The effect of scattering calculations on non-LTE modelling of the C3O and C5O abundances in TMC-1

Abstract

Tricarbon and pentacarbon monoxides have been detected towards TMC-1 with relatively important abundances. Understanding their chemical formation of these molecules requires interpreting their observational spectra by mean of non-local thermodynamical equilibrium modelling. For this purpose, we report rate coefficients of C3O and C5O induced by collision with He for temperatures up to 100 K. These data are obtained by calculating inelastic cross sections for the 31 low-lying rotational levels of C3O and C5O using the close-coupling approach. The comparison of the new rate coefficients with those of HC3N and HC5N, previously used to interpret the observational spectra of C3O and C5O, reveals differences of up to an order of magnitude. The effect of the new collisional rate coefficients in radiative transfer calculations is checked by computing the excitation temperatures for some transitions and simulating the C3O and C5O column densities observed towards TMC-1. Our findings suggest that the use of HCnN as template for CnO may lead to local thermodynamic equilibrium conditions for gas densities as low as ∼103 cm−3. Regarding the interpretation of the observational spectra, using radiative transfer modelling based on the actual CnO collisional rate coefficients instead of rotational diagram analysis leads to underestimate the column densities reported in the literature by up to 25% and accordingly the C3O/ C5O abundance ratio by up to 50%. We expect that the new rate coefficients and the radiative transfer calculations presented in this work will encourage further modellings of the CnO abundance and accordingly constrain the chemistry.

BASECOL2023 scientific content

“…One recommended dataset is available for C 3 O and one for C 3 S. BASECOL2023 was updated with a dataset (Bop et al 2022b) for the rotational de-excitation of C 3 O by He (31 levels; T = 5-150 K) calculated with the PES of Khadri & Hammami (2019), and a dataset (Sahnoun et al 2020) for the rotational deexcitation of C 3 S by He (11 levels; T = 2-25 K); in the latter calculation the authors calculated a new PES.…”

“…In both cases the respective authors calculated a new PES. In addition, it has been updated with a dataset (Bop et al 2022b) for the rotational de-excitation of C 5 O by He (31 levels; T = 5-150 K). These calculations were performed with the PES of Khadri et al (2022a).…”