Linking ice and gas in the Serpens low-mass star-forming region^★

¹ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5−7, 1350 Copenhagen K., Denmark
e-mail: giulia.perotti@nbi.ku.dk
² School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 6 April 2020
Accepted: 30 July 2020

Abstract

Context. The interaction between dust, ice, and gas during the formation of stars produces complex organic molecules. While observations indicate that several species are formed on ice-covered dust grains and are released into the gas phase, the exact chemical interplay between solid and gas phases and their relative importance remain unclear.

Aims. Our goal is to study the interplay between dust, ice, and gas in regions of low-mass star formation through ice- and gas-mapping and by directly measuring gas-to-ice ratios. This provides constraints on the routes that lead to the chemical complexity that is observed in solid and gas phases.

Methods. We present observations of gas-phase methanol (CH₃OH) and carbon monoxide (¹³CO and C¹⁸O) at 1.3 mm towards ten low-mass young protostars in the Serpens SVS 4 cluster from the SubMillimeter Array (SMA) and the Atacama Pathfinder EXperiment (APEX) telescope. We used archival data from the Very Large Telescope (VLT) to derive abundances of ice H₂O, CO, and CH₃OH towards the same region. Finally, we constructed gas-ice maps of SVS 4 and directly measured CO and CH₃OH gas-to-ice ratios.

Results. The SVS 4 cluster is characterised by a global temperature of 15 ± 5 K. At this temperature, the chemical behaviours of CH₃OH and CO are anti-correlated: larger variations are observed for CH₃OH gas than for CH₃OH ice, whereas the opposite is seen for CO. The gas-to-ice ratios (N_gas/N_ice) range from 1–6 for CO and 1.4 × 10⁻⁴–3.7 × 10⁻³ for CH₃OH. The CO gas-maps trace an extended gaseous component that is not sensitive to the effect of freeze-out. Because of temperature variations and dust heating around 20 K, the frozen CO is efficiently desorbed. The CH₃OH gas-maps, in contrast, probe regions where methanol is predominantly formed and present in ices and is released into the gas phase through non-thermal desorption mechanisms.

Conclusions. Combining gas- and ice-mapping techniques, we measure gas-to-ice ratios of CO and CH₃OH in the SVS 4 cluster. The CH₃OH gas-to-ice ratio agrees with values that were previously reported for embedded Class 0/I low-mass protostars. We find that there is no straightforward correlation between CO and CH₃OH gas with their ice counterparts in the cluster. This is likely related to the complex morphology of SVS 4: the Class 0 protostar SMM 4 and its envelope lie in the vicinity, and the outflow associated with SMM 4 intersects the cluster. This study serves as a pathfinder for future observations with ALMA and the James Webb Space Telescope (JWST) that will provide high-sensitivity gas-ice maps of molecules more complex than methanol. Such comparative maps will be essential to constrain the chemical routes that regulate the chemical complexity in star-forming regions.