Chemical complexity induced by efficient ice evaporation in the Barnard 5 molecular cloud

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300-RA Leiden The Netherlands
e-mail: taquet@strw.leidenuniv.nl
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Astrochemistry Laboratory, Mailstop 691, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20770, USA
⁴ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
⁷ Institut de Radioastronomie Millimétrique, 38400 Grenoble, France

Received: 7 November 2016
Accepted: 6 June 2017

Abstract

Cold gas-phase water has recently been detected in a cold dark cloud, Barnard 5 located in the Perseus complex, by targeting methanol peaks as signposts for ice mantle evaporation. Observed morphology and abundances of methanol and water are consistent with a transient non-thermal evaporation process only affecting the outermost ice mantle layers, possibly triggering a more complex chemistry. Here we present the detection of the complex organic molecules (COMs) acetaldehyde (CH₃CHO) and methyl formate (CH₃OCHO), as well as formic acid (HCOOH) and ketene (CH₂CO), and the tentative detection of di-methyl ether (CH₃OCH₃) towards the “methanol hotspot” of Barnard 5 located between two dense cores using the single dish OSO 20 m, IRAM 30 m, and NRO 45 m telescopes. The high energy cis-conformer of formic acid is detected, suggesting that formic acid is mostly formed at the surface of interstellar grains and then evaporated. The detection of multiple transitions for each species allows us to constrain their abundances through LTE and non-LTE methods. All the considered COMs show similar abundances between ~ 1 and ~ 10% relative to methanol depending on the assumed excitation temperature. The non-detection of glycolaldehyde, an isomer of methyl formate, with a [glycolaldehyde]/[methyl formate] abundance ratio lower than 6%, favours gas phase formation pathways triggered by methanol evaporation. According to their excitation temperatures derived in massive hot cores, formic acid, ketene, and acetaldehyde have been designated as “lukewarm” COMs whereas methyl formate and di-methyl ether were defined as “warm” species. Comparison with previous observations of other types of sources confirms that lukewarm and warm COMs show similar abundances in low-density cold gas whereas the warm COMs tend to be more abundant than the lukewarm species in warm protostellar cores. This abundance evolution suggests either that warm COMs are indeed mostly formed in protostellar environments and/or that lukewarm COMs are efficiently depleted by increased hydrogenation efficiency around protostars.