Decrease of the organic deuteration during the evolution of Sun-like
  protostars: the case of SVS13-A
  We present the results of formaldehyde and methanol deuteration measurements
towards the Class I low-mass protostar SVS13-A, in the framework of the IRAM
30-m ASAI (Astrochemical Surveys At IRAM) project. We detected emission lines
of formaldehyde, methanol, and their deuterated forms (HDCO, D2CO, CHD2OH,
CH3OD) with Eup up to 276 K. The formaldehyde analysis indicates Tkin = 15 - 30
K, n (H2) >= 10^6 cm^-3, and a size of about 1200 AU suggesting an origin in
the protostellar envelope. For methanol we find two components: (i) a high
temperature (Tkin = 80 K) and very dense (> 10^8 cm^-3}) gas from a hot corino
(radius about 35 AU), and (ii) a colder Tkin <= 70 K) and more extended (radius
about 350 AU) region.
  The deuterium fractionation is 9 10^-2 for HDCO, 4 10^-3 for D2CO, and 2 - 7
10^-3 for CH2DOH, up to two orders of magnitude lower than the values measured
in Class 0 sources. We derive also formaldehyde deuteration in the outflow: 4
10^-3, in agreement with what found in the L1157-B1 protostellar shock.
Finally, we estimate [CH2DOH]/[CH3OD] about 2. The decrease of deuteration in
the Class I source SVS13-A with respect to Class 0 sources can be explained by
gas-phase processes. Alternatively, a lower deuteration could be the effect of
a gradual collapse of less deuterated external shells of the protostellar
evelope. The present measurements fill in the gap between prestellar cores and
protoplanetary disks in the context of organics deuteration measurements.
