Searcharxiv⌕ Search

arXiv subjects

Gilles Chabrier

Publications and source records attributed to Gilles Chabrier.

At least 73 records · Page 4Linked to original sources
arXiv2010

Is tidal heating sufficient to explain bloated exoplanets? Consistent calculations accounting for finite initial eccentricity

In this paper, we present the consistent evolution of short-period exoplanets coupling the tidal and gravothermal evolution of the planet. Contrarily to previous similar studies, our calculations are based on the complete tidal evolution equations of the Hut model, valid at any order in eccentricity, obliquity and spin. We demonstrate, both analytically and numerically, that, except if the system was formed with a nearly circular orbit (e<0.2), solving consistently the complete tidal equations is mandatory to derive correct tidal evolution histories. We show that calculations based on tidal models truncated at second order in eccentricity, as done in all previous studies, lead to erroneous tidal evolutions. As a consequence, tidal energy dissipation rates are severely underestimated in all these calculations and the characteristic timescales for the various orbital parameters evolutions can be wrong by up to three orders in magnitude. Based on these complete, consistent calculations, we revisit the viability of the tidal heating hypothesis to explain the anomalously large radius of transiting giant planets. We show that, even though tidal dissipation does provide a substantial contribution to the planet's heat budget and can explain some of the moderately bloated hot-Jupiters, this mechanism can not explain alone the properties of the most inflated objects, including HD 209458b. Indeed, solving the complete tidal equations shows that enhanced tidal dissipation and thus orbit circularization occur too early during the planet's evolution to provide enough extra energy at the present epoch. In that case another mechanisms, such as stellar irradiation induced surface winds dissipating in the planet's tidal bulges, or inefficient convection in the planet's interior must be invoked, together with tidal dissipation, to provide all the pieces of the abnormally large exoplanet puzzle.

astro-ph.EP↗
arXiv2009

Plasma physics and planetary astrophysics

In this review, I briefly summarize the present status of experimental and theoretical investigations of the properties of matter under conditions characteristic of planetary interiors, from terrestrial to jovian planets. I first focus on the two lightest elements, hydrogen and helium, and discuss recent theoretical and experimental investigations of their properties at high pressure and temperature. Then, I discuss the impact of these properties, as well as of the equation of state of heavier elements, on planetary interiors. Finally, I highlight the importance of exoplanet transit observations and of the inferred mass-radius relationships to determine the planetary interior compositions.

astro-ph.EP↗
arXiv2009

Planetary Formation and Evolution Revealed with a Saturn Entry Probe: The Importance of Noble Gases

The determination of Saturn's atmospheric noble gas abundances are critical to understanding the formation and evolution of Saturn, and giant planets in general. These measurements can only be performed with an entry probe. A Saturn probe will address whether enhancement in heavy noble gases, as was found in Jupiter, are a general feature of giant planets, and their ratios will be a powerful constraint on how they form. The helium abundance will show the extent to which helium has phase separated from hydrogen in the planet's deep interior. Jupiter's striking neon depletion may also be tied to its helium depletion, and must be confirmed or refuted in Saturn. Together with Jupiter's measured atmospheric helium abundance, a consistent evolutionary theory for both planets, including "helium rain" will be possible. We will then be able to calibrate the theory of the evolution of all giant planets, including exoplanets. In addition, high pressure H/He mixtures under giant planet conditions are an important area of condensed matter physics that are beyond the realm of experiment.

astro-ph.EP↗
arXiv2009

Analytical theory for the initial mass function: II. Properties of the flow

Recently, Hennebelle and Chabrier (2008) derived an analytical theory for the mass spectrum of non self-gravitating clumps associated with overdensities in molecular clouds and for the initial mass function of gravitationally bound prestellar cores, as produced by the turbulent collapse of the cloud. In this companion paper, we examine the effects of the non-isothermality of the flow, of the turbulence forcing and of local fluctuation of the velocity dispersion, on the mass function. In particular, we investigate the influence of a polytropic equation of state and of the effective adiabatic exponent $γ$ and find that it has a drastic influence on the low mass part of the IMF. We also consider a barotropic equation of state (i.e. a piecewise polytropic eos) that mimics the thermal behaviour of the molecular gas and compare the prediction of our theory with the results of numerical simulations and with the observationally-derived IMF, for cloud parameters which satisfy Larson's type relations. We find that for clouds whose density is, at all scales, almost an order of magnitude larger than the density inferred for the CO clumps in the Galaxy, a good agreement is obtained between the theory and the observed IMF, suggesting that star formation preferentially occurs in high density environments. We derive an analytical expression for the IMF which generalizes the expression previously obtained for the isothermal case. This easy-to-implement analytical IMF should serve as a template to compare observational or numerical results with the theory.

astro-ph.GA↗
arXiv2008

Evolution of very low mass pre-main sequence stars and young brown dwarfs under accretion: A phenomenological approach

In the poster presented in Cool Star 15, we analyzed the effect of disk accretion on the evolution of very low mass pre-main sequence stars and young brown dwarfs and the resulting uncertainties on the determination of masses and ages. We use the Lyon evolutionary 1-D code assuming a magnetospheric accretion process, i.e., the material falls covering a small area of the radiative surface, and we take into account the internal energy added from the accreted material as a free parameter $ε$. Even if the approach to this problem is phenomenological, our formalism provides important hints about characteristics of disk accretion, which are useful for improved stellar interior calculations. Using the accretion rates derived from observations our results show that accretion does not affect considerably the position of theoretical isochrones as well as the luminosity compared with standard non-accreting models. See more discussions in a forthcoming paper by Gallardo, Baraffe and Chabrier (2008).

astro-ph↗
arXiv2008

Analytical theory for the initial mass function: CO clumps and prestellar cores

We derive an analytical theory of the prestellar core initial mass function based on an extension of the Press-Schechter statistical formalism. With the same formalism, we also obtain the mass spectrum for the non self-gravitating clumps produced in supersonic flows. The mass spectrum of the self-gravitating cores reproduces very well the observed initial mass function and identifies the different mechanisms responsible for its behaviour. The theory predicts that the shape of the IMF results from two competing contributions, namely a power-law at large scales and an exponential cut-off (lognormal form) centered around the characteristic mass for gravitational collapse. The cut-off exists already in the case of pure thermal collapse, provided that the underlying density field has a lognormal distribution. Whereas pure thermal collapse produces a power-law tail steeper than the Salpeter value, dN/dlog M\propto M^{-x}, with x=1.35, this latter is recovered exactly for the (3D) value of the spectral index of the velocity power spectrum, n\simeq 3.8, found in observations and in numerical simulations of isothermal supersonic turbulence. Indeed, the theory predicts that x=(n+1)/(2n-4) for self-gravitating structures and x=2-n'/3 for non self-gravitating structures, where n' is the power spectrum index of log(rho). We show that, whereas supersonic turbulence promotes the formation of both massive stars and brown dwarfs, it has an overall negative impact on star formation, decreasing the star formation efficiency. This theory provides a novel theoretical foundation to understand the origin of the IMF and to infer its behaviour in different environments. It also provides a complementary approach and useful guidance to numerical simulations exploring star formation, while making testable predictions.

astro-ph↗
arXiv2008

Model X-ray Spectra of Magnetic Neutron Stars with Hydrogen Atmospheres

We construct partially ionized hydrogen atmosphere models for magnetized neutron stars in radiative equilibrium with fixed surface fields between B=10^12 and 2x10^13 G and effective temperatures logT_eff=5.5-6.8, as well as with surface B and T_eff distributions around these values. The models are based on the latest equation of state and opacity results for magnetized, partially ionized hydrogen plasmas. The atmospheres directly determine the characteristics of thermal emission from the surface of neutron stars. We also incorporate these model spectra into XSPEC, under the model name NSMAX, thus allowing them to be used by the community to fit X-ray observations of neutron stars.

astro-ph↗
arXiv2008

Protostellar collapse: A comparison between SPH and AMR calculations

The development of parallel supercomputers allows today the detailed study of the collapse and the fragmentation of prestellar cores with increasingly accurate numerical simulations. Thanks to the advances in sub-millimeter observations, a wide range of observed initial conditions enable us to study the different modes of low-mass star formation. The challenge for the simulations is to reproduce the observational results. Two main numerical methods, namely AMR and SPH, are widely used to simulate the collapse and the fragmentation of prestellar cores. We compare thoroughly these two methods within their standard framework. We use the AMR code RAMSES and the SPH code DRAGON. Our physical model is as simple as possible and consists of an isothermal sphere rotating around the z-axis. We first study the conservation of angular momentum as a function of the resolution. Then, we explore a wide range of simulation parameters to study the fragmentation of prestellar cores. There seems to be a convergence between the two methods, provided resolution in each case is sufficient. Resolution criteria adapted to our physical cases, in terms of resolution per Jeans mass, for an accurate description of the formation of protostellar cores are deduced from the present study. This convergence is encouraging for future work in simulations of low-mass star formation, providing the aforementioned criteria are fulfilled. Higher resolution figures can be downloaded at http://www-dapnia.cea.fr/Projets/COAST/paper_amrvssph.pdf

astro-ph↗
arXiv2007

The Thermal Evolution of the Donors in AM CVn Binaries

(Abridged) We calculate the full stellar-structural evolution of donors in AM CVn systems formed through the WD channel coupled to the binary's evolution. Contrary to assumptions made in prior modelling, these donors are not fully convective over much of the AM CVn phase and do not evolve adiabatically under mass loss indefinitely. Instead, we identify three distinct phases of evolution: a mass transfer turn-on phase (during which the orbital period continues to decrease even after contact, the donor contracts, and the mass transfer rate accelerates to its maximum), a phase in which the donor expands adiabatically in response to mass loss, and a cooling phase beginning at orbital periods of approximately 45--55 minutes during which the donor contracts. The physics that determines the behaviour in the first and third phases, both of which are new outcomes of this study, are discussed in some detail. We find the overall duration of the turn-on phase to be between $\sim 10^4$-$10^6$ yrs, significantly longer than prior estimates. We predict the donor's luminosity and effective temperature. During the adiabatic expansion phase (ignoring irradiation effects), the luminosity is approximately $10^{-6}$--$10^{-4} L_\odot$ and the effective temperature is approximately 1000--1800 K. However, the flux generated in the accretion flow dominates the donor's intrinsic light at all times. The impact of irradiation on the donor extends the phase of adiabatic expansion to longer orbital periods and alters the donor's observational characteristics. Irradiated donors during the adiabatic phase can attain a surface luminosity of up to $\approx10^{-2} L_\odot$. We argue that the turn-on and cooling phases both will leave significant imprints on the AM CVn population's orbital period distribution.

astro-ph↗
arXiv2007

Evolution of low-mass star and brown dwarf eclipsing binaries

We examine the evolution of low-mass star and brown dwarf eclipsing binaries. These objects are rapid rotators and are believed to shelter large magnetic fields. We suggest that reduced convective efficiency, due to fast rotation and large field strengths, and/or to magnetic spot coverage of the radiating surface significantly affect their evolution, leading to a reduced heat flux and thus larger radii and cooler effective temperatures than for regular objects. We have considered such processes in our evolutionary calculations, using a phenomenological approach. This yields mass-radius and effective temperature-radius relationships in agreement with the observations. We also reproduce the effective temperature ratio and the radii of the two components of the recently discovered puzzling eclipsing brown dwarf system. These calculations show that fast rotation and/or magnetic activity may significantly affect the evolution of eclipsing binaries and that the mechanical and thermal properties of these objects depart from the ones of non-active low-mass objects. We find that, for internal field strengths compatible with the observed surface value of a few kiloGauss, convection can be severely inhibited. The onset of a central radiative zone for rapidly rotating active low-mass stars might thus occur below the usual $\sim 0.35 \msol$ limit.

astro-ph↗
arXiv2007

Heat transport in giant (exo)planets: a new perspective

We explore the possibility that large-scale convection be inhibited over some regions of giant planet interiors, as a consequence of a gradient of composition inherited either from their formation history or from particular events like giant impacts or core erosion during their evolution. Under appropriate circumstances, the redistribution of the gradient of molecular weight can lead to double diffusive layered or overstable convection. This leads to much less efficient heat transport and compositional mixing than large-scale adiabatic convection. We show that this process can explain the abnormally large radius of the transit planet HD209458b and similar objects, and may be at play in some giant planets, with short-period planets offering the most favorable conditions. Observational signatures of this transport mechanism are a large radius and a reduced heat flux output compared with uniformly mixed objects. If our suggestion is correct, it bears major consequences on our understanding of giant planet formation, structure and evolution, including possibly our own jovian planets.

astro-ph↗
arXiv2006

Tidal dissipation within hot Jupiters: a new appraisal

Eccentricity or obliquity tides have been proposed as the missing energy source that may explain the anomalously large radius of some transiting ``hot Jupiters''. To maintain a non-zero and large obliquity, it was argued that the planets can be locked in a Cassini state, i.e. a resonance between spin and orbital precessions. We compute the tidal heating within ``inflated'' close-in giant planets with a non-zero eccentricity or obliquity. We further inspect whether the spin of a ``hot Jupiter'' could have been trapped and maintained in a Cassini state during its early despinning and migration. We estimate the capture probability in a spin-orbit resonance between $\sim$ 0.5 AU (a distance where tidal effects become significant) and 0.05 AU for a wide range of secular orbital frequencies and amplitudes of gravitational perturbations. Numerical simulations of the spin evolution are performed to explore the influence of tidal despinning and migration processes on the resonance stability. We find that tidal heating within a non-synchronous giant planet is about twice larger than previous estimates based on the hypothesis of synchronization. Chances of capture in a spin-orbit resonance are very good around 0.5 AU but they decrease dramatically with the semi-major axis. Furthermore, even if captured, both tidal despinning and migration processes cause the tidal torque to become large enough that the obliquity ultimately leaves the resonance and switches to near $0^{\circ}$. Locking a ``hot Jupiter'' in an isolated spin-orbit resonance is unlikely at 0.05 AU but could be possible at larger distances. Another mechanism is then required to maintain a large obliquity and create internal heating through obliquity tides

astro-ph↗
arXiv2006

Thermal Evolution of AM CVn Binary Donors

We discuss results of our study on AM CVn binaries formed with donors that never ignited He before contact. For the first time, we treat the donor's in these systems in the context of a full stellar structure evolution theory and find that the binary's evolution can described in terms of 3 phases: contact, adiabatic donor expansion, and late-time donor cooling. Details of the first and third phase are new results from this study and we focus on generally characterizing these two phases. Finally, we present our predictions for the donor's light in these systems.

astro-ph↗
arXiv2006

Formation and structure of the three Neptune-mass planets system around HD69830

Since the discovery of the first giant planet outside the solar system in 1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been discovered. With improving detection capabilities, a new class of planets with masses 5-20 times larger than the Earth, at close distance from their parent star is rapidly emerging. Recently, the first system of three Neptune-mass planets has been discovered around the solar type star HD69830 (Lovis et al. 2006). Here, we present and discuss a possible formation scenario for this planetary system based on a consistent coupling between the extended core accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al. 2004,2006). We show that the innermost planet formed from an embryo having started inside the iceline is composed essentially of a rocky core surrounded by a tiny gaseous envelope. The two outermost planets started their formation beyond the iceline and, as a consequence, accrete a substantial amount of water ice during their formation. We calculate the present day thermodynamical conditions inside these two latter planets and show that they are made of a rocky core surrounded by a shell of fluid water and a gaseous envelope.

astro-ph↗
arXiv2005

Large-scale alpha^2-dynamo in low-mass stars and brown dwarfs

We develop a model based on 3D mean-field MHD for the generation of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous planets. The dynamo process is of alpha^2 type and thus differs from the shell-dynamo at work in more massive stars.The alpha^2 dynamo becomes supercritical for Rossby numbers Ro\la 10. It generates a large-scale, non-axisymmetric, steady field that is symmetric with respect to the equatorial plane. Saturation of the alpha^2-generated field at the equipartition yields strengths of several kGauss, in agreement with observations of active M dwarfs, and provides a qualitative explanation for the observed activity saturation in late M stars. For brown dwarfs with a conductive core, as occurs at the center of the most massive and oldest of these objects, we have also studied an alpha^2-Omega dynamo, i.e. the effect of differential rotation. In this case the field is predominantly toroidal, axisymmetric and oscillatory, like the solar field. The topology of the field in the fully convective objects exhibits a high order multipole character that differs from the aligned dipole field generated by the alpha-Omega dynamo. The strong reduction of the dipolar component due to the field non-axisymmetry should considerably reduce the Alfven radius and thus the efficiency of magnetic braking, providing an appealing explanation for the decreasing angular momentum loss rate observed in low-mass stars and brown dwarfs. This may have also implications for cataclysmic variables below the period gap. In spite of this large-scale field, the decreasing conductivity in the dominantly neutral atmosphere of these objects prevents the current generation necessary to support a chromosphere and thus activity. (Abridged)

astro-ph↗
arXiv2005

Source blending effects on microlensing time-histograms and optical depth determination

Source blending in microlensing experiments is known to modify the Einstein time of the observed events. In this paper, we have conducted Monte-Carlo calculations, using the analytical relationships derived by Han (1999) to quantify the effect of blending on the observed event time distribution and optical depth. We show that short-time events are affected significantly by source blending and that, for moderately blended sources, the optical depth $τ$ is globally overestimated, because of an underestimation of the exposure. For high blending situations, on the opposite, blending leads to an {\it under}estimation of the optical depth. Our results are in agreement with the most recent optical depth determinations toward the Galactic Center of the MACHO collaboration (Popowski et al. 2004) and the OGLE-II collaboration (Sumi et al. 2005) that use clump giants (less affected by the blending effect) as sources. The blending-corrected, lower optical depth toward the Galactic Bulge is now in good agreement with the value inferred from galactic models, reconciling theoretical and observational determinations.

astro-ph↗
arXiv2004

Free-energy model for fluid helium at high density

We present a semi-analytical free-energy model aimed at characterizing the thermodynamic properties of dense fluid helium, from the low-density atomic phase to the high-density fully ionized regime. The model is based on a free-energy minimization method and includes various different contributions representative of the correlations between atomic and ionic species and electrons. This model allows the computation of the thermodynamic properties of dense helium over an extended range of density and temperature and leads to the computation of the phase diagram of dense fluid helium, with its various temperature and pressure ionization contours. One of the predictions of the model is that pressure ionization occurs abruptly at $ρ\simgr 10$ g cm$^{-3}$, {\it i.e.} $P\simgr 20$ Mbar, from atomic helium He to fully ionized helium He$^{2+}$, or at least to a strongly ionized state, without He$^{+}$ stage, except at high enough temperature for temperature ionization to become dominant. These predictions and this phase diagram provide a guide for future dynamical experiments or numerical first-principle calculations aimed at studying the properties of helium at very high density, in particular its metallization. Indeed, the characterization of the helium phase diagram bears important consequences for the thermodynamic, magnetic and transport properties of cool and dense astrophysical objects, among which the solar and the numerous recently discovered extrasolar giant planets.

physics.plasm-ph↗