Decrease of the organic deuteration during the evolution of Sun-like protostars: the case of SVS13-A
Résumé
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_H_2 \ge 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 (Tkin ∼ 80 K) and very dense (>108 cm-3) gas from a hot corino (radius ≃ 35 au), and (II) a colder (Tkin ≤ 70 K) and more extended (radius ≃ 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 also derive formaldehyde deuteration in the outflow: 4 × 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.
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