arXiv subjects

Gilles Chabrier

Publications and source records attributed to Gilles Chabrier.

At least 19 recordsLinked to original sources

arXiv2026

The slope of the power spectrum of the density field in isothermal supersonic compressible turbulence

The power spectrum (PS) of the density field in supersonic turbulence is a fundamental quantity that characterizes the statistical properties of the structures formed in compressible flows. It is also widely used to estimate the Mach number in the interstellar medium from simulation-derived relations. In this paper, we provide a first quantitative explanation for the evolution of the slope of the PS of the density field with the Mach number in homogeneous isotropic isothermal turbulence using a time-invariant quantity derived by Chandrasekhar (1951). For simulated turbulent flows, the model reproduces the measured slopes for different widths of the inertial range and density variances very well. Our model also provides a comprehensive interpretation of the characteristic slopes of the PS of the density field measured in the interstellar medium. Based on these results, we stress that the Mach number cannot be reliably deduced from the slope of the PS of the density field. In closing, we discuss a resolution criterion that must be fulfilled to correctly simulate a turbulent flow with a given density PS slope.

astro-ph.GA↗

arXiv2026

Roadmap for warm dense matter physics

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations \& experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments and contemporary challenges that are faced by theoretical methods, and experimental techniques needed to create and diagnose warm dense matter. A large part of this roadmap is dedicated to specific warm dense matter systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

physics.plasm-ph↗

arXiv2025

Do tides play a role in the determination of the pre-stellar core mass function?

Recent studies have examined the role of tides in the star formation process. They suggest, notably, that the tides determine the characteristic mass of the stellar initial mass function (IMF) by preventing the collapse of density fluctuations that would become gravitationally unstable in the absence of the tidal field generated by a neighboring central mass. However, most of these studies consider the tidal collapse condition as a 1D process or use a scalar virial condition and thus neglect the anisotropy of the tidal field and its compressive effects. In the present paper, we consider a turbulence-induced density perturbation formed in the envelope of a central core. This perturbation is subject to a tidal field generated by the central core. We study its evolution taking dynamical effects and the anisotropy of the tides into account. Based on the general tensorial virial equations, we determine a new collapse condition that takes these mechanisms into account. We identify two regimes: (i) a weak tidal regime in which the dynamics of the perturbation is only slightly modified by the action of the tides and (ii) a strong tidal regime in which the density threshold for collapse can potentially be increased due to the combined effects of the tides and the rotational support generated by the tidal synchronization of the perturbation with the orbital motion. In the case of a turbulence-induced density perturbation formed in the vicinity of a first Larson core, we show that the density threshold above which the perturbation collapses is increased only for low-mass perturbations (less than 2.7 solar mass) and only by at most a factor of 1.5. We conclude that tides likely do not play a major role in the process of star formation or in the determination of the characteristic mass of the IMF. We propose an alternative explanation for the observed value of the characteristic mass of the IMF.

astro-ph.GA↗

arXiv2025

Brown Dwarf Formation Through Gravitational Collapse: Insights From 3D Numerical Simulations

The formation mechanism of Brown Dwarfs (BDs), whether akin to stars or ejected planetary-mass objects, remains debated. We present the first 3D radiation-MHD simulations of magnetized, turbulent, gravitationally unstable low-mass cores ($0.05-0.1\ \mathrm{M_{\odot}}$) collapsing into proto-BDs. Using the {\ttfamily RAMSES} code with adaptive mesh refinement, we model the full dynamical range ($10^{5}~-10^{22}\ \mathrm{cm^{-3}}$), including radiative transfer (flux limited diffusion) and non-ideal MHD (ambipolar diffusion). Our simulations self-consistently follow the isothermal collapse, first hydrostatic core formation, H$_{2}$ dissociation, and BD birth. The resulting BDs have initial radii $\approx 0.75\ \mathrm{R_{\odot}}$ and masses $\approx 0.8\ \mathrm{M_{Jup}}$, growing via accretion as we follow the early evolution of the object. Crucially, we find that BDs may form similarly to low-mass stars but with a prolonged first-core phase, supporting a star-like formation scenario.

astro-ph.SR↗

arXiv2025

A mass invariant in a compressible turbulent medium

Predicting the measurable statistical properties of density fluctuations in a supersonic compressible turbulent flow is a major challenge in physics. In 1951, Chandrasekhar derived an invariant under the assumption of the statistical homogeneity and isotropy of the turbulent density field and stationarity of the background density. Recently, Jaupart & Chabrier (2021) extended this invariant to non-isotropic flows in a time-evolving background and showed that it has the dimension of a mass. This invariant $M_{\rm inv}$ is defined by $M_{\rm inv} = \mathbb{E}(ρ)\text{Var}\left(\fracρ{\mathbb{E}(ρ)}\right)(l_{\rm c}^ρ)^3$ where $ρ$ is the density field and $l_{\rm c}^ρ$ is the correlation length. In this article, we perform numerical simulations of homogeneous and isotropic compressible turbulence to test the validity of this invariant in a medium subject to isotropic decaying turbulence. We study several input configurations, namely different Mach numbers, injection lengths of turbulence and equations of state. We confirm that $M_{\rm inv}$ remains constant during the decaying phase of turbulence. Furthermore, we develop a theoretical model of the density field statistics which predicts without any free parameter the evolution of the correlation length with the variance of the logdensity field beyond the assumption of the gaussian field for the logdensity. Noting that $M_{\rm inv}$ is independent of the Mach number, we show that this invariant can be used to relate the non-gaussian evolution of the logdensity probability distribution function to its variance with no free parameters.

physics.flu-dyn↗

arXiv2025

Impact of the shape of the prestellar density fluctuations on the core mass function

It is well known that departure from sphericity in the geometry of primordial dark matter halos modifies their mass function. The ellipsoidal collapse model yields a better agreement with simulations of hierarchical clustering than the original, spherical model. In the present paper, we examine the same issue in the context of star formation by studying the impact of non-sphericity of density perturbations in a gravoturbulent medium on the prestellar core mass function (CMF). An important question, notably, is to find out how ellipsoidal, instead of spherical, initial density fluctuations modify both the high-mass and low-mass tails of the CMF. Our study shows that triaxial density fluctuations indeed depart from a purely spherical form but the deformation (prolateness and ellipticity) remains modest, suggesting that the usual hypothesis of spherical collapse in existing theories of the IMF is reasonable. We find that, as in the cosmological case, the departure from sphericity increases the collapse barrier, stabilizing the prestellar cores. The striking difference between the stellar case and the cosmological one for the ellipsoidal collapse model is that, although in both cases the less dense structures are the most deformed, they correspond to small scales, thus low mass halos in cosmology but to large scales, thus large mass cores in star formation. As a result, the high mass range of the CMF is the most affected by the ellipsoidal collapse, resulting in a slightly less steep slope than the one predicted with the spherical hypothesis and a peak slightly shifted toward lower masses.

astro-ph.GA↗

arXiv2024

A consistent explanation for the unusual initial mass function and star formation rate in the Central Molecular Zone (CMZ)

We examine various physical processes that may explain the shallow high-mass slope of the IMF as well as the low SFR in star-forming molecular clouds (MCs) in the Central Molecular Zone (CMZ). We show that the strong tidal field and the tidal shear experienced by the CMZ have opposite effects on the collapse of density fluctuations and nearly compensate, but in any case have a negligible impact and can not explain these unusual properties. Similarly, we show that the intense magnetic field in the CMZ provides a negligible pressure support and, for the high densities at play should not modify the probability density function (PDF) of the turbulent gas flow in the clouds, thus affecting negligibly the slope of the IMF. However, we show that, in contrast to MCs in the Galactic disk, the ones in the CMZ experience only one single episode of turbulence injection at large scale, most likely due dominantly to bar gas inflow. Indeed, their rather short lifetime, due to their high mean densities, is similar to one typical turbulence crossing time. Consequently, according to the Hennebelle-Chabrier theory of star formation, within this 'single turbulence episode' scenario, the cloud experiences one single field of turbulence induced density fluctuations, leading eventually to gravitationally unstable prestellar cores. As shown in Hennebelle & Chabrier (2013}, this yields a flatter IMF than usual and leads to the correct observed slope for the CMZ star-forming clouds. Similarly, this single large scale turbulence event within the cloud lifetime yields a 5 to 6 lower SFR than under usual MW cloud conditions, again in agreement with the observed values. Therefore, we suggest that this 'single large scale turbulence injection' episode can explain both the shallow IMF high-mass slope and low SFR of clouds in the CMZ.

astro-ph.GA↗

arXiv2023

Evidence of Radius Inflation in Radiative GCM Models of WASP-76b due to the Advection of Potential Temperature

Understanding the discrepancy between the radii of observed hot Jupiters and standard 'radiative-convective' models remains a hotly debated topic in the exoplanet community. One mechanism which has been proposed to bridge this gap, and which has recently come under scrutiny, is the vertical advection of potential temperature from the irradiated outer atmosphere deep into the interior, heating the deep, unirradiated, atmosphere, warming the internal adiabat, and resulting in radius inflation. Specifically, a recent study which explored the atmosphere of WASP-76b using a 3D, non-grey, GCM suggested that their models lacked radius inflation, and hence any vertical enthalpy advection. Here we perform additional analysis of these, and related, models, focusing on an explicit analysis of vertical enthalpy transport and the resulting heating of the deep atmosphere compared with 1D models. Our results indicate that, after any evolution linked with initialisation, all the WASP-76b models considered here exhibit significant vertical enthalpy transport, heating the deep atmosphere significantly when compared with standard 1D models. Furthermore, comparison of a long time-scale (and hence near steady-state) model with a Jupiter-like internal-structure model suggests not only strong radius-inflation, but also that the model radius, $1.98 \mathrm{R_{J}}$, may be comparable with observations ($1.83\pm0.06 \mathrm{R_{J}}$). We thus conclude that the vertical advection of potential temperature alone is enough to explain the radius inflation of WASP-76b, and potentially other irradiated gas giants, albeit with the proviso that the exact strength of the vertical advection remains sensitive to model parameters, such as the inclusion of deep atmospheric drag.

astro-ph.EP↗

arXiv2023

Probing the Milky Way stellar and brown dwarf initial mass function with modern microlensing observations

We use recent microlensing observations toward the central bulge of the Galaxy to probe the overall stellar plus brown dwarf initial mass function (IMF) in these regions well within the brown dwarf domain. We find that the IMF is consistent with the same Chabrier (2005) IMF characteristic of the Galactic disk. In contrast, other IMFs suggested in the literature overpredict the number of short-time events, thus of very-low mass stars and brown dwarfs, compared with observations. This, again, supports the suggestion that brown dwarfs and stars form predominantly via the same mechanism. We show that claims for different IMFs in the stellar and substellar domains rather arise from an incorrect parameterization of the IMF. Furthermore, we show that the IMF in the central regions of the bulge seems to be bottom-heavy, as illustrated by the large number of short-time events compared with the other regions. This recalls our previous analysis of the IMF in massive early type galaxies and suggests the same kind of two-phase formation scenario, with the central bulge initially formed under more violent, burst-like conditions than the rest of the Galaxy.

astro-ph.GA↗

arXiv2022

Statistical properties and correlation length in star-forming molecular clouds: I. Formalism and application to observations

The proper characterization of the general statistical behavior of these fluctuations, from a limited sample of observations or simulations, is of prime importance to understand the process of star formation. In this article, we use the ergodic theory for any random field of fluctuations, as commonly used in statistical physics, to derive rigorous statistical results. We outline how to evaluate the autocovariance function (ACF) and the characteristic correlation length of these fluctuations. We then apply this statistical approach to astrophysical systems characterized by a field of density fluctuations, notably star-forming clouds. When it is difficult to determine the correlation length from the empirical ACF, we show alternative ways to estimate the correlation length. We show that the statistics of the column-density field is hampered by biases introduced by integration effects along the line of sight and we explain how to reduce these biases. The statistics of the probability density function (PDF) ergodic estimator also yields the derivation of the proper statistical error bars. We provide a method that can be used by observers and numerical simulation specialists to determine the latter. We show that they (i) cannot be derived from simple Poisson statistics and (ii) become increasingly large for increasing density contrasts, severely hampering the accuracy of the low and high end part of the PDF because of a sample size that is too small. As templates of various stages of star formation in MCs, we then examine the case of the Polaris and Orion B clouds in detail. We calculate, from the observations, the ACF and the correlation length in these clouds and show that the latter is on the order of $\sim$1\% of the size of the cloud.

astro-ph.GA↗

arXiv2022

Statistical properties and correlation length in star-forming molecular clouds: II. Gravitational potential and virial parameter

In the first article of this series, we have used the ergodic theory to assess the validity of a statistical approach to characterize various properties of star-forming molecular clouds (MCs) from a limited number of observations or simulations. This allows the proper determination of confidence intervals for various volumetric averages of statistical quantities obtained form observations or numerical simulations. In this joint paper, we apply the same formalism to a different kind of (observational or numerical) study of MCs. Indeed, as observations cannot fully unravel the complexity of the inner density structure of star forming clouds, it is important to know whether global observable estimates, such as the total mass and size of the cloud, can give an accurate estimation of various key physical quantities that characterize the dynamics of the cloud. Of prime importance is the correct determination of the total gravitational (binding) energy and virial parameter of a cloud. We show that, whereas for clouds that are not in a too advanced stage of star formation, such as Polaris or Orion B, the knowledge of only their mass and size is sufficient to yield an accurate determination of the aforementioned quantities from observations (i.e. in real space). In contrast, we show that this is no longer true for numerical simulations in a periodic box. We derive a relationship for the ratio of the virial parameter in these two respective cases.

astro-ph.GA↗

arXiv2021

Generalised transport equation of the Autocovariance Function of the density field and mass invariant in star-forming clouds

In this Letter, we study the evolution of the autocovariance function (ACF) of density field fluctuations in star-forming clouds and thus of the correlation length $l_c(ρ)$ of these fluctuations, which can be identified as the average size of the most correlated structures within the cloud. Generalizing the transport equation derived by Chandrasekhar (1951) for static, homogeneous turbulence, we show that the mass contained within these structures is an invariant, i.e. that the average mass contained in the most correlated structures remains constant during the evolution of the cloud, whatever dominates the global dynamics (gravity or turbulence). We show that the growing impact of gravity on the turbulent flow yields an increase of the variance of the density fluctuations and thus a drastic decrease of the correlation length. Theoretical relations are successfully compared to numerical simulations. This picture brings a robust support to star formation paradigms where the mass concentration in turbulent star-forming clouds evolves from initially large, weakly correlated filamentary structures to smaller, denser more correlated ones, and eventually to small, tightly correlated prestellar cores. We stress that the present results rely on a pure statistical approach of density fluctuations and do not involve any specific condition for the formation of prestellar cores. Interestingly enough, we show that, under average conditions typical of Milky Way molecular clouds, this invariant average mass is about a solar mass, providing an appealing explanation for the apparent universality of the IMF under such environments.

astro-ph.GA↗

arXiv2021

A new equation of state for dense hydrogen-helium mixtures II: taking into account hydrogen-helium interactions

In a recent paper (Chabrier et al. 2019), we have derived a new equation of state (EOS) for dense hydrogen/helium mixtures which covers the temperature-density domain from solar-type stars to brown dwarfs and gaseous planets. This EOS is based on the so-called additive volume law and thus does not take into account the interactions between the hydrogen and helium species. In the present paper, we go beyond these calculations by taking into account H/He interactions, derived from quantum molecular dynamics simulations. These interactions, which eventually lead to H/He phase separation, become important at low temperature and high density, in the domain of brown dwarfs and giant planets. The tables of this new EOS are made publicly available.

astro-ph.SR↗

arXiv2020

Evolution of the density PDF in star forming clouds: the role of gravity

We derive an analytical theory of the PDF of density fluctuations in supersonic turbulence in the presence of gravity in star-forming clouds. The theory is based on a rigorous derivation of a combination of the Navier-Stokes continuity equations for the fluid motions and the Poisson equation for the gravity. It extends upon previous approaches first by including gravity, second by considering the PDF as a dynamical system, not a stationary one. We derive the transport equations of the density PDF, characterize its evolution and determine the density threshold above which gravity strongly affects and eventually dominates the dynamics of turbulence. We demonstrate the occurence of {\it two} power law tails in the PDF, with two characteristic exponents, corresponding to two different stages in the balance between turbulence and gravity. Another important result of this study is to provide a procedure to relate the observed {\it column density} PDFs to the corresponding {\it volume density} PDFs. This allows to infer, from the observation of column-densities, various physical parameters characterizing molecular clouds, notably the virial parameter. Furthermore, the theory offers the possibility to date the clouds in units of ${t}_{\rm coll}$, the time since a statistically significant fraction of the cloud started to collapse. The theoretical results and diagnostics reproduce very well numerical simulations and observations of star-forming clouds. The theory provides a sound theoretical foundation and quantitative diagnostics to analyze observations or numerical simulations of star-forming regions and to characterize the evolution of the density PDF in various regions of molecular clouds.

astro-ph.GA↗

arXiv2020

What is the role of stellar radiative feedback in setting the stellar mass spectrum?

In spite of decades of theoretical efforts, the physical origin of the stellar initial mass function (IMF) is still debated. Particularly crucial is the question of what sets the peak of the distribution. To investigate this issue we perform high resolution numerical simulations with radiative feedback exploring in particular the role of the stellar and accretion luminosities. We also perform simulations with a simple effective equation of state (eos) and we investigate 1000 solar mass clumps having respectively 0.1 and 0.4 pc of initial radii. We found that most runs, both with radiative transfer or an eos, present similar mass spectra with a peak broadly located around 0.3-0.5 M$_\odot$ and a powerlaw-like mass distribution at higher masses. However, when accretion luminosity is accounted for, the resulting mass spectrum of the most compact clump tends to be moderately top-heavy. The effect remains limited for the less compact one, which overall remains colder. Our results support the idea that rather than the radiative stellar feedback, this is the transition from the isothermal to the adiabatic regime, which occurs at a gas density of about 10$^{10}$ cm$^{-3}$, that is responsible for setting the peak of the initial mass function. This stems for the fact that $i)$ extremely compact clumps for which the accretion luminosity has a significant influence are very rare and $ii)$ because of the luminosity problem, which indicates that the effective accretion luminosity is likely weaker than expected.

astro-ph.GA↗

arXiv2020

Protostellar collapse: regulation of the angular momentum and onset of an ionic precursor

Through the magnetic braking and the launching of protostellar outflows, magnetic fields play a major role in the regulation of angular momentum in star formation, which directly impacts the formation and evolution of protoplanetary disks and binary systems. The aim of this paper is to quantify those phenomena in the presence of non-ideal magnetohydrodynamics effects, namely the Ohmic and ambipola r diffusion. We perform three-dimensional simulations of protostellar collapses varying the mass of the prestellar dense core, the thermal support (the $α$ ratio) and the dust grain size-distribu tion. The mass mostly influences the magnetic braking in the pseudo-disk, while the thermal support impacts the accretion rate and hence the properties of the disk. Removing the grains smaller than 0. 1 $μ$m in the Mathis, Rumpl, Nordsieck (MRN) distribution enhances the ambipolar diffusion coefficient. Similarly to previous studies, we find that this change in the distribution reduces the magnet ic braking with an impact on the disk. The outflow is also significantly weakened. In either case, the magnetic braking largely dominates the outflow as a process to remove the angular momentum from t he disk. Finally, we report a large ionic precursor to the outflow with velocities of several km s$^{-1}$, which may be observable.

astro-ph.SR↗

arXiv2020

A library of ATMO forward model transmission spectra for hot Jupiter exoplanets

We present a grid of forward model transmission spectra, adopting an isothermal temperature-pressure profile, alongside corresponding equilibrium chemical abundances for 117 observationally significant hot exoplanets (Equilibrium Temperatures of 547-2710 K). This model grid has been developed using a 1D radiative-convective-chemical equilibrium model termed ATMO, with up-to-date high temperature opacities. We present an interpretation of observations of ten exoplanets, including best fit parameters and $χ^{2}$ maps. In agreement with previous works, we find a continuum from clear to hazy/cloudy atmospheres for this sample of hot Jupiters. The data for all the 10 planets are consistent with sub-solar to solar C/O ratio, 0.005 to 10 times solar metallicity and water rather than a methane dominated infrared spectra. We then explore the range of simulated atmospheric spectra for different exoplanets, based on characteristics such as temperature, metallicity, C/O-ratio, haziness and cloudiness. We find a transition value for the metallicity between 10 and 50 times solar, which leads to substantial changes in the transmission spectra. We also find a transition value of C/O ratio, from water to carbon species dominated infrared spectra, as found by previous works, revealing a temperature dependence of this transition point ranging from $\sim$0.56 to $\sim$1-1.3 for equilibrium temperatures from $\sim$900 to $\sim$2600 K. We highlight the potential of the spectral features of HCN and C$_2$H$_2$ to constrain the metallicities and C/O ratios of planets, using JWST observations. Finally, our entire grid ($\sim$460,000 simulations) is publicly available and can be used directly with the JWST simulator PandExo for planning observations.

astro-ph.EP↗

arXiv2020

The Parallax of VHS J1256-1257 from CFHT and Pan-STARRS 1

We present new parallax measurements from the CFHT Infrared Parallax Program and the Pan-STARRS 3$π$ Steradian Survey for the young ($\approx150-300$ Myr) triple system VHS J125601.92$-$125723.9. This system is composed of a nearly equal-flux binary ("AB") and a wide, possibly planetary-mass companion ("b"). The system's published parallactic distance ($12.7\pm1.0$ pc) implies absolute magnitudes unusually faint compared to known young objects and is in tension with the spectrophotometric distance for the central binary ($17.2\pm2.6$ pc). Our CFHT and Pan-STARRS parallaxes are consistent, and the more precise CFHT result places VHS J1256-1257 at $22.2^{+1.1}_{-1.2}$ pc. Our new distance results in higher values for the companion's mass ($19\pm5$ M$_{\rm Jup}$) and temperature ($1240\pm50$ K), and also brings the absolute magnitudes of all three components into better agreement with known young objects.

astro-ph.SR↗