Extrasolar comets : the origin of dust in exozodiacal disks?

Comets have been invoked in numerous studies as a potentially important source of dust and gas around stars, but none has studied the thermo-physical evolution, out-gassing rate, and dust ejection of these objects in such stellar systems. We investigate the thermo-physical evolution of comets in exo-planetary systems in order to provide valuable theoretical data required to interpret observations of gas and dust. We use a quasi 3D model of cometary nucleus to study the thermo-physical evolution of comets evolving around a single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to 70 solar luminosities. This paper provides mass ejection, lifetimes, and the rate of dust and water gas mass productions for comets as a function of the distance to the star and stellar luminosity. Results show significant physical changes to comets at high stellar luminosities. The models are presented in such a manner that they can be readily applied to any planetary system. By considering the examples of the Solar System, Vega and HD 69830, we show that dust grains released from sublimating comets have the potential to create the observed (exo)zodiacal emission. We show that observations can be reproduced by 1 to 2 massive comets or by a large number of comets whose orbits approach close to the star. Our conclusions depend on the stellar luminosity and the uncertain lifetime of the dust grains. We find, as in previous studies, that exozodiacal dust disks can only survive if replenished by a population of typically sized comets renewed from a large and cold reservoir of cometary bodies beyond the water ice line. These comets could reach the inner regions of the planetary system following scattering by a (giant) planet.Comment: 21 pages, 10 figure

Elemental ratios in stars vs planets

Context. The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same. Aims. Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O. Methods. A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances. Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.Comment: 8 pages, 5 figures, 3 tables. Accepted for publication in A&

Expectations for the Deep Impact collision from cometary nuclei modelling

Using the cometary nucleus model developed by Espinasse et al. (1991), we calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of 360 years. Starting from an initially amorphous cometary nucleus which incorporates an icy mixture of H2O and CO, we show that, at the time of Deep Impact collision, the crater is expected to form at depths where ice is in its crystalline form. Hence, the subsurface exposed to space should not be primordial. We also attempt an order-of-magnitude estimate of the heating and material ablation effects on the crater activity caused by the 370 Kg projectile released by the DI spacecraft. We thus show that heating effects play no role in the evolution of crater activity. We calculate that the CO production rate from the impacted region should be about 300-400 times higher from the crater resulting from the impact with a 35 m ablation than over the unperturbed nucleus in the immediate post-impact period. We also show that the H2O production rate is decreased by several orders of magnitude at the crater base just after ablation

From stellar nebula to planetesimals

Solar and extrasolar comets and extrasolar planets are the subject of numerous studies in order to determine their chemical composition and internal structure. In the case of planetesimals, their compositions are important as they govern in part the composition of future planets. The present works aims at determining the chemical composition of icy planetesimals, believed to be similar to present day comets, formed in stellar systems of solar chemical composition. The main objective of this work is to provide valuable theoretical data on chemical composition for models of planetesimals and comets, and models of planet formation and evolution. We have developed a model that calculates the composition of ices formed during the cooling of the stellar nebula. Coupled with a model of refractory element formation, it allows us to determine the chemical composition and mass ratio of ices to rocks in icy planetesimals throughout in the protoplanetary disc. We provide relationships for ice line positions (for different volatile species) in the disc, and chemical compositions and mass ratios of ice relative to rock for icy planetesimals in stellar systems of solar chemical composition. From an initial homogeneous composition of the nebula, a wide variety of chemical compositions of planetesimals were produced as a function of the mass of the disc and distance to the star. Ices incorporated in planetesimals are mainly composed of H2O, CO, CO2, CH3OH, and NH3. The ice/rock mass ratio is equal to 1+-0.5 in icy planetesimals following assumptions. This last value is in good agreement with observations of solar system comets, but remains lower than usual assumptions made in planet formation models, taking this ratio to be of 2-3

From stellar nebula to planets: the refractory components

We computed the abundance of refractory elements in planetary bodies formed in stellar systems with solar chemical composition by combining models for chemical composition and planet formation. We also consider the formation of refractory organic compounds, which have been ignored in previous studies on this topic. We used the commercial software package HSC Chemistry in order to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e. organic compounds are formed in the gas phase and are nonreactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets, with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides; pyroxene and olivine in similar quantities are formed. The model predicts planets that are similar in composition to those of the Solar system. It also shows that, starting from a common initial nebula composition, a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.Comment: 18 pages, 29 figures. Accepted for publication in A&

Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere

We describe a scenario of Titan's formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titan's atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding 50 K during their migration within Saturn's subnebula. The observed deficiencies of Titan's atmosphere in krypton and xenon could result from other processes that may have occurred both prior or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH3+ complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellite's surface or in its atmospheric haze.Comment: Accepted for publication in The Astrophysical Journa

Évolution physico-chimique des objets transneptuniens

TransNeptunians Objects (TNOs) and short period comets are often considered as direct relies from the primordial nebula from which the Solar System originated. Their dynamical and collisional history seems to imply that these objects may have differentiated. The goal of the present work is to determine whether the ``primordial'' matter included in the TNOs has lost its pristine characteristics due to physico-chemical transformations induced by repeated collisions. We first analyse the formation conditions and the structure of ices included in planetesimals. We show that the initial chemical composition of the gaz in the protoplanetary disk strongly influences the composition of planetesimals made of various crystalline ices, as well as the thermal evolution of the final objects (extrasolar low-mass icy planets, giant planets, satellites, comets, ...) in the outer reach of the protoplanetary disks. For instance, low-mass planets formed in a cold environment may be ``cold'' or ``warm'' ocean-plantes or carbonaceous planets.We next setup an evolutionary numerical model of planetesimals with simplified physico-chemical composition which ensure proper conservation of physical quantities (mass and energy), thus allowing long term study of transneptunians planetesimals. This model describes a porous matrix made of refractory elements and a mix of various ices. Phase transitions and energy and mass fluxes are accounted for. The choice of the mathematical framework for solving the conservation equations as well as the integration numerical scheme is discussed next. We consider the actual (non-)conservation of energy and mass in the numerical integration and compare our new model to previously published works. We show that the use of the finite volume method allows us to reduce the error by an order of magnitude compared to previous works.Improvements on planetesimal's modelling allows for more reliable long term studies of the differenciation and thus the study of collisional evolution. Thanks to this, we can analyse the effects of initial the physical and chemical composition, as well as other physical parameters, on the thermal, physical and chemical evolution of transneptunian planetesimals. Finally, we determne the collision frequency required to generate a deep phase transition in planetesimals, starting from a given chemical composition. Our results indicate that TNOs are unlikely to be significantly thermodynamically affected by collisions.Les Objets Transneptuniens (OTN) et les comètes à courte période sont considérés comme les vestiges directs de la nébuleuse primitive qui a donné naissance à notre système solaire. L'histoire dynamique et collisionnelle de ces objets pourrait laisser penser qu'ils ont été différenciés physico-chimiquement. L'objet du travail présenté ici est de déterminer si la matière dite ``primordiale'', incorporée dans les OTN, a perdu, d'une certaine manière, la mémoire de ses origines, en subissant des transformations physico-chimiques profondes lors de collisions successives.Nous analysons dans une première étape les conditions de formation et la structure des glaces incorporées dans les planétésimaux. Nous montrons que la composition chimique initiale de la phase gazeuse du disque protoplanétaire a une incidence non négligeable sur la composition des planétésimaux formés de diverses glaces cristallines, et sur l'évolution thermique de l'ensemble des objets formés (planètes extrasolaires de faible masse et glacées, planètes géantes, satellites, comètes, ...) dans la région externe des disques protoplanétaires. Ainsi, les planètes extrasolaires de faible masse formées à l'origine dans un environnement froid peuvent être du type planètes ``océan'' ou ``carbonées''. Nous réalisons ensuite un modèle numérique de planétésimal, à la composition physico-chimique simplifiée, qui assure la conservation des quantités physiques (masse et énergie), et permet l'étude à long terme des planétésimaux dans la région transneptunienne. Ce modèle représente une matrice poreuse composée d'éléments réfractaires et d'un mélange de différentes glaces. L'ensemble des processus physiques tels que les changements de phase (sublimation/condensation, cristallisation) et la modélisation des transferts thermiques et de masse y sont pris en compte. Le choix de la méthode d'intégration numérique et du cadre mathématique de résolution des équations de conservation (masse et énergie) est ensuite discuté. Le problème de la conservation des quantités physiques (masse et énergie) est abordé et l'erreur sur la conservation de la masse obtenue avec notre modèle est comparée à celle obtenue avec des modèles antérieurs. Nous montrons que l'erreur sur le bilan de masse obtenu par la méthode des volumes finis, utilisée dans ce modèle, permet de gagner au moins un ordre de grandeur sur celui des modèles antérieurs.Les améliorations apportées au modèle de planétésimal permettent d'obtenir une représentation de sa différenciation physico-chimique plus fiable sur le long terme et permettent l'étude de l'influence de collisions successives. Grâce à cela, nous pouvons analyser l'influence de la composition physico-chimique et celle de l'ensemble des paramètres physiques sur l'évolution thermique et physico-chimique de planétésimaux situés dans la région transneptunienne.Enfin, nous déterminons les laps de temps nécessaires entre deux collisions pour engendrer une évolution physico-chimique des objets cibles, à partir d'une composition originelle imposée. Nos résultats infirment l'hypothèse selon laquelle les Objets Transneptuniens pourraient être significativement affectés par le processus collisionnel

How to link the relative abundances of gas species in coma of comets to their initial chemical composition?

International audienceComets are expected to be the most primitive objects in the Solar System. The chemical composition of these objects is frequently assumed to be directly provided by the observations of the abundances of volatile molecules in the coma. The present work aims to determine the relationship between the chemical composition of the coma, the outgassing profile of volatile molecules and the internal chemical composition, and water ice structure of the nucleus, and physical assumptions on comets. To do this, we have developed a quasi 3D model of a cometary nucleus which takes into account all phase changes and water ice structures (amorphous, crystalline, clathrate, and a mixture of them); we have applied this model to the Comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission. We find that the outgassing profile of volatile molecules is a strong indicator of the physical and thermal properties (water ice structure, thermal inertia, abundances, distribution, physical differentiation) of the solid nucleus. Day/night variations of the rate of production of species helps to distinguish the clathrate structure from other water ice structures in nuclei, implying different thermodynamic conditions of cometary ice formation in the protoplanetary disc. The relative abundance (to H2O) of volatile molecules released from the nucleus interior varies by some orders of magnitude as a function of the distance to the Sun, the volatility of species, their abundance and distribution between the "trapped" and "condensed" states, the structure of water ice, and the thermal inertia and other physical assumptions (dust mantle, …) on the nucleus. For the less volatile molecules such as CO2 and H2S, the relative (to H2O) abundance of species in coma remain similar to the primitive composition of the nucleus (relative deviation less than 25%) only around the perihelion passage (in the range -3 to -2 to +2-3 AU), whatever is the water ice structure and chemical composition, and under the conditions that the nucleus is not fully covered by a dust mantle. The relative (to H2O) abundance of highly volatile molecules such as CO and CH4 in the coma remain approximately equal to the primitive nucleus composition only for nuclei made of clathrates. The nucleus releases systematically lower relative abundances of highly volatile species (up to one order of magnitude) around perihelion (in the range -3 to -2 to +2-3 AU) in the cases of the crystalline and amorphous water ice structures in the nuclei. The rate of production, the outgassing profile and the relative abundances (to H2O) of volatile molecules are the key parameters allowing one to retrieve the chemical composition and thermodynamic conditions of cometary ice formation in the early Solar System. The coming observations of the coma and nucleus by the Rosetta mission instruments (VIRTIS, MIRO, …) should provide the necessary constraints to the model to allow it to infer the primordial ice structure and composition of the comet

Radial drift and concurrent ablation of boulder-sized objects

Context. The composition of a protoplanetary disk at a given location does not only depend on temperature and pressure but also on the time dependent transport of matter, such as radial drift of solid bodies, which could release water and other volatile species before disintegration or accretion onto a larger body with potentially considerable implications for the composition of planets. Aims. We performed a parameter study focused on the water depletion of different sized bodies able to cross the water snowline by gas-induced radial drift. Methods. Either the analytical Hertz–Knudsen–Langmuir sublimation formula assuming equilibrium temperature within the body or a more involved, numerical model for the internal thermal evolution was coupled with an α-disk model. Different properties of the disk and the embedded body were explored. Results. Bodies with radii up to 100 m drift faster toward the central star than the water snowline, and can therefore cross it. The region that can be reached before complete disintegration – and is therefore polluted with H₂O ice – extends to 10% closer to the star than the snowline location. The extent of this polluted region could be multiple times larger in the presence of a dust mantle, which is, however, unlikely to form due to frequent collisions with objects smaller than a centimeter. Conclusions. Given a significant abundance of meter-sized boulders in protoplanetary disks, the transport of water by radial drift of these bodies toward regions closer to the star than the snowline is not negligible and this flux of volatiles can be estimated for a given distribution of solid body sizes and compositions. A simple expression for surface sublimation is applicable for a homogeneous body consisting of only dust and water ice without a dust mantle

Can collisional activity produce a crystallization of Edgeworth-Kuiper Belt comets?

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