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 T-kin similar to 15-30 K, H-n(2) >= 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 (T-kin similar to 80 K) and very dense (> 10(8) cm(-3)) gas from a hot corino ( radius similar or equal to 35 au), and (ii) a colder (T-kin <= 70 K) and more extended (radius similar or equal to 350 au) region. The deuterium fractionation is 9 x 10(-2) for HDCO, 4 x 10(-3) for D2CO and 2-7 x 10(-3) for CH2DOH, up to two orders of magnitude lower than the values measured in Class 0 sources. We also derive formaldehyde deuteration in the outflow: 4 x 10(-3), in agreement with what found in the L1157- B1 protostellar shock. Finally, we estimate [CH2DOH]/[CH3OD] similar or equal to 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 envelope. The present measurements fill in the gap between pre-stellar cores and protoplanetary discs in the context of organic deuteration measurements.