139 research outputs found
The interstellar gas-phase chemistry of HCN and HNC
We review the reactions involving HCN and HNC in dark molecular clouds to
elucidate new chemical sources and sinks of these isomers. We find that the
most important reactions for the HCN-HNC system are Dissociative Recombination
(DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC
reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new
reactions: H + CCN and C + HNC. We test the effect of the new rate constants
and branching ratios on the predictions of gas-grain chemical models for dark
cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio
significantly above one as long as the carbon atom abundance remains high.
However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize
HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio
close to or slightly greater than 1. This agrees well with observations in
TMC-1 and L134N taking into consideration the overestimation of HNC abundances
through the use of the same rotational excitation rate constants for HNC as for
HCN in many radiative transfer models.Comment: Accepted for publication in MNRA
From Prestellar to Protostellar Cores II. Time Dependence and Deuterium Fractionation
We investigate the molecular evolution and D/H abundance ratios that develop
as star formation proceeds from a dense-cloud core to a protostellar core, by
solving a gas-grain reaction network applied to a 1-D radiative hydrodynamic
model with infalling fluid parcels. Spatial distributions of gas and ice-mantle
species are calculated at the first-core stage, and at times after the birth of
a protostar. Gas-phase methanol and methane are more abundant than CO at radii
AU in the first-core stage, but gradually decrease with time,
while abundances of larger organic species increase. The warm-up phase, when
complex organic molecules are efficiently formed, is longer-lived for those
fluid parcels in-falling at later stages. The formation of unsaturated carbon
chains (warm carbon-chain chemistry) is also more effective in later stages;
C, which reacts with CH to form carbon chains, increases in abundance
as the envelope density decreases. The large organic molecules and carbon
chains are strongly deuterated, mainly due to high D/H ratios in the parent
molecules, determined in the cold phase. We also extend our model to simulate
simply the chemistry in circumstellar disks, by suspending the 1-D infall of a
fluid parcel at constant disk radii. The species CHOCH and HCOOCH
increase in abundance in yr at the fixed warm temperature; both
also have high D/H ratios.Comment: accepted to ApJ. 55 pages, 7 figures, 3 table
The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration
The deuterium fraction [ND]/[NH], may provide information
about the ages of dense, cold gas structures, important to compare with
dynamical models of cloud core formation and evolution. Here we introduce a
complete chemical network with species containing up to three atoms, with the
exception of the Oxygen chemistry, where reactions involving HO and its
deuterated forms have been added, significantly improving the consistency with
comprehensive chemical networks. Deuterium chemistry and spin states of H
and H isotopologues are included in this primarily gas-phase chemical
model. We investigate dependence of deuterium chemistry on model parameters:
density (), temperature, cosmic ray ionization rate, and gas-phase
depletion factor of heavy elements (). We also explore the effects
of time-dependent freeze-out of gas-phase species and dynamical evolution of
density at various rates relative to free-fall collapse. For a broad range of
model parameters, the timescales to reach large values of , observed in some low- and high-mass starless cores, are
relatively long compared to the local free-fall timescale. These conclusions
are unaffected by introducing time-dependent freeze-out and considering models
with evolving density, unless the initial 10. For fiducial
model parameters, achieving requires
collapse to be proceeding at rates at least several times slower than that of
free-fall collapse, perhaps indicating a dynamically important role for
magnetic fields in the support of starless cores and thus the regulation of
star formation.Comment: 23 pages, 18 figures, accepted by Ap
The gas-phase chemistry of carbon chains in dark cloud chemical models
We review the reactions between carbon chain molecules and radicals, namely
Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and
branching ratios for these processes have been re-evaluated using experimental
and theoretical literature data. In total 8 new species have been introduced,
41 new reactions have been proposed and 122 rate coefficients from
kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of
the new rate constants and branching ratios on the predictions of gas-grain
chemical models for dark cloud conditions using two different C/O elemental
ratios. We show that the new rate constants produce large differences in the
predicted abundances of carbon chains since the formation of long chains is
less effective. The general agreement between the model predictions and
observed abundances in the dark cloud TMC-1 (CP) is improved by the new network
and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement
for different times. The general agreement for L134N (N) is not significantly
changed. The current work specifically highlights the importance of O + CnH and
N + CnH reactions. As there are very few experimental or theoretical data for
the rate constants of these reactions we highlight the need for experimental
studies of the O + CnH and N + CnH reactions, particularly at low temperature.Comment: Accepted for publication in MNRA
Kinetic Study of the Gas-Phase Reaction between Atomic Carbon and Acetone. Low Temperature Rate Constants and Hydrogen Atom Product Yields
The reactions of ground state atomic carbon, C(3P), are likely to be
important in astrochemistry due to the high abundance levels of these atoms in
the dense interstellar medium. Here we present a study of the gas-phase
reaction between C(3P) and acetone, CH3COCH3. Experimentally, rate constants
were measured for this process over the 50 to 296 K range using a
continuous-flow supersonic reactor, while secondary measurements of H(2S) atom
formation were also performed over the 75 to 296 K range to elucidate the
preferred product channels. C(3P) atoms were generated by In-situ pulsed
photolysis of carbon tetrabromide, while both C(3P) and H(2S) atoms were
detected by pulsed laser induced fluorescence. Theoretically, quantum chemical
calculations were performed to obtain the various complexes, adducts and
transition states involved in the C(3P) + CH3COCH3 reaction over the 3A''
potential energy surface, allowing us to better understand the reaction
pathways and help to interpret the experimental results. The derived rate
constants are large, (2-3) x 10-10 cm3 s-1 , displaying only weak temperature
variations; a result that is consistent with the barrierless nature of the
reaction. As this reaction is not present in current astrochemical networks,
its influence on simulated interstellar acetone abundances is tested using a
gas-grain dense interstellar cloud model. For interstellar modelling purposes,
the use of a temperature independent value for the rate constant, k(C+CH3COCH3
)= 2.2 x 10-10 cm3 s-1, is recommended. The C(3P) + CH3COCH3 reaction decreases
gas-phase CH3COCH3 abundances by as much as two orders of magnitude at early
and intermediate cloud ages.Comment: Accepted for publication in ACS Earth and Space Chemistry. 55 pages
including S
Caractérisation physico-chimique des premières phases de formation des disques protoplanétaires
Les étoiles de type solaire se forment par l'effondrement d'un nuage moléculaire, durant lequel la matière s'organise autour de l'étoile en formation sous la forme d'un disque, appelé disque protoplanétaire. Dans ce disque se forment les planètes, comètes et autres objets du système stellaire. La nature de ces objets peut donc avoir un lien avec l'histoire de la matière du disque.J'ai étudié l'évolution chimique et physique de cette matière, du nuage au disque, à l'aide du code de chimie gaz-grain Nautilus.Une étude de sensibilité à divers paramètres du modèle (comme les abondances élémentaires et les paramètres de chimie de surface) a été réalisée. Notamment, la mise à jour des constantes de vitesse et des rapports de branchement des réactions de notre réseau chimique s'est avérée influente sur de nombreux points, comme les abondances de certaines espèces chimiques, et la sensibilité du modèle à ses autres paramètres.Plusieurs modèles physiques d'effondrement ont également été considérés. L'approche la plus complexe et la plus consistante a été d'interfacer notre code de chimie avec le code radiatif magnétohydrodynamique de formation stellaire RAMSES, pour modéliser en trois dimensions l'évolution physique et chimique de la formation d'un jeune disque. Notre étude a démontré que le disque garde une trace de l'histoire passée de la matière, et sa composition chimique est donc sensible aux conditions initiales.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.BORDEAUX1-Bib.electronique (335229901) / SudocBORDEAUX1-Observatoire (331672201) / SudocSudocFranceF
The C(3P) + NH3 reaction in interstellar chemistry: II. Low temperature rate constants and modeling of NH, NH2 and NH3 abundances in dense interstellar clouds
A continuous supersonic flow reactor has been used to measure rate constants
for the C + NH3 reaction over the temperature range 50 to 296 K. C atoms were
created by the pulsed laser photolysis of CBr4. The kinetics of the title
reaction were followed directly by vacuum ultra-violet laser induced
fluorescence (VUV LIF) of C loss and through H formation. The experiments show
unambiguously that the reaction is rapid at 296 K, becoming faster at lower
temperatures, reaching a value of 1.8 10-10 cm3 molecule-1 s-1 at 50 K. As this
reaction is not currently included in astrochemical networks, its influence on
interstellar nitrogen hydride abundances is tested through a dense cloud model
including gas-grain interactions. In particular, the effect of the
ortho-to-para ratio of H2 which plays a crucial role in interstellar NH3
synthesis is examined
Investigating the hot molecular core, G10.47+0.03: A pit of nitrogen-bearing complex organic molecules
Recent observations have shown that Nitrogen-bearing complex organic species
are present in large quantities in star-forming regions. Thus, investigating
the N-bearing species in a hot molecular core, such as G10.47+0.03, is crucial
to understanding the molecular complexity in star-forming regions. They also
allow us to investigate the chemical and physical processes that determine the
many phases during the structural and chemical evolution of the source in
star-forming regions. The aim of this study is to investigate the spatial
distribution and the chemical evolution states of N-bearing complex organic
molecules in the hot core G10.47+0.03. We used the ALMA archival data of the
hot molecular core G10.47+0.03. The extracted spectra were analyzed assuming
LTE. Furthermore, robust methods such as MCMC and rotational diagram methods
are implemented for molecules for which multiple transitions were identified to
constrain the temperature and column density. Finally, we used the Nautilus
gas-grain code to simulate the nitrogen chemistry in the hot molecular core. We
carried out both 0D and 1D simulations of the source and compared with
observational results. We report various transitions of nitrogen-bearing
species (NH2CN, HC3N, HC5N, C2H3CN, C2H5CN, and H2NCH2CN) together with some of
their isotopologues and isomers. Besides this, we also report the
identification of CH3CCH and one of its isotopologues. The emissions
originating from vinyl cyanide, ethyl cyanide, cyanoacetylene, and cyanamide
are compact, which could be explained by our astrochemical modeling. Our 0D
model shows that the chemistry of certain N-bearing molecules can be very
sensitive to initial local conditions such as density or dust temperature. In
our 1D model, simulated higher abundances of species such as HCN, HC3N, and
HC5N toward the inner shells of the source confirm the observational findings.Comment: 40 pages, 30 figure
The Si + SO2 collision and an extended network of neutral–neutral reactions between silicon and sulphur bearing species
International audienceThe Si + SO2 reaction is investigated to verify its impact on the abundances of molecules with astrochemical interest, such as SiS, SiO, SO, and others. According to our results Si(3P) and SO2 react barrierlessly yielding only the monoxides SO and SiO as products. No favourable pathway has been found leading to other products, and this reaction should not contribute to SiS abundance. Furthermore, it is predicted that SiS is stable in collisions with O2, and that S(3P) + SiO2 and O(3P)+OSiS will also produce SO + SiO. Using these results and gathering further experimental and computational data from the literature, we provide an extended network of neutral-neutral reactions involving Si- and S-bearing molecules. The effects of these reactions were examined in a protostellar shock model, using the NAUTILUS gas-grain code. This consisted in simulating the physicochemical conditions of a shocked gas evolving from (i) primeval cold core, (ii) the shock region itself, (iii) and finally the gas bulk conditions after the passage of the shock. Emphasizing on the cloud ages and including systematically these chemical reactions, we found that [SiS/H2] can be of the order of ~10-8 in shocks that evolves from clouds of t = 1 × 106 yr, whose values are mostly affected by the SiS + O SiO + S reaction. Perspectives on further models along with observations are discussed in the context of sources harbouring molecular outflows
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