Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) III. CH${}_3$OH isotopic fractionation in the low-mass protostar BHR71-IRS1

Abstract

[Abridged] Methanol is a complex organic molecule detected in both ice and gas toward star-forming regions. Due to its high abundance, its rarer isotopologues, including the deuterated ones, are also commonly detected. Our aim is to determine the column density and deuterium fractionation of methanol in the low-mass protostar BHR71-IRS1. We analyzed data from a large spectral survey of BHR71-IRS1 carried out as part of the ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). We used the CASSIS software to identify the lines of the methanol isotopologues and constrain their column densities under the assumption of local thermodynamic equilibrium. We detected CH${}_3$OH, ${}^{13}$CH${}_3$OH, CH${}_3$${}^{18}$OH, CH${}_3$OD, CH${}_2$DOH, CHD${}_2$OH, and CD${}_3$OH at the continuum peak position. In addition, CH${}_3$${}^{17}$OH is tentatively detected, while CD${}_3$OD is not detected, although a few faint lines could be present toward a position redshifted from the continuum peak. The column densities of the main isotopologue obtained from ${}^{13}$CH${}_3$OH and CH${}_3$${}^{18}$OH are consistent, assuming nominal ${}^{12}$C/${}^{13}$C and ${}^{16}$O/${}^{18}$O ratios of 68 and 557, respectively. The CH${}_2$DOH/CH${}_3$OD column density ratio is equal to 7$\pm$2. The statistically corrected D/H ratio is higher for CH${}_2$DOH (1.9$\pm$0.6 %) than for CH${}_3$OD (0.8$\pm$0.2 %). The level of deuteration after statistical correction increases with the number of D atoms in the methyl group, reaching 23$\pm$5 % for CD${}_3$OH/CH${}_3$OH. These trends are similar to other low-mass protostars such as IRAS~16293--2422 and HOPS 373SW, but seem to differ from the eruptive young star V883-Ori. Similar studies with comparable datasets will be carried out for the other COMPASS sources to determine if factors such as environmental conditions and evolutionary states impact the deuteration of methanol.

Accepted in A&A