Methanol deuteration in high-mass protostars

Abstract

Context . The deuteration of molecules forming in the ices such as methanol (CH 3 OH) is sensitive to the physical conditions during their formation in dense cold clouds and can be probed through observations of deuterated methanol in hot cores. Aims . The aim is to determine the D/H ratio of methanol for a large sample of 99 high-mass protostars and to link this to the physical conditions during the formation of methanol in the prestellar phases. Methods . Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) containing transitions of CH 3 OH, CH 2 DOH, CHD 2 OH, 13 CH 3 OH, and CH 3 18 OH are investigated. The column densities of CH 2 DOH, CHD 2 OH, and CH 3 OH are determined for all sources, where the column density of CH 3 OH is derived from optically thin 13 C and 18 O isotopologues. Consequently, the D/H ratio of methanol is derived taking statistical effects into account. Results . Singly deuterated methanol (CH 2 DOH) is detected at the 3σ level toward 25 of the 99 sources in our sample of the high-mass protostars. Including upper limits, the (D/H) CH 3 OH ratio inferred from N CH 2 DOH / N CH 3 OH was derived for 38 of the 99 sources and varies between ~10−3-10−2. Including other high-mass hot cores from the literature, the mean methanol D/H ratio is 1.1 ± 0.7 × 10−3. This is more than one order of magnitude lower than what is seen for low-mass protostellar systems (2.2 ± 1.2 × 10−2). Doubly deuterated methanol (CHD 2 OH) is detected at the 3σ level toward 11 of the 99 sources. Including upper limits for 15 sources, the (D/H) CH 2 DOH ratios derived from N CHD 2 OH / N CH 2 DOH are more than two orders of magnitude higher than (D/H) CH 3 OH with an average of 2.0 ± 0.8 × 10−1 which is similar to what is found for low-mass sources. Comparison with literature GRAINOBLE models suggests that the high-mass prestellar phases are either warm (>20 K) or live shorter than the free-fall timescale. In contrast, for low-mass protostars, both a low temperature of <15 K and a prestellar phase timescale longer than the free-fall timescale are necessary. Conclusions . The (D/H) CH 3 OH ratio drops by more than an order of magnitude between low-mass and high-mass protostars due to either a higher temperature during the prestellar phases or shorter prestellar phases. However, successive deuteration toward CHD 2 OH seems equally effective between low-mass and high-mass systems.

Topics & Concepts