Hyperfine excitation of CN by para- and ortho-H2

Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of cold dark molecular clouds, and especially prestellar cores. Modelling of CN emission spectra from these dense molecular clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. We calculate fine- and hyperfinestructure-resolved excitation rate coefficients of CN(X2+) by para- and ortho-H2. The calculations are based on a new potential energy surface obtained recently from highly correlated ab initio calculations. State-to-state rate coefficients between fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100 K. The new results are compared to available CN rate coefficients. Significant differences are found between the different sets of rate coefficients. This comparison shows that the new CN–H2 rate coefficients have to be used for observations interpretations. We expect that their use will help significantly to have a new insight into the physical conditions of prestellar cores

Formation of interstellar SH+^+ from vibrationally excited H2_2: Quantum study of S+^+ + H2_2 ⇄\rightleftarrows SH+^+ + H reactions and inelastic collisions

The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A''$ and $^2A''$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3\Sigma^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H$_2$ reaction is endothermic by $\approx$ 9860 K, and requires at least two vibrational quanta on the H$_2$ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H$_2$($v=2$) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for $v=3$, 4, and 5 to cover all vibrationally excited H$_2$ levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH$^+$ column densities, in agreement with those inferred from recent ALMA observations of the Orion bar.Comment: 8 pages, 7 figure

Molecules as Diagnostic Tools in the Interstellar Medium

International audienceAnalysis of light emission from different regions of the interstellar medium and circumstellar environments gives crucial information about the chemical composition and the physical conditions in these regions. Interpretation of the observed spectra requires the knowledge of collisional excitation rates as well as radiative rates participating to the line formation. In a first part, this paper focuses on collisional excitation rates of molecules relevant to the interstellar medium. It discusses currently available data and outlines new work carried out by the authors. Due to the use of accurate ab initio potential energy surfaces, the new rate coefficients differ significantly from previously published ones. In a second part, the paper analyses from two examples how the use of the new rates could lead to important changes in the interpretation of molecular emission emerging from molecular clouds

Collisional energy transfer in the HeH + –H reactive system

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Fine and hyperfine excitation of CCS isotopologues: isotopic effect on astrophysical modeling

International audienceMolecular spectra often serve as the sole source of information about astrophysical media. To interpret these spectra, collisional rate coefficients are essential. The CCS molecule has been detected in several molecular clouds, protostellar envelopes, and the circumstellar envelope of IRC+10216. Despite its abundance in these media, no reliable rate coefficients exist for this molecule yet. This is due to its complex fine structure, which complicates the computation of data. Additionally, various isotopologues of CCS, such as 13CCS, C13CS, CC34S, and recently CC33S, have been detected in molecular clouds. Having collisional data for all these isotopologues will aid in discussing isotopic fractionation in such media and understanding the formation mechanism of CCS through the analysis of the 13CCS and C13CS abundance ratio. Nevertheless, 13C and 33S isotopologues have non-zero molecular spin, resulting in the splitting of fine structure levels into 2 hyperfine levels for 13C-based isotopologues and 4 hyperfine levels for the CC33S isotopologue. In this study, we present the first CCS-He potential energy surface computed with CCSD(T)/aVQZ and mid-bond functions level of theory, as well as the first accurate CCS-He rate coefficients. These were compared to previously computed values, showing a global agreement within a factor of 2-10, and up to a factor of 100. The derived abundances in TMC-1 differed by 20% and were 2.5 times different from the CCS abundances derived from the ISA (https://isa.astrochem-tools.org/) database.Furthermore, we computed fine and hyperfine rate coefficients for 13CCS, C13CS, CC34S, and CC33S isotopologues. The isotopic effect on rate coefficients was found to be relatively weak, with differences in line intensity of 13C-based isotopologues primarily induced by a factor of 2.5 difference in abundances (not 4.2 as suggested by Sakai et al., 2007), rather than by collisional effects. Therefore, the formation of CCS cannot be induced by a symmetric molecule.In this study, accurate rate coefficients were computed for 5 CCS isotopologues, allowing us to revise the CCS abundance in TMC-1, derive 13CCS, C13CS, CC34S, and CC33S abundances, and provide insight into the CCS formation pathway

Hyperfine excitation of CN by para- and ortho-H2

Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of cold dark molecular clouds, and especially prestellar cores. Modelling of CN emission spectra from these dense molecular clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. We calculate fine- and hyperfinestructure-resolved excitation rate coefficients of CN(X2+) by para- and ortho-H2. The calculations are based on a new potential energy surface obtained recently from highly correlated ab initio calculations. State-to-state rate coefficients between fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100 K. The new results are compared to available CN rate coefficients. Significant differences are found between the different sets of rate coefficients. This comparison shows that the new CN–H2 rate coefficients have to be used for observations interpretations. We expect that their use will help significantly to have a new insight into the physical conditions of prestellar cores

Ortho-para-H-2 conversion by hydrogen exchange: Comparison of theory and experiment

International audienceWe report fully-quantum time-independent calculations of cross sections and rate coefficients for the collisional (de) excitation of H-2 by H. Our calculations are based on the H-3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. The reactive hydrogen exchange channels are taken into account. We show that the ortho-para and para-ortho conversion of H-2 are significant processes at temperatures above similar to 300 K and for the last process we provide the first comparison with available experimental rate coefficients between 300 and 444 K. The good agreement between theory and experiment is a new illustration of our detailed understanding of the simplest chemical reaction. The importance of the ortho-para-H-2 conversion by hydrogen exchange in astrophysics is discussed