Chimie interstellaire des hydrures d'azote : modélisation - observations

The new spectroscopic window opened by the advent of the Herschel Space Observatory,has enabled the detection of simple nitrogen species, the nitrogen hydrides NH, NH2, and NH3,in the cold envelope of protostars. These envelopes are made of dense cold gas characteristicof the physico-chemical conditions of molecular clouds. The observation of nitrogen hydrides insuch environments has brought new constraints on the interstellar chemistry of these kind ofclouds, and gives, in particular, the opportunity to revisit the chemistry of nitrogen.The aim of my thesis was to comprehensively analyse the interstellar chemistry of nitrogen,focussing on the gas-phase formation of the simplest polyatomic species, namely nitrogen hydrides.Under dense, cold gas conditions (n = 104 cm−3, T = 10 K), the chemistry of theselatter is initiated by a slow chemistry (the conversion from N to N2 with neutral-neutral reactions),in contrast to their carbonated and oxygenated analogues. We have investigated andrevisited this specific part of the nitrogen chemistry in the light of recent theoretical and experimentalwork carried out by several expert teams (Bordeaux, Besançon). In addition, recentwork about the ortho-para conversion of molecular hydrogen and new calculations of nuclearspin branching ratios for the production pathways of nitrogen hydrides in their ortho and paraconfigurations conducted at IPAG, enabled us to treat self-consistently the different spin symmetriesof the nitrogen hydrides together with the ortho and para forms of molecular hydrogen.We were able to develop a new network of chemical nitrogen in which the kinetic rates of criticalreactions involved in the nitrogen chemistry have been updated.This new network is used to model the time evolution of the nitrogen species abundancesin dense cold gas conditions (T ≤ 50 K, 103 < n < 106 cm−3). The steady-state resultsare compared to observations of NH, NH2 and NH3 towards a sample of low-mass protostars,with a special emphasis on the influence of the overall amounts of gaseous carbon, oxygen, andsulphur. Our chemical models reproduced the nitrogen hydrides abundances and their ratios fora gas-phase elemental C/O ratio of ∼ 0.8, provided that the total amount of sulphur is depletedby at least a factor of two. Our predicted ortho-to-para ratios for NH2 and NH3, ∼ 2.3 and∼ 0.7 respectively, are in good agreement with the observations towards cold diffuse clouds.Then, in dark gas conditions, the nitrogen hydride abundances are consistent with a pure gasphasesynthesis. Moreover, our results are based on the fact that NH is coming from a differentpathway than NH2 and NH3. NH is the daughter molecule of N2H+, deriving from the reactionN2+H+3 , while NH2 and NH3 proceed from NH+4 , itself daughter molecule of N+, resulting fromthe dissociative charge exchange N2 + He+.La nouvelle fenêtre spectroscopique dans le sub-millimétrique, ouverte par l’avènement del’observatoire spatial Herschel, a permis la détection d’espèces azotées simples, les hydruresd’azote NH, NH2 et NH3, dans les enveloppes froides de proto-étoiles. Ces enveloppes sontconstituées de gaz dense et froid caractéristique des conditions physico-chimiques des nuagesmoléculaires. L’observation d’hydrures d’azote dans de tels environnements a donc permis d’apporterde nouvelles contraintes sur la chimie interstellaire de ces nuages, et nous a donné enparticulier l’occasion de ré-explorer la chimie de l’azote.L’objectif de mon travail de thèse a été d’analyser en détail cette chimie interstellaire etprincipalement la formation en phase gazeuse d’espèces polyatomiques simples : les hydruresd’azote. Dans des conditions de gaz dense et froid (n = 104 cm−3, T = 10 K), la chimie de cesderniers est initiée par une chimie lente (la conversion de N en N2 par réactions neutre-neutre),contrairement à celles de ses analogues oxygénés et carbonés. Nous nous sommes particulièrementintéressés à cette étape de la chimie de l’azote, au vu des récents travaux théoriqueset expérimentaux menés par plusieurs équipes d’experts (Bordeaux, Besançon). De plus, lesrécents travaux concernant la conversion ortho-para de l’hydrogène moléculaire et les nouveauxcalculs de rapports de branchement de spins nucléaires pour les voies de production des hydruresd’azote dans leurs configurations ortho et para, menés à l’IPAG, nous ont permis d’entreprendrele calcul auto-cohérent des différentes symétries de spin des hydrures d’azote et de l’hydrogènemoléculaire. Nous avons ainsi pu développer un nouveau réseau chimique de l’azote, bénéficiantdes taux cinétiques les plus à jour pour les réactions critiques impliquées dans la chimie deshydrures d’azote.Ce nouveau réseau est utilisé pour modéliser l’évolution temporelle des abondances desespèces azotées dans des conditions de gaz dense et froid ( 103 < n < 106 cm−3, T ≤ 50 K).Les résultats à l’état stationnaire sont comparés aux observations de NH, NH2 et NH3, dans lesenveloppes froides de proto-étoiles de faible masse, en étudiant l’influence des abondances totalesen phase gazeuse du carbone, de l’oxygène et du soufre. Nos modèles chimiques reproduisent lesabondances des hydrures d’azote observés et leurs rapports pour un rapport C/O élementaire, enphase gazeuse, de ∼ 0.8 et à condition que l’abondance totale de soufre soit déplétée d’au moinsun facteur 2. Les rapports ortho/para prédits par nos modèles, pour NH2 et NH3, respectivement∼ 2.3 et ∼ 0.7, sont compatibles avec les observations de ces derniers dans des nuages diffusfroids. Les abondances des hydrures d’azote, dans des conditions de nuages sombres, sont donccohérentes avec une synthèse purement en phase gazeuse. De plus, nos résultats soulignent lefait que NH provient d’une voie de formation différente de celle de NH2 et NH3. NH vient de larecombinaison dissociative de N2H+ alors que la formation de NH2 et NH3 est principalementdue à la recombinaison dissociative de l’ion ammonium (NH+4 ), lui même molécule fille deN+. Ainsi, NH2 et NH3 procèdent de l’échange de charge dissociatif N2 + He+, tandis que NHprovient de la réaction N2 + H+3

THE KEY ROLE OF NUCLEAR-SPIN ASTROCHEMISTRY

Thanks to the new spectroscopic windows opened by the recent_x000d_ generation of telescopes, a large number of molecular lines have been_x000d_ detected. In particular, nuclear-spin astrochemistry has gained_x000d_ interest owing to numerous ortho-to-para ratio (OPR) measurements for species including H$_3^+$, CH$_2$, C$_3$H$_2$, H$_2$O, NH$_3$, NH$_2$, H$_2$S,_x000d_ H$_2$CS, H$_2$O$^+$ and H$_2$Cl$^+$. Any multi-hydrogenated species_x000d_ can indeed present different spin configurations, if some of their_x000d_ hydrogen nuclei are identical, and the species thus exist in distinguishable_x000d_ forms, such as ortho and para. In thermal equilibrium, OPRs are only_x000d_ functions of the temperature and since spontaneous conversion between_x000d_ ortho and para states is extremely slow in comparison with typical_x000d_ molecular cloud lifetimes, OPRs were commonly believed to reflect a_x000d_ ``formation temperature''.~However, observed OPRs are not always consistent with their thermal equilibrium values, as for the NH$_3$ and NH$_2$ cases.~It is thus crucial to understand how interstellar OPRs are formed to constrain the information such new probes can provide.~This involves a comprehensive analysis of the processes governing the interstellar nuclear-spin chemistry, including the formation and possible conversions of the different spin symmetries both in the gas and solid phases. If well understood, OPRs might afford new powerful astrophysical diagnostics on the chemical_x000d_ and physical conditions of their environments, and in particular could trace their thermal history. In this context, observations of non-thermal values for the OPR of the radical NH$_2$ toward four high-mass star-forming regionsfootnote{Persson et al. 2016, A&A,_x000d_ 586, A128}, and a 3:1 value measured for the H$_2$Cl$^+$ OPR toward_x000d_ diffusefootnote{Neufeld et al. 2016, ApJ, 807, 54} and denser gas,_x000d_ led us to develop detailed studies of the mechanisms involved in_x000d_ obtaining such OPRs with the aid of quasi-classical trajectory_x000d_ calculationsfootnote{Le Gal et al. 2016, A&A, 596, A35 and Le Gal et_x000d_ al., in prep}. We will present these new promising results,_x000d_ improving our understanding of the interstellar medium

The ortho-to-para ratio of interstellar NH2_2: Quasi-classical trajectory calculations and new simulations

Based on recent $Herschel$ results, the ortho-to-para ratio (OPR) of NH$_2$ has been measured towards the following high-mass star-forming regions: W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. The OPR at thermal equilibrium ranges from the statistical limit of three at high temperatures to infinity as the temperature tends toward zero, unlike the case of H$_{2}$. Depending on the position observed along the lines-of-sight, the OPR was found to lie either slightly below the high temperature limit of three (in the range $2.2-2.9$) or above this limit ($\sim3.5$, $\gtrsim 4.2$, and $\gtrsim 5.0$). In low temperature interstellar gas, where the H$_{2}$ is para-enriched, our nearly pure gas-phase astrochemical models with nuclear-spin chemistry can account for anomalously low observed NH$_2$-OPR values. We have tentatively explained OPR values larger than three by assuming that spin thermalization of NH$_2$ can proceed at least partially by H-atom exchange collisions with atomic hydrogen, thus increasing the OPR with decreasing temperature. In this paper, we present quasi-classical trajectory calculations of the H-exchange reaction NH$_2$ + H, which show the reaction to proceed without a barrier, confirming that the H-exchange will be efficient in the temperature range of interest. With the inclusion of this process, our models suggest both that OPR values below three arise in regions with temperatures $\gtrsim20-25$~K, depending on time, and values above three but lower than the thermal limit arise at still lower temperatures.Comment: 12 pages, 12 figures. Accepted for publication in A&

Interstellar chemistry of nitrogen hydrides in dark clouds

The aim of the present work is to perform a comprehensive analysis of the interstellar chemistry of nitrogen, focussing on the gas-phase formation of the smallest polyatomic species and in particular nitrogen hydrides. We present a new chemical network in which the kinetic rates of critical reactions have been updated based on recent experimental and theoretical studies, including nuclear spin branching ratios. Our network thus treats the different spin symmetries of the nitrogen hydrides self-consistently together with the ortho and para forms of molecular hydrogen. This new network is used to model the time evolution of the chemical abundances in dark cloud conditions. The steady-state results are analysed, with special emphasis on the influence of the overall amounts of carbon, oxygen, and sulphur. Our calculations are also compared with Herschel/HIFI observations of NH, NH$_2$, and NH$_3$ detected towards the external envelope of the protostar IRAS 16293-2422. The observed abundances and abundance ratios are reproduced for a C/O gas-phase elemental abundance ratio of $\sim0.8$, provided that the sulphur abundance is depleted by a factor larger than 2. The ortho-to-para ratio of H$_2$ in these models is $\sim10^{-3}$. Our models also provide predictions for the ortho-to-para ratios of NH$_2$ and NH$_3$ of $\sim2.3$ and $\sim0.7$ respectively. We conclude that the abundances of nitrogen hydrides in dark cloud conditions are consistent with the gas-phase synthesis predicted with our new chemical network.Comment: Accepted for publication in Astronomy & Astrophysics; 22 pages (9 in Appendix), 7 figures (2 in Appendix), 6 tables (3 in Appendix

High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2

Nearby Photo-Dissociation Regions (PDRs), where the gas and dust are heated by the far UV-irradiation emitted from stars, are ideal templates to study the main stellar feedback processes. With this study we aim to probe the detailed structures at the interfaces between ionized, atomic, and molecular gas in the Orion Bar. This nearby prototypical strongly irradiated PDR will be among the first targets of the James Webb Space Telescope (JWST) within the framework of the PDRs4All Early Release Science program. We employed the sub-arcsec resolution accessible with Keck-II NIRC2 and its adaptive optics system to obtain the most detailed and complete images, ever performed, of the vibrationally excited line H$_2$ 1-0 S(1) at 2.12~$\mu$m, tracing the dissociation front, and the [FeII] and Br$\gamma$ lines, at 1.64 and 2.16~$\mu$m respectively, tracing the ionization front. We obtained narrow-band filter images in these key gas line diagnostic over $\sim 40''$ at spatial scales of $\sim$0.1$''$ ($\sim$0.0002~pc or $\sim$40~AU at 414~pc). The Keck/NIRC2 observations spatially resolve a plethora of irradiated sub-structures such as ridges, filaments, globules and proplyds. A remarkable spatial coincidence between the H$_2$ 1-0 S(1) vibrational and HCO$^+$ J=4-3 rotational emission previously obtained with ALMA is observed. This likely indicates the intimate link between these two molecular species and highlights that in high pressure PDR the H/H$_2$ and C$^+$/C/CO transitions zones come closer as compared to a typical layered structure of a constant density PDR. This is in agreement with several previous studies that claimed that the Orion Bar edge is composed of very small, dense, highly irradiated PDRs at high thermal pressure immersed in a more diffuse environment