Survival of Terrestrial Planets in the Presence of Giant Planet Migration

The presence of ``Hot Jupiters'', Jovian mass planets with very short orbital periods orbiting nearby main sequence stars, has been proposed to be primarily due to the orbital migration of planets formed in orbits initially much further from the parent star. The migration of giant planets would have profound effects on the evolution of inner terrestrial planets in these systems, and previous analyses have assumed that no terrestrial planets survive after migration has occurred. We present numerical simulations showing that a significant fraction of terrestrial planets could survive the migration process, eventually returning to circular orbits relatively close to their original positions. A fraction of the final orbits are in the Habitable Zone, suggesting that planetary systems with close-in giant planets are viable targets for searches for Earth-like habitable planets around other stars.Comment: 5 pages, 3 figures, emulateapj. ApJL in press, referee comments changes and edited for lengt

Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38

We present high contrast images of the hydrogen white dwarf G 29-38 taken in the near infrared with the Hubble Space Telescope and the Gemini North Telescope as part of a high contrast imaging search for substellar objects in orbit around nearby white dwarfs. We review the current limits on planetary companions for G29-38, the only nearby white dwarf with an infrared excess due to a dust disk. We add our recent observations to these limits to produce extremely tight constraints on the types of possible companions that could be present. No objects $>$ 6 M$_{Jup}$ are detected in our data at projected separations $>$ 12 AU, and no objects $>$ 16 M$_{Jup}$ are detected for separations from 3 to 12 AU, assuming a total system age of 1 Gyr. Limits for companions at separations $<$ 3 AU come from a combination of 2MASS photometry and previous studies of G29-38's pulsations. Our imaging with Gemini cannot confirm a tentative claim for the presence of a low mass brown dwarf. These observations demonstrate that a careful combination of several techniques can probe nearby white dwarfs for large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap

Improved approximate inspirals of test-bodies into Kerr black holes

We present an improved version of the approximate scheme for generating inspirals of test-bodies into a Kerr black hole recently developed by Glampedakis, Hughes and Kennefick. Their original "hybrid" scheme was based on combining exact relativistic expressions for the evolution of the orbital elements (the semi-latus rectum p and eccentricity e) with approximate, weak-field, formula for the energy and angular momentum fluxes, amended by the assumption of constant inclination angle, iota, during the inspiral. Despite the fact that the resulting inspirals were overall well-behaved, certain pathologies remained for orbits in the strong field regime and for orbits which are nearly circular and/or nearly polar. In this paper we eliminate these problems by incorporating an array of improvements in the approximate fluxes. Firstly, we add certain corrections which ensure the correct behaviour of the fluxes in the limit of vanishing eccentricity and/or 90 degrees inclination. Secondly, we use higher order post-Newtonian formulae, adapted for generic orbits. Thirdly, we drop the assumption of constant inclination. Instead, we first evolve the Carter constant by means of an approximate post-Newtonian expression and subsequently extract the evolution of iota. Finally, we improve the evolution of circular orbits by using fits to the angular momentum and inclination evolution determined by Teukolsky based calculations. As an application of the improved scheme we provide a sample of generic Kerr inspirals and for the specific case of nearly circular orbits we locate the critical radius where orbits begin to decircularise under radiation reaction. These easy-to-generate inspirals should become a useful tool for exploring LISA data analysis issues and may ultimately play a role in source detection.Comment: 25 pages, 14 figures, some typos corrected, short section on conservative corrections added, minor changes for consistency with published versio

Blue straggler stars in dwarf spheroidal galaxies

Blue straggler star (BSS) candidates have been observed in all old dwarf spheroidal galaxies (dSphs), however whether or not they are authentic BSSs or young stars has been a point of debate. To both address this issue and obtain a better understanding of the formation of BSSs in different environments, we have analysed a sample of BSS candidates in two nearby Galactic dSphs, Draco and Ursa Minor. We have determined their radial and luminosity distributions from wide field multicolour imaging data extending beyond the tidal radii of both galaxies. BSS candidates are uniformly distributed through the host galaxy, whereas a young population is expected to show a more clumpy distribution. Furthermore, the observed radial distribution of BSSs, normalized to both red giant branch (RGB) and horizontal branch (HB) stars, is almost flat, with a slight decrease towards the centre. Such a distribution is at odds with the predictions for a young stellar population, which should be more concentrated. Instead, it is consistent with model predictions for BSS formation by mass transfer in binaries (MT-BSSs). Such results, although not decisive, suggest that these candidates are indeed BSSs and that MT-BSSs form in the same way in Draco and Ursa Minor as in globular clusters. This favours the conclusion that Draco and Ursa Minor are truly ‘fossil' galaxies, where star formation ceased completely more than 8 billion years ag

Blue straggler stars in dwarf spheroidal galaxies - II. Sculptor and Fornax

The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax, there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor, the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax, the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars, and this is consistent with previous studie

A Panchromatic Study of the Globular Cluster NGC 1904. I: The Blue Straggler Population

By combining high-resolution (HST-WFPC2) and wide-field ground based (2.2m ESO-WFI) and space (GALEX) observations, we have collected a multi-wavelength photometric data base (ranging from the far UV to the near infrared) of the galactic globular cluster NGC1904 (M79). The sample covers the entire cluster extension, from the very central regions up to the tidal radius. In the present paper such a data set is used to study the BSS population and its radial distribution. A total number of 39 bright ($m_{218}\le 19.5$) BSS has been detected, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of NGC 1904, in contrast to other clusters (M 3, 47 Tuc, NGC 6752, M 5) studied with the same technique. Such evidences, coupled with the large radius of avoidance estimated for NGC 1904 ($r_{avoid}\sim 30$ core radii), indicate that the vast majority of the cluster heavy stars (binaries) has already sunk to the core. Accordingly, extensive dynamical simulations suggest that BSS formed by mass transfer activity in primordial binaries evolving in isolation in the cluster outskirts represent only a negligible (0--10%) fraction of the overall population.Comment: ApJ accepte

Gravitational-Wave Signature of an Inspiral into a Supermassive Horizonless Object

Event horizons are among the most intriguing of general relativity's predictions. Although on firm theoretical footing, direct indications of their existence have yet to be observed. With this motivation in mind, we explore here the possibility of finding a signature for event horizons in the gravitational waves (GWs) produced during the inspiral of stellar-mass compact objects (COs) into the supermassive ($\sim 10^6 M_\odot$) objects that lie at the center of most galaxies. Such inspirals will be a major source for LISA, the future space-based GW observatory. We contrast supermassive black holes with models in which the central object is a supermassive boson star (SMBS). Provided the COs interact only gravitationally with the SMBS, stable orbits exist not just outside the Schwarzschild radius but also inside the surface of the SMBS as well. The absence of an event horizon allows GWs from these orbits to be observed. Here we solve for the metric in the interior of a fairly generic class of SMBS and evolve the trajectory of an inspiraling CO from the Schwarzschild exterior through the plunge into the exotic SMBS interior. We calculate the approximate waveforms for GWs emitted during this inspiral. Geodesics within the SMBS surface will exhibit extreme pericenter precession and other features making the emitted GWs readily distinguishable from those emitted during an inspiral into a black hole.Comment: 20 pages, 9 figures, submitted to PR

New Evidence of Holocene Mass Wasting Events in Recent Volcanic Lakes from the French Massif Central (Lakes Pavin, Montcineyre and Chauvet) and Implications for Natural Hazards

International audienceHigh-resolution seismic profiling (12 kHz) surveys combined with sediment cores, radiocarbon dating, tephrochronology and multibeam bathymetry (when available) allow documentation of a range of Holocene mass wasting events in nearby contrasting lakes of volcanic origin in the French Massif Central (45°N, 2°E): two deep maar lakes (Pavin and Chauvet) and a shallow lake (Montcineyre) dammed by the growth of a volcano. In these lacustrine environments dominated by authigenic sedimentation, recent slide scars, acoustically transparent to chaotic lens-shaped bodies, slump deposits or reworked regional tephra layers suggest that subaqueous mass wasting processes may have been favoured by gas content in the sediments and lake level changes. While these events may have had a limited impact in both lakes Chauvet and Montcineyre, they apparently favoured the development of lacustrine meromicticity in maar Lake Pavin along with possible subaerial debris flows resulting from crater outburst events

Loss cone: past, present and future

The capture and subsequent in--spiral of compact stellar remnants by central massive black holes, is one of the more interesting likely sources of gravitational radiation detectable by LISA. The relevant stellar population includes stellar mass black holes, and possibly intermediate mass black holes, generally on initially eccentric orbits. Predicted detectable rates of capture are highly uncertain, but may be high enough that source confusion is an issue. Foreground events with relatively high signal-to-noise ratio may provide important tests of general relativity. I review the rate estimates in the literature, and the apparent discrepancy between different authors' estimates, and discuss some of the relevant uncertainties and physical processes. The white dwarf mergers rate are uncertain by a factor of few; the neutron star merger rate is completely uncertain and likely to be small; the black hole merger rate is likely to be dominant for detectable mergers and is uncertain by at least two orders of magnitude, largely due to unknown physical conditions and processes. The primary difference in rate estimates is due to different initial conditions and less directly due to different estimates of key physical processes, assumed in different model scenarios for in-spiral and capture.Comment: 7 pages, revtex twocolumn, Special LISA Issue Classical and Quantum Gravity in pres

LISA observations of massive black hole mergers: event rates and issues in waveform modelling

The observability of gravitational waves from supermassive and intermediate-mass black holes by the forecoming Laser Interferometer Space Antenna (LISA), and the physics we can learn from the observations, will depend on two basic factors: the event rates for massive black hole mergers occurring in the LISA best sensitivity window, and our theoretical knowledge of the gravitational waveforms. We first provide a concise review of the literature on LISA event rates for massive black hole mergers, as predicted by different formation scenarios. Then we discuss what (in our view) are the most urgent issues to address in terms of waveform modelling. For massive black hole binary inspiral these include spin precession, eccentricity, the effect of high-order Post-Newtonian terms in the amplitude and phase, and an accurate prediction of the transition from inspiral to plunge. For black hole ringdown, numerical relativity will ultimately be required to determine the relative quasinormal mode excitation, and to reduce the dimensionality of the template space in matched filtering.Comment: 14 pages, 2 figures. Added section with conclusions and outlook. Matches version to appear in the proceedings of 10th Annual Gravitational Wave Data Analysis Workshop (GWDAW 10), Brownsville, Texas, 14-17 Dec 200