astrometric positions of VB 10 over a timespan of 9 years, which allowed us to compare the expected motion of VB 10 due to parallax and proper motion with the observed motion and to compute precise deviations. The single-epoch precisions of our observations are about 0.1 mas, and the data showed no significant residual trend, while the presence of the planet should have induced an apparent proper motion greater than 10 mas yr-1. Subtraction of the predicted orbital motion from the observed data produces a strong trend in position residuals of VB 10. We estimated the probability that this trend is caused by random noise. After taking all the uncertainties into account and using Monte-Carlo resampling of the data, we are able to reject the existence of VB 10b with the announced mass of 6.4 MJ with a false alarm probability of only 5 ×nbsp;10-4. A 3.2 MJ planet is also rejected with a false alarm probability of 0.023.
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, 349.
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 Hén-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.
Libeskind, N.I., Knebe, A., Hoffman, Y., Gottlö, S., Yepes, G., Steinmetz, M., 2011, "The preferred direction of infalling satellite galaxies in the Local Group," Monthly Notices of the Royal Astronomical Society, 411, 1525-1535.
Using a high-resolution dark matter (DM) simulation of the Local Group, conducted within the framework of the Constrained Local UniversE Simulation (CLUES) project, we investigate the nature of how satellites of the Milky Way (MW) and M31 are accreted. Satellites of these two galaxies are accreted anisotropically on to the main haloes, entering the virial radius of their hosts, from specific .spots. with respect to the large-scale structure. Furthermore, the material which is tidally stripped from these accreted satellites is also, at z= 0, distributed anisotropically and is characterized by an ellipsoidal subvolume embedded in the halo. The angular pattern created by the locus of satellite infall points and the projected z= 0 stripped dark matter is investigated within a coordinate system determined by the location of the Local Group companion and the simulated Virgo cluster across concentric shells ranging from 0.1 to 5 rvir. Remarkably, the principal axis of the ellipsoidal subvolume shows a coherent alignment extending from well within the halo to a few rvir. A spherical harmonics transform applied to the angular distributions confirms the visual impression: namely, the angular distributions of both the satellites entry points and stripped DM for both haloes are dominated by the l= 2 quadrupole term, whose major principal axis is approximately aligned across the shells considered. It follows that the outer (r > 0.5rvir) structure of the main haloes of the Local Group composed of stripped material is closely related to the cosmic web, within which it is embedded. Given the very plausible hypothesis that an important fraction of the stellar halo of the MW has been accreted from satellite galaxies, the present results can be directly applied to the stellar halo of the MW and M31. We predict that the remnants of tidally stripped satellites should be embedded in streams of material composed of dark matter and stars. The present results can therefore shed light on the existence of satellites embedded within larger streams of matter, such as the Segue 2 satellite.
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, 90.
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.
Luan, X., Wang, W., Burns, A., Solomon, S., Zhang, Y., Paxton, L.J., Xu, J., 2011, "Longitudinal variations of nighttime electron auroral precipitation in both the Northern and Southern hemispheres from the TIMED global ultraviolet imager," Journal of Geophysical Research (Space Physics), 116, 03302.
Using 6 years of Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) global ultraviolet imager auroral observations in both hemispheres, we have studied the longitudinal variations of auroral precipitation during the magnetic nighttime period of 2100.0300 magnetic local time. There was a strong seasonal dependence of the longitudinal variations of the aurora: (1) During solstices and for both hemispheres, auroral precipitation peaked between magnetic longitude (MLON) 210°E and 360°E in June and between MLON 120°E and 300°E in December. (2) In the equinoxes, the auroral longitudinal pattern was generally similar to that in local summer in each hemisphere, except that in the Northern Hemisphere the maximum precipitation was usually located in more westward longitudes in equinox than in summer. (3) The ratios between the maximum and the minimum of the precipitation energy flux along longitudes varied between 1.3 and 1.9, which were similar to those in previous studies. These features of the auroral longitudinal patterns did not change much from Kp = 1 to Kp = 4 conditions. Since the longitudinal distribution of auroral precipitation changed greatly with season in each hemisphere, the longitudinal variations of the magnetic field strength, which do not change with season, might not be the only process that caused the observed longitudinal variations of the aurora. Further data analysis shows that there was a significant negative correlation (coefficient |r = ~0.4.0.8) between the peak auroral precipitation intensity and the solar-EUV-produced ionospheric conductivity of the same hemisphere (in summer and equinox) or of the conjugate hemisphere (in winter). These results indicate the important effects of solar-EUV-produced ionospheric conductivity, which has significant longitudinal variations, on the longitudinal patterns of the aurora at magnetic nighttime. Our results also suggest that the interhemispheric coupling during solstices might be an important factor that contributes to the longitudinal variations of the nighttime aurora. Our correlation analysis indicates that the hemispheric differences in the conjugate magnetic field strengths also contribute to the longitudinal variations of the aurora, although they appear not to be a major factor.
Lykawka, P.S., Horner, J., Jones, B.W., Mukai, T., 2011, "Origin and dynamical evolution of Neptune Trojans - II. Long-term evolution," Monthly Notices of the Royal Astronomical Society, 412, 537-550.
Following our earlier work studying the formation of the Neptunian Trojan population during the planet's migration, we present results examining the eventual fate of the Trojan clouds produced in that work. A large number of Trojans were followed under the gravitational influence of the giant planets for a period of at least 1 Gyr. We find that the stability of Neptunian Trojans seems to be strongly correlated to their initial post-migration orbital elements, with those objects that survive as Trojans for billions of years, displaying negligible orbital evolution. The great majority of these survivors began the integrations with small eccentricities (e < 0.2) and small libration amplitudes (A < 30°-40°). The survival rate of 'pre-formed' Neptunian Trojans [which in general survived on dynamically cold orbits (e < 0.1, i < 5°-10°)] varied between ~5 and 70 per cent, depending on the precise detail of their initial orbits. In contrast, the survival rate of 'captured' Trojans (on final orbits spread across a larger region of the e-i element space) was markedly lower, ranging between 1-10 per cent after 4 Gyr. Taken in concert with our earlier work and the broad i-distribution of the observed Trojan population, we note that planetary formation scenarios, which involve the slow migration (a few tens of millions of years) of Neptune from an initial planetary architecture that is both resonant and compact (aN < 18 au), provide the most promising fit of those we considered to the observed Trojan population. In such scenarios, we find that the present-day Trojan population would number ~1 per cent of that which was present at the end of the planet's migration (i.e. survival rate of ~1 per cent), with the bulk being sourced from captured, rather than pre-formed objects. We note, however, that even those scenarios still fail to reproduce the presently observed portion of the Neptune Trojan population moving on orbits with e < 0.1 but i > 20°. Dynamical integrations of the presently observed Trojans show that five out of the seven are dynamically stable on time-scales comparable to the age of the Solar system, while 2001 QR322 exhibits significant dynamical instability on time-scales of less than 1 Gyr. The seventh Trojan object, 2008 LC18, was only recently discovered and has such large orbital uncertainties that only future studies will be able to determine its stability.
Machida, M.N., Inutsuka, S.-i., Matsumoto, T., 2011, "Recurrent Planet Formation and Intermittent Protostellar Outflows Induced by Episodic Mass Accretion," The Astrophysical Journal, 729, 42.
The formation and evolution of a circumstellar disk in magnetized cloud cores are investigated from a prestellar core stage until ~104 yr after protostar formation. In the circumstellar disk, fragmentation first occurs due to gravitational instability in a magnetically inactive region, and substellar-mass objects appear. The substellar-mass objects lose their orbital angular momenta by gravitational interaction with the massive circumstellar disk and finally fall onto the protostar. After this fall, the circumstellar disk increases its mass by mass accretion and again induces fragmentation. The formation and falling of substellar-mass objects are repeated in the circumstellar disk until the end of the main accretion phase. In this process, the mass of the fragments remains small, because the circumstellar disk loses its mass by fragmentation and subsequent falling of fragments before it becomes very massive. In addition, when fragments orbit near the protostar, they disturb the inner disk region and promote mass accretion onto the protostar. The orbital motion of substellar-mass objects clearly synchronizes with the time variation of the accretion luminosity of the protostar. Moreover, as the objects fall, the protostar shows a strong brightening for a short duration. The intermittent protostellar outflows are also driven by the circumstellar disk whose magnetic field lines are highly tangled owing to the orbital motion of fragments. The time-variable protostellar luminosity and intermittent outflows may be a clue for detecting planetary-mass objects in the circumstellar disk.
Machida, M.N., Matsumoto, T., 2011, "The origin and formation of the circumstellar disc," Monthly Notices of the Royal Astronomical Society, 284.
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.
Matsuo, T., Traub, W.A., Hattori, M., Tamura, M., 2011, "A New Concept for Direct Imaging and Spectral Characterization of Exoplanets in Multi-planet Systems," The Astrophysical Journal, 729, 50.
We present a novel method for direct detection and characterization of exoplanets from space. This method uses four collecting telescopes, combined with phase chopping and a spectrometer, with observations on only a few baselines rather than on a continuously rotated baseline. Focusing on the contiguous wavelength spectra of typical exoplanets, the (u, v) plane can be simultaneously and uniformly filled by recording the spectrally resolved signal. This concept allows us to perfectly remove speckles from reconstructed images. For a target comprising a star and multiple planets, observations on three baselines are sufficient to extract the position and spectrum of each planet. Our simulations show that this new method allows us to detect an analog Earth around a Sun-like star at 10 pc and to acquire its spectrum over the wavelength range from 8 to 19 mm with a high spectral resolution of 100. This method allows us to fully characterize an analog Earth and to similarly characterize each planet in multi-planet systems.
Messenger, C., Lommen, A., Demorest, P., Ransom, S., 2011, "A Bayesian parameter estimation approach to pulsar time-of-arrival analysis," Classical and Quantum Gravity, 28, 5001.
The increasing sensitivities of pulsar timing arrays to ultra-low frequency (nHz) gravitational waves promise to achieve direct gravitational wave (GW) detection within the next 5-10 years. While there are many parallel efforts being made in the improvement of telescope sensitivity, the detection of stable millisecond pulsars and the improvement of the timing software, there are reasons to believe that the methods used to accurately determine the time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More specifically, the determination of the uncertainties on these TOAs, which strongly affect the ability to detect GWs through pulsar timing, may be unreliable. We propose two Bayesian methods for the generation of pulsar TOAs starting from pulsar 'search-mode' data and pre-folded data. These methods are applied to simulated toy-model examples and in this initial work we focus on the issue of uncertainties in the folding period. The final results of our analysis are expressed in the form of posterior probability distributions on the signal parameters (including the TOA) from a single observation.
Milyukov, V.K., Kravchyuk, V.K., Mironov, A.P., Latynina, L.A., 2011, "Deformation processes in the lithosphere related to the nonuniformity of the Earth's rotation," Izvestiya Physics of the Solid Earth, 47, 246-258.
The series of observations conducted at the Baksan and Protvino deformation stations in the Northern Caucasus and the Central Russian Plain, respectively, and the length-of-day (LOD) data describing the variable rate of the Earth.s rotation are used to study the relation between the deformation processes in the lithosphere and the global geodynamics of the Earth over short time intervals. The methods applied are based on high-resolution spectral analysis, analysis of the coherence of the studied processes, and correlation analysis. A significant (95%) correlation is revealed between the local deformation fields at two remote observation stations, which proves the existence of a global component in the Earth.s deformation field that manifests itself at characteristic time intervals of up to 3.4 weeks. At the same level of significance, the correlation between the local deformation fields and variations in the rate of the Earth.s rotation has also been identified. It is shown that the found correlations in the tidal low-frequency range are caused by the direct impact of the long-period tidal loading (M f and M tm waves) on the lithosphere and the length-of-the-day (LOD). The global mechanisms giving rise to the correlation of these processes in the nontidal range require further study.
Moutou, C., Mayor, M., Lo Curto, G., Séansan, D., Udry, S., Bouchy, F., Benz, W., Lovis, C., Naef, D., Pepe, F., Queloz, D., Santos, N.C., Sousa, S.G., 2011, "The HARPS search for southern extra-solar planets. XXVII. Seven new planetary systems," Astronomy and Astrophysics, 527, 63.
We are conducting a planet search survey with HARPS since seven years. The volume-limited stellar sample includes all F2 to M0 main-sequence stars within 57.5 pc, where extrasolar planetary signatures are systematically searched for with the radial-velocity technics. In this paper, we report the discovery of new substellar companions of seven main-sequence stars and one giant star, detected through multiple Doppler measurements with the instrument HARPS installed on the ESO 3.6 m telescope, La Silla, Chile. These extrasolar planets orbit the stars
Mustill, A.J., Wyatt, M.C., 2011, "A general model of resonance capture in planetary systems: first- and second-order resonances," Monthly Notices of the Royal Astronomical Society, 277.
Mean motion resonances are a common feature of both our own Solar system and of extrasolar planetary systems. Bodies can be trapped in resonance when their orbital semimajor axes change, for instance when they migrate through a protoplanetary disc. We use a Hamiltonian model to thoroughly investigate the capture behaviour for first- and second-order resonances. Using this method, all resonances of the same order can be described by one equation, with applications to specific resonances by appropriate scaling. We focus on the limit where one body is a massless test particle and the other a massive planet. We quantify how the probability of capture into a resonance depends on the relative migration rate of the planet and particle, and the particle.s eccentricity. Resonant capture fails for high migration rates, and has decreasing probability for higher eccentricities, although for certain migration rates, capture probability peaks at a finite eccentricity. More massive planets can capture particles at higher eccentricities and migration rates. We also calculate libration amplitudes and the offset of the libration centres for captured particles, and the change in eccentricity if capture does not occur. Libration amplitudes are higher for larger initial eccentricity. The model allows for a complete description of a particle.s behaviour as it successively encounters several resonances. Data files containing the integration grid output will be available online. We discuss implications for several scenarios: (i) Planet migration through gas discs trapping other planets or planetesimals in resonances: we find that, with classical prescriptions for Type I migration, capture into second-order resonances is not possible, and lower mass planets or those further from the star should trap objects in first-order resonances closer to the planet than higher mass planets or those closer to the star. For fast enough migration, a planet can trap no objects into its resonances. We suggest that the present libration amplitude of planets may be a signature of their eccentricities at the epoch of capture, with high libration amplitudes suggesting high eccentricity (e.g. HD 128311). (ii) Planet migration through a debris disc: we find the resulting dynamical structure depends strongly both on migration rate and on planetesimal eccentricity. Translating this to spatial structure, we expect clumpiness to decrease from a significant level at e ≲ 0.01 to non-existent at e ≳ 0.1. (iii) Dust migration through Poynting.Robertson (PR) drag: we predict that Mars should have its own resonant ring of particles captured from the zodiacal cloud, and that the capture probability is ≲25 per cent that of the Earth, consistent with published upper limits for its resonant ring. To summarize, the Hamiltonian model will allow quick interpretation of the resonant properties of extrasolar planets and Kuiper Belt Objects, and will allow synthetic images of debris disc structures to be quickly generated, which will be useful for predicting and interpreting disc images made with Atacama Large Millimeter Array (ALMA), Darwin/Terrestrial Planet Finder (TPF) or similar missions.
Nascimbeni, V., Piotto, G., Bedin, L.R., Damasso, M., 2011, "TASTE: The Asiago Search for Transit timing variations of Exoplanets. I. Overview and improved parameters for HAT-P-3b and HAT-P-14b," Astronomy and Astrophysics, 527, 85.
A promising method for detecting earth-sized exoplanets is the timing analysis of a known transit. The technique allows a search for variations in either the transit duration or the center induced by the perturbation of a third body, e.g. a second planet or an exomoon. By applying this method, the TASTE (The Asiago search for transit timing variations of Exoplanets) project will collect high-precision, short-cadence light curves for a selected sample of transits by using imaging differential photometry at the Asiago 1.82 m telescope. The first light curves show that our project can achieve a competitive timing accuracy, as well as a significant improvement of the orbital parameters. We derived refined ephemerides for HAT-P-3b and HAT-P-14b with a timing accuracy of 11 and 25 s, respectively. Photometric data is only a vailable in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsweb.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A85
Ouyang, T., Yan, D., 2011, "Periodic solutions with alternating singularities in the collinear four-body problem," Celestial Mechanics and Dynamical Astronomy, 109, 229-239.
This paper gives an analytic proof of the existence of Schubart-like orbit, a periodic orbit with singularities in the symmetric collinear four-body problem. In each period of the Schubart-like orbit, there is a binary collision (BC) between the inner two bodies and a simultaneous binary collision (SBC) of the two clusters on both sides of the origin. The system is regularized and the existence is proved by using a .turning point. technique and a continuity argument on differential equations of the regularized Hamiltonian.
Paschalidis, V., Etienne, Z., Liu, Y.T., Shapiro, S.L., 2011, "Head-on collisions of binary white dwarf-neutron stars: Simulations in full general relativity," Physical Review D, 83, 64002.
We simulate head-on collisions from rest at large separation of binary white dwarf-neutron stars (WDNSs) in full general relativity. Our study serves as a prelude to our analysis of the circular binary WDNS problem. We focus on compact binaries whose total mass exceeds the maximum mass that a cold-degenerate star can support, and our goal is to determine the fate of such systems. A fully general relativistic hydrodynamic computation of a realistic WDNS head-on collision is prohibitive due to the large range of dynamical time scales and length scales involved. For this reason, we construct an equation of state (EOS) which captures the main physical features of neutron stars (NSs) while, at the same time, scales down the size of white dwarfs (WDs). We call these scaled-down WD models .pseudo-WDs (pWDs).. Using pWDs, we can study these systems via a sequence of simulations where the size of the pWD gradually increases toward the realistic case. We perform two sets of simulations; One set studies the effects of the NS mass on the final outcome, when the pWD is kept fixed. The other set studies the effect of the pWD compaction on the final outcome, when the pWD mass and the NS are kept fixed. All simulations show that after the collision, 14%.18% of the initial total rest mass escapes to infinity. All remnant masses still exceed the maximum rest mass that our cold EOS can support (1.92M⊙ ), but no case leads to prompt collapse to a black hole. This outcome arises because the final configurations are hot. All cases settle into spherical, quasiequilibrium configurations consisting of a cold NS core surrounded by a hot mantle, resembling Thorne-Zytkow objects. Extrapolating our results to realistic
Payne, M.J., Ford, E.B., 2011, "An Analysis of Jitter and Transit Timing Variations in the HAT-P-13 System," The Astrophysical Journal, 729, 98.
f the two planets in the HAT-P-13 system are coplanar, the orbital states provide a probe of the internal planetary structure. Previous analyses of radial velocity and transit timing data for the system suggested that the observational constraints on the orbital states were rather small. We reanalyze the available data, treating the jitter as an unknown MCMC parameter, and find that a wide range of jitter values are plausible, hence the system parameters are less well constrained than previously suggested. For slightly increased levels of jitter (~4.5 m s.1), the eccentricity of the inner planet can be in the range 0 < e inner< 0.07, the period and eccentricity of the outer planet can be 440 days < P outer < 470 days and 0.55 < e outer < 0.85, respectively, while the relative pericenter alignment, η, of the planets can take essentially any value .180° < η < +180°. It is therefore difficult to determine whether e inner and η have evolved to a fixed-point state or a limit cycle or to use e inner to probe the internal planetary structure. We perform various transit timing variation (TTV) analyses, demonstrating that current constraints merely restrict e outer < 0.85, and rule out relative planetary inclinations within ~2° of i rel = 90°, but that future observations could significantly tighten the restriction on both these parameters. We demonstrate that TTV profiles can readily distinguish the theoretically favored inclinations of i rel = 0° and 45°, provided that sufficiently precise and frequent transit timing observations of HAT-P-13b can be made close to the pericenter passage of HAT-P-13c. We note the relatively high probability that HAT-P-13c transits and suggest observational dates and strategies.
Pont, F., Aigrain, S., Zucker, S., 2011, "Reassessing the radial-velocity evidence for planets around CoRoT-7," Monthly Notices of the Royal Astronomical Society, 411, 1953-1962.
CoRoT-7 is an 11 th magnitude K-star whose light curve shows transits with a depth of 0.3 mmag and a period of 0.854 d, superimposed on variability at the 1 per cent level, due to the modulation of evolving active regions with the star's 23-d rotation period. In this paper, we revisit the published HARPS radial-velocity (RV) measurements of the object, which were previously used to estimate the companion mass, but have been the subject of ongoing debate.
We build a realistic model of the star's activity during the HARPS observations, by fitting simultaneously the linewidth (as measured by the width of the cross-correlation function) and the line bisector, and use it to evaluate the contribution of activity to the RV variations. The data show clear evidence of errors above the level of the formal uncertainties, which are accounted for neither by activity nor by any plausible planet model and which increase rapidly with a decreasing signal-to-noise ratio (S/N) of the spectra. We cite evidence of similar systematics in mid-S/N spectra of other targets obtained with HARPS and other high-precision RV spectrographs, and discuss possible sources. Allowing for these, we re-evaluate the semi-amplitude of the CoRoT-7b signal, finding Kb= 1.6 ± 1.3 m s−1, a tentative detection with a much reduced significance (1.2σ) compared to previous estimates. We also argue that the combined presence of activity and additional errors precludes a meaningful search for additional low-mass companions, despite previous claims to the contrary.
Taken at face value, our analysis points to a lower density for CoRoT-7b, the 1σ mass range spanning 1.4 M ⊕ and allowing for a wide range of bulk compositions. In particular, an ice-rich composition is compatible with the RV constraints. More generally, this study highlights the importance of a realistic treatment of both activity and uncertainties, particularly in the medium S/N regime, which applies to most small planet candidates from CoRoT and Kepler.
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, 378.
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.
Rambaux, N., van Hoolst, T., Karatekin, Ö, 2011, "Librational response of Europa, Ganymede, and Callisto with an ocean for a non-Keplerian orbit," Astronomy and Astrophysics, 527, 118.
Context. The Galilean satellites Europa, Ganymede, and Callisto are thought to harbor a subsurface ocean beneath an ice shell but its properties, such as the depth beneath the surface, have not been well constrained. Future geodetic observations with, for example, space missions like the Europa Jupiter System Mission (EJSM) of NASA and ESA may refine our knowledge about the shell and ocean.
Aims: Measurement of librational motion is a useful tool for detecting an ocean and characterizing the interior parameters of the moons. The objective of this paper is to investigate the librational response of Galilean satellites, Europa, Ganymede, and Callisto assumed to have a subsurface ocean by taking the perturbations of the Keplerian orbit into account. Perturbations from a purely Keplerian orbit are caused by gravitational attraction of the other Galilean satellites, the Sun, and the oblateness of Jupiter.
Methods: We use the librational equations developed for a satellite with a subsurface ocean in synchronous spin-orbit resonance. The orbital perturbations were obtained from recent ephemerides of the Galilean satellites.
Results: We identify the wide frequency spectrum in the librational response for each Galilean moon. The librations can be separated into two groups, one with short periods close to the orbital period, and a second group of long-period librations related to the gravitational interactions with the other moons and the Sun. Long-period librations can have amplitudes as large as or even larger than the amplitude of the main libration at orbital period for the Keplerian problem, implying the need to introduce them in analyses of observations linked to the rotation. The amplitude of the short-period librations contains information on the interior of the moons, but the amplitude associated with long periods is almost independent of the interior at first order in the low frequency. For Europa, we identified a short-period libration with period close to twice the orbital period, which could have been resonantly amplified in the history of Europa. For Ganymede, we also found a possible resonance between a proper period and a forced period when the icy shell thickness is around 50 km. The librations of Callisto are dominated by solar perturbations.
Reffert, S., Quirrenbach, A., 2011, "Mass constraints on substellar companion candidates from the re-reduced Hipparcos intermediate astrometric data: nine confirmed planets and two confirmed brown dwarfs," Astronomy and Astrophysics, 527, 140.
Context. The recently completed re-reduction of the Hipparcos data by van Leeuwen (2007a, Astrophysics and Space Science Library, 350) makes it possible to search for the astrometric signatures of planets and brown dwarfs known from radial velocity surveys in the improved Hipparcos intermediate astrometric data.
Aims: Our aim is to put more significant constraints on the orbital parameters which cannot be derived from radial velocities alone, i.e. the inclination and the longitude of the ascending node, than was possible before. The determination of the inclination in particular allows to calculate an unambiguous companion mass, rather than the lower mass limit which can be obtained from radial velocity measurements.
Methods: We fitted the astrometric orbits of 310 substellar companions around 258 stars, which were all discovered via the radial velocity method, to the Hipparcos intermediate astrometric data provided by van Leeuwen. <BR /> Results: Even though the astrometric signatures of the companions cannot be detected in most cases, the Hipparcos data still provide lower limits on the inclination for all but 67 of the investigated companions, which translates into upper limits on the masses of the unseen companions. For nine companions the derived upper mass limit lies in the planetary and for 75 companions in the brown dwarf mass regime, proving the substellar nature of those objects. Two of those objects have minimum masses also in the brown dwarf regime and are thus proven to be brown dwarfs. The confirmed planets are the ones around Pollux (β Gem b), ϵ Eri b,ϵ Ret b, μ Ara b, υ And c and d, 47 UMa b, HD 10647 b and HD 147513 b. The confirmed brown dwarfs are HD 137510 b and HD 168443 c. In 20 cases, the astrometric signature of the substellar companion was detected in the Hipparcos data, resulting in reasonable constraints on inclination and ascending node. Of these 20 companions, three are confirmed as planets or lightweight brown dwarfs (HD 87833 b, ι Dra b, and γ Cep b), two as brown dwarfs (HD 106252 b and HD 168443 b), and four are low-mass stars (BD .04 782 b, HD 112758 b, ρ CrB b, and HD169822 b). Of the others, many are either brown dwarfs or very low mass stars. For ϵ Eri, we derive a solution which is very similar to the one obtained using Hubble Space Telescope data.
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, 468.
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.
Ryu, Y.-H., Chang, H.-Y., Park, M.-G., 2011, "Detection probability of a low-mass planet for triple lens events: implication of properties of binary-lens superposition," Monthly Notices of the Royal Astronomical Society, 412, 503-510.
In view of the assumption that any planetary system is likely to be composed of more than one planet, and that a multiple planet system with a large-mass planet has a greater chance of detailed follow-up observations, the multiple planet system may be an efficient way to search for sub-Jovian planets. We study the central region of the magnification pattern for the triple lens system composed of a star, a Jovian mass planet and a low-mass planet to answer the question of if the low-mass planet can be detected in high-magnification events. We compare the magnification pattern of the triple lens system with that of a best-fitted binary system composed of a star and a Jovian mass planet, and check the probability of detecting the low-mass secondary planet whose signature will be superposed on that of the primary Jovian mass planet. Detection probabilities of the low-mass planet in the triple lens system are quite similar to the probability of detecting such a low-mass planet in a binary system with a star and only a low-mass planet, which shows that the signature of a low-mass planet can be effectively detected even when it is concurrent with the signature of the more massive planet, implying that the binary superposition approximation works over a relatively broad range of planet mass ratio and separations, and the inaccuracies thereof do not significantly affect the detection probability of the lower-mass secondary planet. Since the signature of the Jovian mass planet will be larger and lasting longer, thereby warranting more intensive follow-up observations, the actual detection rate of the low-mass planet in a triple system with a Jovian mass can be significantly higher than that in a binary system with a low-mass planet only. We conclude that it may be worthwhile to develop an efficient algorithm to search for 'super-Earth' planets in the paradigm of the triple lens model for high-magnification microlensing events.
Salva, H.R., 2011, "Searching the Allais effect during the total sun eclipse of 11 July 2010," Physical Review D, 83, 67302.
I have measured the precession change of the oscillation plane with an automated Foucault pendulum and found no evidence (within the measurement error) of the Allais effect. The precession speed was registered and, due the variations involved, if the precession speed would changed 0.3 degree per hour (increasing or decreasing the angle of the normal precession speed) during the all eclipse, it would be notice in this measurement.
Southworth, J., Dominik, M., Jøsen, U.G., Rahvar, S., Snodgrass, C., Alsubai, K., Bozza, V., Browne, P., Burgdorf, M., Calchi Novati, S., Dodds, P., Dreizler, S., Finet, F., Gerner, T., Hardis, S., Harpsø., Hellier, C., Hinse, T.C., Hundertmark, M., Kains, N., Kerins, E., Liebig, C., Mancini, L., Mathiasen, M., Penny, M.T., Proft, S., Ricci, D., Sahu, K., Scarpetta, G., Schär, S., Schöeck, F., Surdej, J., 2011, "A much lower density for the transiting extrasolar planet WASP-7," Astronomy and Astrophysics, 527, 8.
We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 ± 0.093 RJup versus RJup) and much lower density (0.41 ± 0.10 ρJup versus ρJup) and surface gravity (13.4 ± 2.6 m s-2 versus m s-2) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass-radius and period-gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.
Stone, N., Loeb, A., 2011, "Prompt tidal disruption of stars as an electromagnetic signature of supermassive black hole coalescence," Monthly Notices of the Royal Astronomical Society, 412, 75-80.
A precise electromagnetic measurement of the sky coordinates and redshift of a coalescing black hole binary holds the key for using its gravitational wave (GW) signal to constrain cosmological parameters and to test general relativity. Here we show that the merger of ∼ 106−7 M⊙ black holes is generically followed by electromagnetic flares from tidally disrupted stars. The sudden recoil imparted to the merged black hole by GW emission promptly fills its loss cone and results in a tidal disruption rate of stars as high as ∼ 0.1 yr−1. The prompt disruption of a single star within a galaxy provides a unique electromagnetic flag of a recent black hole coalescence event, and sequential disruptions could be used on their own to calibrate the expected rate of GW sources for pulsar timing arrays or the proposedLaser Interferometer Space Antenna.
Uzan, J.-P., 2011, "Varying Constants, Gravitation and Cosmology," Living Reviews in Relativity, 14, 2.
Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, solar system observations, meteorite dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with.
von Braun, K., Boyajian, T.S., Kane, S.R., van Belle, G.T., Ciardi, D.R., Ló-Morales, M., McAlister, H.A., Henry, T.J., Jao, W.-C., Riedel, A.R., Subasavage, J.P., Schaefer, G., ten Brummelaar, T.A., Ridgway, S., Sturmann, L., Sturmann, J., Mazingue, J., Turner, N.H., Farrington, C., Goldfinger, P.J., Boden, A.F., 2011, "Astrophysical Parameters and Habitable Zone of the Exoplanet Hosting Star GJ 581," The Astrophysical Journal, 729, L26.
GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be 0.299 ± 0.010 R . Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature T EFF = 3498 ± 56 K and its luminosity L = 0.01205 ± 0.00024 L . From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.
Wahhaj, Z., Liu, M.C., Biller, B.A., Clarke, F., Nielsen, E.L., Close, L.M., Hayward, T.L., Mamajek, E.E., Cushing, M., Dupuy, T., Tecza, M., Thatte, N., Chun, M., Ftaclas, C., Hartung, M., Reid, I.N., Shkolnik, E.L., Alencar, S.H.P., Artymowicz, P., Boss, A., de Gouveia Dal Pino, E., Gregorio-Hetem, J., Ida, S., Kuchner, M., Lin, D.N.C., Toomey, D.W., 2011, "The Gemini NICI Planet-finding Campaign: Discovery of a Substellar L Dwarf Companion to the Nearby Young M Dwarf CD.35 2722," The Astrophysical Journal, 729, 139.
We present the discovery of a wide (67 AU) substellar companion to the nearby (21 pc) young solar-metallicity M1 dwarf CD.35 2722, a member of the 100 Myr AB Doradus association. Two epochs of astrometry from the NICI Planet-Finding Campaign confirm that CD.35 2722 B is physically associated with the primary star. Near-IR spectra indicate a spectral type of L4±1 with a moderately low surface gravity, making it one of the coolest young companions found to date. The absorption lines and near-IR continuum shape of CD.35 2722 B agree especially well the dusty field L4.5 dwarf 2MASS J22244381.0158521, while the near-IR colors and absolute magnitudes match those of the 5 Myr old L4 planetary-mass companion, 1RXS J160929.1.210524 b. Overall, CD.35 2722 B appears to be an intermediate-age benchmark for L dwarfs, with a less peaked H-band continuum than the youngest objects and near-IR absorption lines comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR spectra and find T eff= 1700-1900 K and log(g)= 4.5 ± 0.5. The spectra also show that the radial velocities of components A and B agree to within ±10 km s.1, further confirming their physical association. Using the age and bolometric luminosity of CD.35 2722 B, we derive a mass of 31 ± 8 M Jup from the Lyon/Dusty evolutionary models. Altogether, young late-M to mid-L type companions appear to be overluminous for their near-IR spectral type compared with field objects, in contrast to the underluminosity of young late-L and early-T dwarfs.
Watson, C.A., Littlefair, S.P., Diamond, C., Collier Cameron, A., Fitzsimmons, A., Simpson, E., Moulds, V., Pollacco, D., 2011, "On the alignment of debris discs and their host stars' rotation axis - implications for spin-orbit misalignment in exoplanetary systems," Monthly Notices of the Royal Astronomical Society, L235.
It has been widely thought that measuring the misalignment angle between the orbital plane of a transiting exoplanet and the spin of its host star was a good discriminator between different migration processes for hot-Jupiters. Specifically, well-aligned hot-Jupiter systems (as measured by the Rossiter-McLaughlin effect) were thought to have formed via migration through interaction with a viscous disc, while misaligned systems were thought to have undergone a more violent dynamical history. These conclusions were based on the assumption that the planet-forming disc was well-aligned with the host star. Recent work by a number of authors has challenged this assumption by proposing mechanisms that act to drive the star-disc interaction out of alignment during the pre-main-sequence phase. We have estimated the stellar rotation axis of a sample of stars which host spatially resolved debris discs. Comparison of our derived stellar rotation axis inclination angles with the geometrically measured debris-disc inclinations shows no evidence for a misalignment between the two.
Wen, Z.L., Jenet, F.A., Yardley, D., Hobbs, G.B., Manchester, R.N., 2011, "Constraining the Coalescence Rate of Supermassive Black-hole Binaries Using Pulsar Timing," The Astrophysical Journal, 730, 29.
Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10 ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems.
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, 467.
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.