2015A&A...575A..87F

An ever growing number of observational and theoretical evidence suggests that the deuterated fraction (column density ratio between a species containing D and its hydrogenated counterpart, D_frac) is an evolutionary indicator both in the low- and the high-mass star formation process. However, the role of surface chemistry in these studies has not been quantified from an observational point of view. Because many abundant species, such as NH₃, H₂CO, and CH₃OH, are actively produced on ice mantles of dust grains during the early cold phases, their D_frac is expected to evolve differently from species formed only (or predominantly) in the gas, such as N₂H⁺, HNC, HCN, and their deuterated isotopologues. The differences are expected to be relevant especially after the protostellar birth, in which the temperature rises, causing the evaporation of ice mantles. To compare how the deuterated fractions of species formed only in the gas and partially or uniquely on grain surfaces evolve with time, we observed rotational transitions of CH₃OH, ¹³CH₃OH, CH₂DOH, and CH₃OD at 3mm and 1.3mm, of NH₂D at 3mm with the IRAM-30 m telescope, and the inversion transitions (1, 1) and (2, 2) of NH₃ with the GBT, towards most of the cores already observed in N₂H⁺, N₂D⁺, HNC, and DNC. NH₂D is detected in all but two cores, regardless of the evolutionary stage. D_frac(NH₃) is on average above 0.1 and does not change significantly from the earliest to the most evolved phases, although the highest average value is found in the protostellar phase (∼0.3). Few lines of CH₂DOH and CH₃OD are clearly detected, and then only towards protostellar cores or externally heated starless cores. In quiescent starless cores, we have only one doubtful detection of CH₂DOH. This work clearly confirms an expected different evolutionary trend of the species formed exclusively in the gas (N₂D⁺ and N₂H⁺) and those formed partially (NH₂D and NH₃) or totally (CH₂DOH and CH₃OH) on grain mantles. It also reinforces the idea that D_frac(N₂H⁺) is the best tracer of massive starless cores, while high values of D_frac(CH₃OH) seem fairly good tracers of the early protostellar phases, where the evaporation or sputtering of the grain mantles is most efficient.