Decrease of the organic deuteration during the evolution of Sun-like protostars: the case of SVS13-A

Abstract

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, D<SUB>2</SUB>CO, CHD<SUB>2</SUB>OH, CH<SUB>3</SUB>OD) with E<SUB>up</SUB> up to 276 K. The formaldehyde analysis indicates T<SUB>kin</SUB> ∼ 15-30 K, n_H_2 \ge 10<SUP>6</SUP> cm<SUP>-3</SUP> 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<SUB>kin</SUB> ∼ 80 K) and very dense (>10<SUP>8</SUP> cm<SUP>-3</SUP>) gas from a hot corino (radius ≃ 35 au), and (II) a colder (T<SUB>kin</SUB> ≤ 70 K) and more extended (radius ≃ 350 au) region. The deuterium fractionation is 9 × 10<SUP>-2</SUP> for HDCO, 4 × 10<SUP>-3</SUP> for D<SUB>2</SUB>CO and 2-7 × 10<SUP>-3</SUP> for CH<SUB>2</SUB>DOH, up to two orders of magnitude lower than the values measured in Class 0 sources. We also derive formaldehyde deuteration in the outflow: 4 × 10<SUP>-3</SUP>, in agreement with what found in the L1157-B1 protostellar shock. Finally, we estimate [CH<SUB>2</SUB>DOH]/[CH<SUB>3</SUB>OD] ≃ 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.