Constraints on the non-thermal desorption of methanol in the cold core LDN 429-C^★

¹ Laboratoire d’Astrophysique de Bordeaux (LAB), Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
e-mail: angele.taillard@u-bordeaux.fr
² Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Bât. 520, Rue André Rivière, 91405 Orsay, France
³ Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
⁴ Physique des Interactions Ioniques et Moléculaires, CNRS, Aix-Marseille Univ., 13397 Marseille, France
⁵ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822-1897, USA
⁶ Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

Received: 6 October 2022
Accepted: 14 December 2022

Abstract

Context. Cold cores are one of the first steps of star formation, characterized by densities of a few 10⁴–10⁵ cm⁻³, low temperatures (15 K and below), and very low external UV radiation. In these dense environments, a rich chemistry takes place on the surfaces of dust grains. Understanding the physico-chemical processes at play in these environments is essential to tracing the origin of molecules that are predominantly formed via reactions on dust grain surfaces.

Aims. We observed the cold core LDN 429-C (hereafter L429-C) with the NOEMA interferometer and the IRAM 30 m single dish telescope in order to obtain the gas-phase abundances of key species, including CO and CH₃OH. Comparing the data for methanol to the methanol ice abundance previously observed with Spitzer allows us to put quantitative constraints on the efficiency of the non-thermal desorption of this species.

Methods. With physical parameters determined from available Herschel data, we computed abundance maps of 11 detected molecules with a non-local thermal equilibrium (LTE) radiative transfer model. These observations allowed us to probe the molecular abundances as a function of density (ranging from a few 10³ to a few 10⁶ cm⁻³) and visual extinction (ranging from 7 to over 75), with the variation in temperature being restrained between 12 and 18 K. We then compared the observed abundances to the predictions of the Nautilus astrochemical model.

Results. We find that all molecules have lower abundances at high densities and visual extinctions with respect to lower density regions, except for methanol, whose abundance remains around 4.5 × 10⁻¹⁰ with respect to H₂. The CO abundance spreads over a factor of 10 (from an abundance of 10⁻⁴ with respect to H₂ at low density to 1.8 × 10⁻⁵ at high density) while the CS, SO, and H₂S abundances vary by several orders of magnitude. No conclusion can be drawn for CCS, HC₃N, and CN because of the lack of detections at low densities. Comparing these observations with a grid of chemical models based on the local physical conditions, we were able to reproduce these observations, allowing only the parameter time to vary. Higher density regions require shorter times than lower density regions. This result can provide insights on the timescale of the dynamical evolution of this region. The increase in density up to a few 10⁴ cm⁻³ may have taken approximately 10⁵ yr, while the increase to 10⁶ cm⁻³ occurs over a much shorter time span (10⁴ yr). Comparing the observed gas-phase abundance of methanol with previous measurements of the methanol ice, we estimate a non-thermal desorption efficiency between 0.002 and 0.09%, increasing with density. The apparent increase in the desorption efficiency cannot be reproduced by our model unless the yield of cosmic-ray sputtering is altered due to the ice composition varying as a function of density.