Linking ice and gas in the Coronet cluster in Corona Australis

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 $\hat{I}»$ 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.

20 pages, 17 figures, accepted for publication in Astronomy & Astrophysics