Papers by Ugo Hincelin
Water is usually the main component of ice mantles, which cover the cores of dust grains in cold ... more Water is usually the main component of ice mantles, which cover the cores of dust grains in cold portions of dense interstellar clouds. When molecular hydrogen is adsorbed onto an icy mantle through physisorption, a common assumption in gas-grain rate equation models is to use an adsorption energy for molecular hydrogen on a pure water substrate. However, at high density and low temperature, when H2 is efficiently adsorbed onto the mantle, its surface abundance can be strongly overestimated if this assumption is still used. Unfortunately, the more detailed microscopic Monte Carlo treatment cannot be used to study the abundance of H2 in ice mantles if a full gas-grain network is utilized. We present a numerical method adapted for rate-equation models that takes into account the possibility that an H2 molecule can, while diffusing on the surface, find itself bound to another hydrogen molecule, with a far weaker bond than the H2-water bond, which can lead to more efficient desorption. We label the ensuing desorption "encounter desorption". The method is implemented first in a simple system consisting only of hydrogen molecules at steady state between gas and dust using the rate-equation approach and comparing the results with the results of a microscopic Monte Carlo calculation. We then discuss the use of the rate-equation approach with encounter desorption embedded in a complete gas-grain chemical network. For both systems, the rate-equation model with encounter desorption reproduces the H2 granular coverage computed by the microscopic Monte Carlo model. The method is especially useful for dense and cold environments, and for time-dependent physical conditions, such as occur in the collapse of dense cores and the formation of protoplanetary disks. It is not significantly CPU time consuming, so can be used for example with complex 3D chemical-hydrodynamical simulations.
Monthly Notices of the Royal Astronomical Society, 2014
Understanding water deuterium fractionation is important for constraining the mechanisms of water... more Understanding water deuterium fractionation is important for constraining the mechanisms of water formation in interstellar clouds. Observations of HDO and H2(18)O transitions were carried out towards the high-mass star-forming region G34.26+0.15 with the HIFI instrument onboard the Herschel Space Observatory, as well as with ground-based single-dish telescopes. Ten HDO lines and three H2(18)O lines covering a broad range of upper energy levels (22-204 K) were detected. We used a non-LTE 1D analysis to determine the HDO/H2O ratio as a function of radius in the envelope. Models with different water abundance distributions were considered in order to reproduce the observed line profiles. The HDO/H2O ratio is found to be lower in the hot core (3.5x10^(-4) - 7.5x10^(-4)) than in the colder envelope (1.0x10^(-3) - 2.2x10^(-3)). This is the first time that a radial variation of the HDO/H2O ratio has been found to occur in a high-mass source. The chemical evolution of this source was modeled as a function of its radius and the observations are relatively well reproduced. The comparison between the chemical model and the observations leads to an age of 10^5 years after the infrared dark cloud stage.
The Astrophysical Journal, Jul 29, 2013
An outstanding question of astrobiology is the link between the chemical composition of planets, ... more An outstanding question of astrobiology is the link between the chemical composition of planets, comets and other Solar System bodies and the molecules formed in the interstellar medium. Understanding the chemical and physical evolution of the matter leading to the formation of protoplanetary disks is an important step for this. We bring some new stones to this longstanding problem using three-dimensional chemical simulations of the early phases of disk formation: we interfaced the full gas-grain chemical model Nautilus with the radiation-magneto-hydrodynamic model RAMSES, for different configurations and intensities of magnetic field. Our results show that the chemical content (gas and ices) is globally conserved during the collapsing process, from the parent molecular cloud to the young disk surrounding the first Larson core. A qualitative comparison with cometary composition suggests that comets are constituted of different phases, some molecules being direct tracers of interstellar chemistry, while others, including complex molecules, seem to have been formed in disks, where higher densities and temperatures allow for an active grain surface chemistry. The latter phase, and its connection with the formation of the first Larson core, remains to be modelled.
Proceedings of the National Academy of Sciences, 2012
Many chemical models of dense interstellar clouds predict that the majority of gas-phase elementa... more Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N2, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N2 is difficult to detect spectroscopically through infrared or millimetre-wavelength transitions so its abundance is often inferred indirectly through its reaction product N2H+. Two main formation mechanisms each involving two radical-radical reactions are the source of N2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction down to 56 K. The effect of the measured rate constants for this reaction and those recently determined for two other reactions implicated in N2 formation are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N2 formation are inefficient, we argue that N2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.
Astronomy & Astrophysics, 2011
""Context.
Dark cloud chemical models usually predict large amounts of O2, often above observati... more ""Context.
Dark cloud chemical models usually predict large amounts of O2, often above observational limits.
Aims. We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion.
Methods.
We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts.
Results.
The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H+3 + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10−4.
Conclusions.
The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well
constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.""
Suivi photométrique des candidats exoplanètes du satellite CoRoT Sous la direction de Daniel ROUA... more Suivi photométrique des candidats exoplanètes du satellite CoRoT Sous la direction de Daniel ROUAN Mars -Juin 2009 Remerciements Je tiens tout d'abord à remercier Daniel Rouan pour le bon encadrement du stage. Malgré un emploi du temps relativement chargé, il a su prendre le temps de répondre à mes questions et m'a beaucoup aidé dans l'utilisation des outils informatiques. Les moments de réflexion que nous avons eu sur les résultats et les améliorations des analyses des images furent très enrichissants. Enfin, sa sympathie a beaucoup joué sur le bon déroulement de ce stage. Je tiens à remercier Pascal Bordé pour m'avoir accueilli à l'IAS, et ce durant quasiment une demi-journée. Il m'a donné de précieux conseils dans la recherche de thèse, et j'ai pu faire connaissance avec une partie de l'équipe de l'IAS travaillant sur les exoplanètes. Cette entrevue m'a permis de découvrir certains projets en cours sur l'étude et la détection d'exoplanètes, et je pense pouvoir dire que c'est à Pascal que je dois l'obtention du stage avec Daniel. Je remercie aussi Franck Selsis, mon maître de stage de Master1, qui m'a guidé au début de l'année du Master2 dans les recherches de stage sur la détection d'exoplanètes.
Je tiens tout d'abord à remercier Thibault Cavalié et Franck Selsis pour leur bon encadrement tou... more Je tiens tout d'abord à remercier Thibault Cavalié et Franck Selsis pour leur bon encadrement tout au long de ce stage et leur aide dans les difficultés que j'ai pu rencontrer, pour les conseils qu'ils m'ont donnés dans l'élaboration des programmes informatiques et en particulier Thibault pour la rédaction de mon rapport. Je remercie aussi Michel Dobrijevic pour son aide dans l'élaboration des programmes, dans l'utilisation du modèle photochimique et ses commentaires sur les différents résultats que j'ai pu obtenir durant ces deux mois. Je tiens enfin à les remercier tous les trois pour leur disponibilité et leur sympathie.
Thesis Chapters by Ugo Hincelin
Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls i... more Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls in the center of the cloud, creating a protoplanetary disk surrounding a protostar. Planets and other solar system bodies will be formed in the disk.The chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these objects.I studied the chemical and physical evolution of this matter, from the cloud to the disk, using the chemical gas-grain code Nautilus.A sensitivity study to some parameters of the code (such as elemental abundances and parameters of grain surface chemistry) has been done. More particularly, the updates of rate coefficients and branching ratios of the reactions of our chemical network showed their importance, such as on the abundances of some chemical species, and on the code sensitivity to others parameters.Several physical models of collapsing dense core have also been considered. The more complex and solid approach has been to interface our chemical code with the radiation-magneto-hydrodynamic model of stellar formation RAMSES, in order to model in three dimensions the physical and chemical evolution of a young disk formation. Our study showed that the disk keeps imprints of the past history of the matter, and so its chemical composition is sensitive to the initial conditions.
Talks by Ugo Hincelin
Interstellar matter is not inert, but is constantly evolving. On the one hand, its physical chara... more Interstellar matter is not inert, but is constantly evolving. On the one hand, its physical characteristics such as its density and its temperature, and on the other hand, its chemical characteristics such as the abundances of the species and their distribution, can change drastically. The phases of this evolution spread over different timescales, and this matter evolves to create very different objects such as molecular clouds (T∼10 K, n~10^4 cm−3, t∼10^6 years), collapsing prestellar cores (inner core : T∼1000 K, n∼10^16 cm^−3, t∼10^4 years), protostellar cores (inner core : T∼10^5 K, n∼10^24 cm^−3, t∼10^6 years), or protoplanetary disks (T∼10−1000 K, n∼10^9−10^12 cm^−3, t∼10^7 years). These objects are the stages of the star formation process. Starting from the diffuse cloud, matter evolves to form molecular clouds. Then, matter can condense to form prestellar cores, which can collapse to form a protostar surrounded by a protoplanetary disk. The protostar can evolve in a star, and planets and comets can be formed in the disk. Thus, modeling of astrochemistry during star formation should consider chemical and physical evolution in parallel. We present a new gas-grain chemical network involving deuterated species, which takes into account ortho, para, and meta states of H2, D2, H3+, H2D+, D2H+, and D3+. It includes high temperature gas phase reactions, and some ternary reactions for high density, so that it should be able to simulate media with temperature equal to [10;800]~K and density equal to [∼10^4;∼10^12]~cm^−3. We apply this network to the modeling of low-mass and high-mass star formation, using a gas-grain chemical code coupled to a time dependent physical structure. Comparisons with observational constraints, such as the HDO/H2O ratio in high mass star forming region, give good agreement which is promising. Besides, high density conditions have highlighted some limitations of our grain surface modeling. We present a numerical technique to model in a more realistic way H2 diffusion and desorption in high density conditions.
Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls i... more Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls in the center of the cloud, creating a protoplanetary disk surrounding a protostar. Planets and other Solar System bodies will be formed in the disk. The chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these planets and other bodies. In order to study the chemical evolution during the early phases of Solar System formation, one needs to know the evolution of the density and the temperature of the matter during the collapse process. Indeed, the chemical composition and its evolution depend mostly on these quantities.
We present a tri-dimensional physical and chemical model of the collapse of a prestellar dense core up to the first Larson core formation [1,2]. We have interfaced the state-of-the-art radiation magneto-hydrodynamic (RMHD) model of star formation RAMSES [3,4,5] with the full gas-grain chemical model NAUTILUS [6,7]. NAUTILUS computes the chemical evolution of gas and ices using the time dependent physical structure derived by RAMSES.
Within the structure of the object, we identify the different components: the central core, the outflow, the rotating disk, the pseudodisk and the envelope [8]. Our results show that the chemical composition of interstellar matter computed in the initial cloud does not evolve much during the formation of the first Larson core. However, the ice/gas ratio of a given chemical species depends to the considered component, which can be explained by the temperature and density conditions of the component.
Low mass stars, like our Sun, are born from the collapse of a molecular cloud, which is composed... more Low mass stars, like our Sun, are born from the collapse of a molecular cloud, which is composed of interstellar matter. This matter (gas and grain) falls in the center of the cloud, creating a protostar and a protoplanetary disk. Planets and other solar system bodies will be formed in the disk, so the chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these objects. We study the disk chemistry using the gas-grain code Nautilus (Hersant et al. 2009) developed at the Laboratoire d'Astrophysique de Bordeaux, based on the models from the Ohio State University (Eric Herbst's team). The change in physical conditions during the formation of disk is not well constrained (by observations or theory) up to now. We thus assume several scenarii for the possible thermal and density history of the gas and dust during the formation of the disk, partly based on Visser et al. (2009). One goal is to understand the importance of initial conditions for disk chemistry and to quantify the fraction of the parent cloud material that survives the disk formation. Our first results show that the disk chemical evolution will depend on the initial conditions (parent cloud composition). Changing for instance the age (104 to 106 yr) of the initial molecular cloud can modify by several orders of magnitude the chemical composition of ice mantles (CH3OH, CH3OCH3, H2CO, H2O2, H2S...) in a 10^5 yr old protoplanetary disk.
Posters by Ugo Hincelin
Introduction: Low mass stars, like our Sun, are born from the collapse of a molecular cloud which... more Introduction: Low mass stars, like our Sun, are born from the collapse of a molecular cloud which is composed of interstellar matter. This matter, during the formation process, forms the protostar and protoplanetary disk. Planets and other solar system bodies are formed in the disk, so the chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these objects. We study the chemical composition of the disk during its formation, using the gas-grain chemical model Nautilus (Hersant et al. 2009) and results of hydrodynamical models of protostar and disk formation. One of the purposes is to understand the influence of initial conditions for disk chemistry.
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Papers by Ugo Hincelin
Dark cloud chemical models usually predict large amounts of O2, often above observational limits.
Aims. We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion.
Methods.
We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts.
Results.
The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H+3 + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10−4.
Conclusions.
The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well
constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.""
Thesis Chapters by Ugo Hincelin
Talks by Ugo Hincelin
We present a tri-dimensional physical and chemical model of the collapse of a prestellar dense core up to the first Larson core formation [1,2]. We have interfaced the state-of-the-art radiation magneto-hydrodynamic (RMHD) model of star formation RAMSES [3,4,5] with the full gas-grain chemical model NAUTILUS [6,7]. NAUTILUS computes the chemical evolution of gas and ices using the time dependent physical structure derived by RAMSES.
Within the structure of the object, we identify the different components: the central core, the outflow, the rotating disk, the pseudodisk and the envelope [8]. Our results show that the chemical composition of interstellar matter computed in the initial cloud does not evolve much during the formation of the first Larson core. However, the ice/gas ratio of a given chemical species depends to the considered component, which can be explained by the temperature and density conditions of the component.
Posters by Ugo Hincelin
Dark cloud chemical models usually predict large amounts of O2, often above observational limits.
Aims. We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion.
Methods.
We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts.
Results.
The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H+3 + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10−4.
Conclusions.
The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well
constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.""
We present a tri-dimensional physical and chemical model of the collapse of a prestellar dense core up to the first Larson core formation [1,2]. We have interfaced the state-of-the-art radiation magneto-hydrodynamic (RMHD) model of star formation RAMSES [3,4,5] with the full gas-grain chemical model NAUTILUS [6,7]. NAUTILUS computes the chemical evolution of gas and ices using the time dependent physical structure derived by RAMSES.
Within the structure of the object, we identify the different components: the central core, the outflow, the rotating disk, the pseudodisk and the envelope [8]. Our results show that the chemical composition of interstellar matter computed in the initial cloud does not evolve much during the formation of the first Larson core. However, the ice/gas ratio of a given chemical species depends to the considered component, which can be explained by the temperature and density conditions of the component.