arXiv subjects

Philippe Thébault

Publications and source records attributed to Philippe Thébault.

At least 19 recordsLinked to original sources

arXiv2026

Dust Growth in Binary Systems: Inhibition of dust settling and growth in circumbinary discs

Stellar multiplicity alters the density structure of protoplanetary discs and thereby the initial conditions for planet formation. Yet, the interplay between companion-disc interactions and dust growth remains poorly understood. The goal of this work is to investigate to what extent the density structure of a disc undergoing tidal interactions with a companion star promotes or inhibits the growth of dust grains. We perform a set of hydrodynamical simulations of protoplanetary discs orbiting one or both stars of a binary, including dust growth and fragmentation. We explore a range of companion orbits and compare the results with a single-star reference case. We find that dust growth is mainly driven by local accumulations of dust. In circumbinary discs, the maximum grain size is up to five times smaller than in isolated discs. This result likely originates from the perturbations caused by the inner binary, which prevent dust grains from properly settling and drifting. As a consequence, the conditions required to trigger strong clumping driven by the streaming instability are difficult to achieve. In contrast, circumstellar discs in binary systems exhibit grain sizes similar to those in isolated discs, leading to comparable conditions for strong clumping by the streaming instability. Planet formation through core accretion seems challenging in circumbinary discs harbouring binaries larger than a few au, suggesting that circumbinary planets observed near the dynamical stability limit did not form in situ. Conversely, perturbations from external companions only marginally affect density-driven dust growth compared to isolated systems.

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arXiv2025

The vertical structure of debris discs and the role of disc gravity: A primer using a simplified model

Debris discs provide valuable insights into the formation and evolution of exoplanetary systems. Their structures are commonly attributed to planetary perturbations, serving as probes of as-yet-undetected planets. However, most studies of planet-debris disc interactions ignore the disc's gravity, treating it as a collection of massless planetesimals. Here, using an analytical model, we investigate how the vertical structure of a back-reacting debris disc responds to secular perturbations from an inner, inclined planet. Considering the disc's axisymmetric potential, we identify two dynamical regimes: planet-dominated and disc-dominated, which may coexist, separated by a secular-inclination resonance. In the planet-dominated regime ($M_d/m_p\ll1$), we recover the classical result: a transient warp propagates outward until the disc settles into a box-like structure centered around the planetary orbit's initial inclination $I_p(0)$, with a distance-independent aspect ratio $\mathcal{H}(R)\approx I_p(0)$. In contrast, in the disc-dominated regime ($M_d/m_p\gtrsim1$), the disc exhibits dynamical rigidity, remaining thin and misaligned, with significantly suppressed inclinations and a sharply declining aspect ratio, $\mathcal{H}(R)\propto I_p(0)R^{-7/2}$. In the intermediate regime ($M_d/m_p\lesssim1$), the system exhibits a secular-inclination resonance, leading to long-lived, warp-like structures and a bimodal inclination distribution, containing both dynamically hot and cold populations. We provide analytic formulae describing these effects as a function of system parameters. We also find that the vertical density profile is intrinsically non-Gaussian and recommend fitting observations with non-zero slopes of $\mathcal{H}(R)$. Our results may be used to infer planetary parameters and debris disc masses based on observed warps and scale heights, as demonstrated for HD 110058 and $β$ Pic.

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arXiv2024

An impact-free mechanism to deliver water to terrestrial planets and exoplanets

To date, the most widespread scenario is that the Earth originated without water and was brought to the planet mainly due to impacts by wet asteroids coming from further out in space. However, many uncertainties remain regarding the exact processes that supply water to inner terrestrial planets. This article explores a new mechanism that would allow water to be efficiently transported to planets without impacts. We propose that primordial asteroids were icy and that when the ice sublimated, it formed a gaseous disk that could then reach planets and deliver water. We have developed a new model that follows the sublimation of asteroids and evolves the subsequent gas disk using a viscous diffusion code. We can then quantify the amount of water that can be accreted onto each planet in a self-consistent manner. We find that this new disk-delivery mechanism can explain the water content on Earth as well as on other planets. Our model shows most of the water being delivered between 20 and 30 Myr after the birth of the Sun. Our scenario implies the presence of a gaseous water disk with substantial mass for 100s Myr, which could be one of the key tracers of this mechanism. We show that such a watery disk could be detected in young exo-asteroid belts with ALMA. We propose that viscous water transport is inevitable and more generic than the impact scenario. We also suggest it is a universal process that may also occur in extrasolar systems. The conditions required for this scenario to unfold are indeed expected to be present in most planetary systems: an opaque proto-planetary disk that is initially cold enough for ice to form in the exo-asteroid belt region, followed by a natural outward-moving snow line that allows this initial ice to sublimate after the dissipation of the primordial disk, creating a viscous secondary gas disk and leading to the accretion of water onto the exoplanets.

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arXiv2024

The near-infrared degree of polarization in debris disks. Toward a self-consistent approach to model scattered light observations

Debris disks give us the unique opportunity to probe the properties of small $μ$m-sized particles, allowing us to peer into the constituents of their parent bodies, young analogs of comets and asteroids of our solar system. In the past, studies of the total intensity phase function have proven powerful to constrain the main characteristics of the dust particles in debris disks. Nonetheless, there can remain some degeneracies in the modeling that can be alleviated when considering polarized intensity observations. We obtained new near-IR scattered light observations of four young debris disks which allow us to constrain the degree of linear polarization as a function of the scattering angle. All four debris disks are detected in polarized intensity, and three are also recovered in total intensity. We measured peak degree of polarization of $\lesssim 40$\% for all three disks. We find that the particles must consist of highly refractive and absorbing material. For HD129590, by measuring the polarization fraction beyond the birth ring, we constrain the width of the size distribution to be smaller and smaller, compatible with the effect of radiation pressure. We put these findings to the test and present a self-consistent approach to produce synthetic images, assuming different profiles for the radiation pressure strength, and accounting for the presence of unbound grains. We find the contribution of these grains to be especially critical to reproduce the increasing degree of polarization with stellocentric distances. Some of our results might seem difficult to reconcile with our understanding of cosmic dust but since similar results have been obtained for other disks, we discuss the current limitation of available light scattering models as well as possible avenues to alleviate these unfortunate limitations.

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arXiv2023

The polarisation properties of the HD 181327 debris ring. Evidence for sub-micron particles from scattered light observations

Polarisation is a powerful remote-sensing tool to study the nature of particles scattering the starlight. It is widely used to characterise interplanetary dust particles in the Solar System and increasingly employed to investigate extrasolar dust in debris discs' systems. We aim to measure the scattering properties of the dust from the debris ring around HD 181327 at near-infrared wavelengths. We obtained high-contrast polarimetric images of HD 181327 in the H band with the SPHERE / IRDIS instrument on the Very Large Telescope (ESO). We complemented them with archival data from HST / NICMOS in the F110W filter reprocessed in the context of the Archival Legacy Investigations of Circumstellar Environments (ALICE) project. We developed a combined forward-modelling framework to simultaneously retrieve the scattering phase function in polarisation and intensity. We detected the debris disc around HD 181327 in polarised light and total intensity. We measured the scattering phase function and the degree of linear polarisation of the dust at 1.6 micron in the birth ring. The maximum polarisation is 23.6% +/- 2.6% and occurs between a scattering angle of 70 deg and 82 deg. We show that compact spherical particles made of a highly refractive and relatively absorbing material in a differential power-law size distribution of exponent $-3.5$ can simultaneously reproduce the polarimetric and total intensity scattering properties of the dust. This type of material cannot be obtained with a mixture of silicates, amorphous carbon, water ice, and porosity, and requires a more refracting component such as iron-bearing minerals. We reveal a striking analogy between the near-infrared polarisation of comets and that of HD 181327. The methodology developed here combining VLT/SPHERE and HST/NICMOS may be applicable in the future to combine the polarimetric capabilities of SPHERE with the sensitivity of JWST.

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arXiv2023

Apocenter pile-up and arcs: a narrow dust ring around HD 129590

Observations of debris disks have significantly improved over the past decades, both in terms of sensitivity and spatial resolution. At near-infrared wavelengths, new observing strategies and post-processing algorithms allow us to drastically improve the final images, revealing faint structures in the disks. These structures inform us about the properties and spatial distribution of the small dust particles. We present new $H$-band observations of the disk around HD 129590, which display an intriguing arc-like structure in total intensity but not in polarimetry, and propose an explanation for the origin of this arc. Assuming geometric parameters for the birth ring of planetesimals, our model provides the positions of millions of particles of different sizes to compute scattered light images. We demonstrate that if the grain size distribution is truncated or strongly peaks at a size larger than the radiation pressure blow-out size we are able to produce an arc quite similar to the observed one. If the birth ring is radially narrow, given that particles of a given size have similar eccentricities, they will have their apocenters at the same distance from the star. Since this is where the particles will spend most of their time, this results in a "apocenter pile-up" that can look like a ring. Due to more efficient forward scattering this arc only appears in total intensity observations and remains undetected in polarimetric data. This scenario requires sharp variations either in the grain size distribution or for the scattering efficiencies $Q_\mathrm{sca}$. Alternative possibilities such as a wavy size distribution and a size-dependent phase function are interesting candidates to strengthen the apocenter pile-up. We also discuss why such arcs are not commonly detected in other systems, which can mainly be explained by the fact that most parent belts are usually broad.

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arXiv2023

Morphology of the gas-rich debris disk around HD 121617 with SPHERE observations in polarized light

Debris disks are the signposts of collisionally eroding planetesimal circumstellar belts, whose study can put important constraints on the structure of extrasolar planetary systems. The best constraints on the morphology of disks are often obtained from spatially resolved observations in scattered light. Here, we investigate the young (~16 Myr) bright gas-rich debris disk around HD121617. We use new scattered-light observations with VLT/SPHERE to characterize the morphology and the dust properties of this disk. From these properties we can then derive constraints on the physical and dynamical environment of this system, for which significant amounts of gas have been detected. The disk morphology is constrained by linear-polarimetric observations in the J band. Based on our modeling results and archival photometry, we also model the SED to put constraints on the total dust mass and the dust size distribution. We explore different scenarios that could explain these new constraints. We present the first resolved image in scattered light of the debris disk HD121617. We fit the morphology of the disk, finding a semi-major axis of 78.3$\pm$0.2 au, an inclination of 43.1$\pm$0.2° and a position angle of the major axis with respect to north, of 239.8$\pm$0.3°, compatible with the previous continuum and CO detection with ALMA. Our analysis shows that the disk has a very sharp inner edge, possibly sculpted by a yet-undetected planet or gas drag. While less sharp, its outer edge is steeper than expected for unperturbed disks, which could also be due to a planet or gas drag, but future observations probing the system farther from the main belt would help explore this further. The SED analysis leads to a dust mass of 0.21$\pm$0.02 M$_{\oplus}$ and a minimum grain size of 0.87$\pm$0.12 $μ$m, smaller than the blowout size by radiation pressure, which is not unexpected for very bright col...

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arXiv2022

Stellar winds can affect gas dynamics in debris disks and create observable belt winds

Context: Gas is now detected in many extrasolar systems around mature stars aged between 10 Myr to $\sim$ 1 Gyr with planetesimal belts. Gas in these mature disks is thought to be released from planetesimals and has been modelled using a viscous disk approach. At low densities, this may not be a good assumption as the gas could be blown out by the stellar wind instead. Methods: We developed an analytical model for A to M stars that can follow the evolution of gas outflows and target when the transition occurs between a disk or a wind. The crucial criterion is the gas density for which gas particles stop being protected from stellar wind protons impacting at high velocities on radial trajectories. Results: We find that: 1) Belts of radial width $ΔR$ with gas densities $< 7 \, (ΔR/50 {\rm \, au})^{-1}$ cm$^{-3}$ would create a wind rather than a disk, which would explain the recent outflowing gas detection in NO Lup. 2) The properties of this belt wind can be used to measure stellar wind properties such as their densities and velocities. 3) Debris disks with low fractional luminosities $f$ are more likely to create gas winds, which could be observed with current facilities. Conclusions: The systems containing low gas masses such as Fomalhaut or TWA 7 or more generally, debris disks with fractional luminosities $f \lesssim 10^{-5} (L_\star/L_\odot)^{-0.37} $ or stellar luminosity $\gtrsim 20 \, L_\odot$ (A0V or earlier) would rather create gas outflows (or belt winds) than gas disks. Gas observed to be outflowing at high velocity in the young system NO Lup could be an example of such belt winds. The detection of these gas winds is possible with ALMA (CO and CO$^+$ could be good wind tracers) and would allow us to constrain the stellar wind properties of main-sequence stars, which are otherwise difficult to measure.

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arXiv2022

The halo around HD 32297: $μ$m-sized cometary dust

The optical properties of the second generation dust that we observe in debris disks remain quite elusive, whether it is the absorption efficiencies at millimeter wavelengths or the (un)polarized phase function at near-infrared wavelengths. Thankfully the same particles are experiencing forces that are size dependent (e.g., radiation pressure), and with high angular resolution observations we can take advantage of this natural spatial segregation. Observations at different wavelengths probe different ranges of sizes, and there is therefore a great synergy in multi-wavelength observations to better constrain the optical properties of the particles. We present a new approach to simultaneously model SPHERE and ALMA observations and apply it to the debris disk around HD\,32297, putting the emphasis on the spatial distribution of the grains with different $β$ values. This modeling approach requires few assumptions on the actual sizes of the particles and the interpretation can therefore be done a posteriori. We find that the ALMA observations are best reproduced with a combination of small and large $β$ values ($0.03$ and $0.42$) while the SPHERE observations require several intervals of $β$ values. We discuss the nature of the halo previously reported in ALMA observations, and hypothesize it could be caused by over-abundant $μ$m-sized particles (the over-abundance being the consequence of their extended lifetime). We model the polarized phase function at near-infrared wavelengths and fluffy aggregates larger than a few $μ$m provide the best solution. Comparing our results with comets of the solar system, we postulate that the particles released in the disk originate from rather pristine cometary bodies (to avoid compaction of the fluffy aggregates) and are then set on highly eccentric orbits, which could explain the halo detected at long wavelengths.

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arXiv2022

The vertical structure of debris disks and the impact of gas

The vertical structure of debris disks provides clues about their dynamical evolution and the collision rate of the unseen planetesimals. Thanks to the ever-increasing angular resolution of contemporary instruments and facilities, we are beginning to constrain the scale height of a handful of debris disks, either at near-infrared or millimeter wavelengths. Nonetheless, this is often done for individual targets only. We present here the geometric modeling of eight disks close to edge-on, all observed with the same instrument (SPHERE) and using the same mode (dual-beam polarimetric imaging). Motivated by the presence of CO gas in two out of the eight disks, we then investigate the impact that gas can have on the scale height by performing N-body simulations including gas drag and collisions. We show that gas can quickly alter the dynamics of particles (both in the radial and vertical directions), otherwise governed by gravity and radiation pressure. We find that, in the presence of gas, particles smaller than a few tens of microns can efficiently settle toward the midplane at the same time as they migrate outward beyond the birth ring. For second generation gas ($M_\mathrm{gas} \leq 0.1$ $M_\oplus$), the vertical settling should be best observed in scattered light images compared to observations at millimeter wavelengths. But if the gas has a primordial origin ($M_\mathrm{gas} \geq 1$ $M_\oplus$), the disk will appear very flat both at near-infrared and sub-mm wavelengths. Finally, far beyond the birth ring, our results suggest that the surface brightness profile can be as shallow as $\sim -2.25$.

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arXiv2021

Faint objects in motion: the new frontier of high precision astrometry

Sky survey telescopes and powerful targeted telescopes play complementary roles in astronomy. In order to investigate the nature and characteristics of the motions of very faint objects, a flexibly-pointed instrument capable of high astrometric accuracy is an ideal complement to current astrometric surveys and a unique tool for precision astrophysics. Such a space-based mission will push the frontier of precision astrometry from evidence of Earth-mass habitable worlds around the nearest stars, to distant Milky Way objects, and out to the Local Group of galaxies. As we enter the era of the James Webb Space Telescope and the new ground-based, adaptive-optics-enabled giant telescopes, by obtaining these high precision measurements on key objects that Gaia could not reach, a mission that focuses on high precision astrometry science can consolidate our theoretical understanding of the local Universe, enable extrapolation of physical processes to remote redshifts, and derive a much more consistent picture of cosmological evolution and the likely fate of our cosmos. Already several missions have been proposed to address the science case of faint objects in motion using high precision astrometry missions: NEAT proposed for the ESA M3 opportunity, micro-NEAT for the S1 opportunity, and Theia for the M4 and M5 opportunities. Additional new mission configurations adapted with technological innovations could be envisioned to pursue accurate measurements of these extremely small motions. The goal of this White Paper is to address the fundamental science questions that are at stake when we focus on the motions of faint sky objects and to briefly review instrumentation and mission profiles.

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arXiv2021

Revealing asymmetrical dust distribution in the inner regions of HD 141569

We obtained polarimetric differential imaging of a gas-rich debris disk around HD 141569A with SPHERE in the H-band to compare the scattering properties of the innermost ring at 44 au with former observations in total intensity with the same instrument. In polarimetric imaging, we observed that the intensity of the ring peaks in the south-east, mostly in the forward direction, whereas in total intensity imaging, the ring is detected only at the south. This noticeable characteristic suggests a non-uniform dust density in the ring. We implemented a density function varying azimuthally along the ring and generated synthetic images both in polarimetry and in total intensity, which are then compared to the actual data. We find that the dust density peaks in the south-west at an azimuthal angle of $220^{\circ} \sim 238^{\circ}$ with a rather broad width of $61^{\circ} \sim 127^{\circ}$. Although there are still uncertainties that remain in the determination of the anisotropic scattering factor, the implementation of an azimuthal density variation to fit the data proved to be robust. Upon elaborating on the origin of this dust density distribution, we conclude that it could be the result of a massive collision when we account for the effect of the high gas mass that is present in the system on the dynamics of grains. Using the outcome of this modelization, we further measured the polarized scattering phase function for the observed scattering angle between 33$^{\circ}$ and 147$^{\circ}$ as well as the spectral reflectance of the southern part of the ring between 0.98 $μ$m and 2.1 $μ$m. We tentatively derived the grain properties by comparing these quantities with MCFOST models and assuming Mie scattering. Our preliminary interpretation indicates a mixture of porous sub-micron sized astro-silicate and carbonaceous grains.

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arXiv2019

A multi-wavelength study of the debris disc around 49 Cet

In a multi-wavelength study of thermal emission and scattered light images we analyse the dust properties and structure of the debris disc around the A1-type main sequence star 49~Cet. As a basis for this study, we present new scattered light images of the debris disc known to possess both a high amount of dust and gas. The outer region of the disc is revealed in former coronagraphic H-band and our new Y-band images from the Very Large Telescope SPHERE instrument. We use the knowledge of the disc's radial extent inferred from ALMA observations and the grain size distribution found by SED fitting to generate semi-dynamical dust models of the disc. We compare the models to scattered light and thermal emission data and find that a disc with a maximum of the surface density at 110~au and shallow edges can describe both thermal emission and scattered light observations. This suggests that grains close to the blow-out limit and large grains stem from the same planetesimal population and are mainly influenced by radiation pressure. The influence of inwards transport processes could not be analysed in this study.

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arXiv2019

Hot exozodiacal dust: an exocometary origin?

We aim to explore two exozodiacal dust production mechanisms, first re-investigating the Poynting-Robertson drag pile-up scenario, and then elaborating on the less explored, but promising exocometary dust delivery scenario. We developped a new versatile, numerical model that calculates the dust dynamics, with non orbit-averaged equations for the grains close to the star. The model includes dust sublimation and incorporates a radiative transfer code for direct comparison to the observations. We consider in this study four stellar types, three dust compositions, and we assume a parent belt at 50 au. We find that, in the case of the Poynting-Robertson drag pile-up scenario, it is impossible to produce long-lived submicron-sized grains close to the star. The inward drifting grains fill in the region between the parent belt and the sublimation distance, producing an unrealistically strong mid-infrared excess compared to the near-infrared excess. The dust pile-up at the sublimation radius is by far insufficient to boost the near-IR flux of the exozodi to the point where it dominates over the mid-infrared excess. In the case of the exocometary dust delivery scenario, we find that a narrow ring can form close to the sublimation zone, populated with large grains several tens to several hundred of micrometers in radius. Although not perfect, this scenario provides a better match to the observations, especially if the grains are carbon-rich. We also find that the required number of active exocomets to sustain the observed dust level is reasonable. We conclude that the hot exozodiacal dust detected by near-infrared interferometry is unlikely to result from inwards grains migration by Poynting-Robertson drag from a distant parent belt, but could instead have an exocometary origin. [Abridged]

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arXiv2018

Searching for Biosignatures in Exoplanetary Impact Ejecta

With the number of confirmed rocky exoplanets increasing steadily, their characterisation and the search for exoplanetary biospheres is becoming an increasingly urgent issue in astrobiology. We aim to investigate the possibility of characterising an exoplanet (in terms of habitability, geology, presence of life etc.) by studying material ejected from the surface during an impact event. For given parameters characterising the impact event, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica or ejected microorganisms could be detected. For a 20 km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust. The collisional evolution is best modelled by considering two independent dust populations, a spalled population consisting of non-melted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapour evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.

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arXiv2017

Expelled grains from an unseen parent body around AU Mic

Recent observations of the debris disk of AU Mic have revealed asymmetric, fast outward-moving arch-like structures above the disk midplane. No model can readily explain the characteristics of these features. We present a model aiming to reproduce the dynamics of these structures, more specifically their high projected speeds and their apparent position. We test the hypothesis of dust emitted by a point source and then expelled from the system by the strong stellar wind of this young, M-type star. In this model, we make the assumption that the dust grains follow the same dynamics as the structures. We perform numerical simulations of test particle trajectories to explore the available parameter space, in particular the radial location $R_{0}$ of the dust producing parent body and the size of the dust grains as parameterized by $β$ (ratio of stellar wind and radiation pressure forces over gravitation). We consider both the case of a static and an orbiting parent body. We find that, for all considered scenarii, there is always a set of ($R_0, β$) parameters able to fit the observed features. The common characteristics of these solutions is that they all require a high value of $β$, of around 6. This means that the star is probably very active and the grains composing the structures are sub-micronic, in order to reach such high $β$ values. As for the location of the hypothetical parent body, we constrain it to lie around 8 au (orbiting case) or 28 au (static case). We show that the scenario of sequential dust releases by an unseen, punctual parent body is able to explain the radial behaviour of the observed structures. We predict the evolution of the structures to help future observations to discriminate between the different parent body configurations that have been considered. We expect new structures to appear on the northwest side of the disk in the coming years.

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arXiv2013

LIDT-DD: A new self-consistent debris disc model including radiation pressure and coupling collisional and dynamical evolution

In most current debris disc models, the dynamical and the collisional evolutions are studied separately, with N-body and statistical codes, respectively, because of stringent computational constraints. We present here LIDT-DD, the first code able to mix both approaches in a fully self-consistent way. Our aim is for it to be generic enough so as to be applied to any astrophysical cases where we expect dynamics and collisions to be deeply interlocked with one another: planets in discs, violent massive breakups, destabilized planetesimal belts, exozodiacal discs, etc. The code takes its basic architecture from the LIDT3D algorithm developed by Charnoz et al.(2012) for protoplanetary discs, but has been strongly modified and updated in order to handle the very constraining specificities of debris discs physics: high-velocity fragmenting collisions, radiation-pressure affected orbits, absence of gas, etc. In LIDT-DD, grains of a given size at a given location in a disc are grouped into "super-particles", whose orbits are evolved with an N-body code and whose mutual collisions are individually tracked and treated using a particle-in-a-box prescription. To cope with the wide range of possible dynamics, tracers are sorted and regrouped into dynamical families depending on their orbits. The code retrieves the classical features known for debris discs, such as the particle size distributions in unperturbed discs, the outer radial density profiles (slope in -1.5) outside narrow collisionally active rings, and the depletion of small grains in "dynamically cold" discs. The potential of the new code is illustrated with the test case of the violent breakup of a massive planetesimal within a debris disc. The main potential future applications of the code are planet/disc interactions, and more generally any configurations where dynamics and collisions are expected to be intricately connected.

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arXiv2009

Can gas in young debris disks be constrained by their radial brightness profiles?

Disks around young stars are known to evolve from optically thick, gas-dominated protoplanetary disks to optically thin, almost gas-free debris disks. It is thought that the primordial gas is largely removed at ages of ~10 Myr, but it is difficult to discern the true gas densities from gas observations. This suggests using observations of dust: it has been argued that gas, if present with higher densities, would lead to flatter radial profiles of the dust density and surface brightness than those actually observed. However, here we show that these profiles are surprisingly insensitive to variation of the parameters of a central star, location of the dust-producing planetesimal belt, dustiness of the disk and - most importantly - the parameters of the ambient gas. This result holds for a wide range of gas densities (three orders of magnitude), for different radial distributions of the gas temperature, and different gas compositions. The brightness profile slopes of -3...-4 we find are the same that were theoretically found for gas-free debris disks, and they are the same as actually retrieved from observations of many debris disks. Our specific results for three young (10-30 Myr old), spatially resolved, edge-on debris disks (beta Pic, HD 32297, and AU Mic) show that the observed radial profiles of the surface brightness do not pose any stringent constraints on the gas component of the disk. We cannot exclude that outer parts of the systems may have retained substantial amounts of primordial gas which is not evident in the gas observations (e.g. as much as 50 Earth masses for beta Pic). However, the possibility that gas, most likely secondary, is only present in little to moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in the beta Pic disk), remains open, too.

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