To determine the chemical composition of gases in molecular clouds, the oxygen-bearing systems CnO are needed as probe elements. The pentacarbon monoxide C5O was recently detected in TMC-1, and in order to derive accurate physical conditions from its rotational transitions, calculation of rate coefficients of C5O(1Σ+) induced by collision with He are performed for thermal temperature below 100 K. These calculations are based on a new two-dimensional potential energy surface (2D-PES) obtained from the explicit correlated coupled cluster with single, double and pertubative triple excitation (ccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C5O–He PES presents two minima below its dissociation limit with a well depths of −59.321 cm−1 and −53.059 cm−1. By mean of this PES, the integral cross sections are calculated in the close-coupling quantum time independant formalism for E ≤ 500 cm−1 and J ≤ 20. The de-excitation rate coefficients are obtained after averaging these cross sections at low temperatures. We expect that the new collisional data will allow accurate determination of the C5O abundance in the interstellar medium, as well as the interpretation of its emission lines. These new data are crucial to understand the chemistry of carbon chain species in the interstellar gas.
The C5S molecule is the largest member of the series of sulfur-containing carbon chains CnS observed in space. Given the lack of data concerning this molecule, we computed rate coefficients of C5S(1Σ+) induced by collision with He. These rates are obtained for thermal temperature below 100 K by mean of a new two-dimensional potential energy surface (PES) calculated with the explicit correlated coupled cluster with single, double and pertubative triple excitation (ccsd(t)-f12) ab initio approach and the aug-cc-pVTZ basis sets. The C5S–He PES presents three minimums of −59.726, −55.355 and −36.506 cm−1 below it’s dissociation limit. Using this PES, the integral cross sections are performed in the close-coupling (CC) and coupled-state (CS) quantum time independant formalisms for Ec ≤ 500 cm−1 and J ≤ 13 (for CC) and J ≤ 50 (for CS). By averaging these cross sections we obtained the downward rate coefficients. The new collisional data are used to simulate the excitation of C5S in the circumstellar gas. We obtain the excitation and brightness temperatures of the four lines observed towards the IRC +10216 which confirms the necessity of using radiative transfer calculations to accurately determine C5S abundance since the local thermodynamic equilibrium (LTE) conditions are not fulfilled. The new collisional data should help to estimate the abundance of C5S in several interstellar regions.
The CCN radical has been recently detected in the interstellar medium. Accurate modeling of its abundance in such media requires one to model its excitation by both radiation and collisions. Here, we report the first quantum mechanical close-coupling study of CCN-He collisions. Calculations of fine-structure resolved excitation cross sections of CCN(XΠ) induced by collision with He are performed for kinetic energies below 500 cm. The calculations are based on new two-dimensional potential energy surfaces obtained from coupled cluster approaches. We found that the inelastic cross sections for spin-orbit conserving transitions are larger than those for spin-orbit changing transitions. The new collisional data should significantly help the interpretation of interstellar CCN emission lines observed with current and future telescopes and we expect that they will allow accurate determination of the CCN abundance in the interstellar medium, which is crucial to understand the chemistry of carbon chain species in the interstellar gas.
An appropriate estimation of the abundance of the observed C5 radical in the interstellar medium requires accurate radiative and collisional rate coefficients.
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