Linking ice and gas in the Coronet cluster in Corona Australis

TL;DR

This study investigates the relationship between ice and gas-phase methanol in protostellar envelopes within the Coronet cluster, revealing that methanol chemistry is largely unaffected by local irradiation levels and is established during the prestellar phase.

Contribution

It provides the first detailed comparison of gas-to-ice methanol ratios in a strongly irradiated cluster with less irradiated regions, highlighting the robustness of methanol chemistry against environmental variations.

Findings

Methanol gas-to-ice ratios vary by an order of magnitude in the Coronet cluster.

Ratios are similar across different regions despite varying UV radiation fields.

Methanol chemistry is likely set during the prestellar stage, independent of current physical conditions.

Abstract

During the journey from the cloud to the disc, the chemical composition of the protostellar envelope material can be either preserved or processed to varying degrees depending on the surrounding physical environment. This works aims to constrain the interplay of solid (ice) and gaseous methanol (CH$_3$OH) in the outer regions of protostellar envelopes located in the Coronet cluster in Corona Australis (CrA), and assess the importance of irradiation by the Herbig Ae/Be star R CrA. CH$_3$OH is a prime test-case as it predominantly forms as a consequence of the solid-gas interplay (hydrogenation of condensed CO molecules onto the grain surfaces) and it plays an important role in future complex molecular processing. We present 1.3 mm Submillimeter Array (SMA) and Atacama Pathfinder Experiment (APEX) observations towards the envelopes of four low-mass protostars in the Coronet. Eighteen molecular transitions of seven species are identified. We calculate CH$_3$OH gas-to-ice ratios in this strongly irradiated cluster and compare them with ratios determined towards protostars located in less irradiated regions such as the Serpens SVS 4 cluster in Serpens Main and the Barnard 35A cloud in the $\lambda$ Orionis region. The CH$_3$OH gas-to-ice ratios in the Coronet vary by one order of magnitude (from 1.2$\times$10$^{-4}$ to 3.1$\times$10$^{-3}$) which is similar to less irradiated regions as found in previous studies. We find that the CH$_3$OH gas-to-ice ratios estimated in these three regions are remarkably similar despite the different UV radiation field intensities and formation histories. This result suggests that the overall CH$_3$OH chemistry in the outer regions of low-mass envelopes is relatively independent of variations in the physical conditions and hence that it is set during the prestellar stage.