CoCCoA: Complex Chemistry in hot Cores with ALMA. Selected oxygen-bearing species

Abstract

Complex organic molecules (COMs) have been observed to be abundant in the gas phase toward protostars. Deep line surveys have been carried out only for a limited number of well-known high-mass star forming regions using the Atacama Large Millimeter/submillimeter Array (ALMA), which has unprecedented resolution and sensitivity. Statistical studies on oxygen-bearing COMs (O-COMs) in high-mass protostars using ALMA are still lacking. With the recent CoCCoA survey, we are able to determine the column density ratios of six O-COMs with respect to methanol (CH${}_3$OH) in a sample of 14 high-mass protostellar sources to investigate their origin through ice and/or gas-phase chemistry. The selected species are: acetaldehyde (CH${}_3$CHO), ethanol (C${}_2$H${}_5$OH), dimethyl ether (DME, CH${}_3$OCH${}_3$), methyl formate (MF, CH${}_3$OCHO), glycolaldehyde (GA, CH${}_2$OHCHO), and ethylene glycol (EG, (CH${}_2$OH)${}_2$). DME and MF have the highest and most constant ratios within one order of magnitude, while the other four species have lower ratios and exhibit larger scatter by 1-2 orders of magnitude. We compare the O-COM ratios of high-mass CoCCoA sources with those of 5 low-mass protostars available from the literature, along with the results from experiments and simulations. We find that the O-COM ratios with respect to methanol are on the same level in both the high- and low-mass samples, which suggests that these species are mainly formed in similar environments during star formation, probably in ice mantles on dust grains during early pre-stellar stages. Current simulations and experiments can reproduce most observational trends with a few exceptions, and hypotheses exist to explain the differences between observations and simulations/experiments, such as the involvement of gas-phase chemistry and different emitting areas of molecules.

Accepted for publication in Astronomy & Astrophysics. 17 pages and 7 figures in the main body, 15 pages and 8 figures in the appendix

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