2017MNRAS.467.3011B

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 E_up up to 276 K. The formaldehyde analysis indicates T_kin ∼ 15-30 K, n_ H₂ ≥10⁶ 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 ∼ 80 K) and very dense (>10⁸ cm^–3) gas from a hot corino (radius ≃ 35 au), and (ii) a colder (T_kin <= 70 K) and more extended (radius ≃ 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] ≃ 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.