  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, Dfrac) 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. In order 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, CH3OD at 3 and
1.3~mm, and of NH2D at 3~mm with the IRAM-30m telescope, and the inversion
transitions (1,1) and (2,2) of NH3 with the GBT, towards most of the cores
already observed by Fontani et al.~(2011, 2014) in N2H+, N2D+, HNC, DNC. NH2D
is detected in all but two cores, regardless of the evolutionary stage.
Dfrac(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 only towards protostellar cores or externally heated
starless cores. 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.
The study also reinforces the idea that Dfrac(N2H+) is the best tracer of
massive starless cores, while high values of Dfrac(CH3OH) seem rather good
tracers of the early protostellar phases, at which the evaporation/sputtering
of the grain mantles is most efficient.
