Deuteration and evolution in the massive star formation process The role of surface chemistry

Context. 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 highmass star formation process. However, the role of surface chemistry in these studies has not been quantified from an observational point of view. Aims. Because many abundant species, such as NH3, H2CO3 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 tor predominantly) in the gas, such as N2H+, HNC, HCN, and their deuterated isotopologues. The differences are expected to be ides ant especially after the protostellar birth, in which the temperature rises, causing the evaporation of ice mantles. Methods. 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, (CH3OH)-C-13, CH2DOH, and CH3OD at 3 mm and 1.3 mm, of NH2D at 3 mm with the IRAM-30 in 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. Results. 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 (similar to 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. Conclusions. 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.