The Herschel and IRAM CHESS Spectral Surveys of the Protostellar Shock L1157-B1: Fossil Deuteration

Abstract

We present the first study of deuteration toward the protostellar shock L1157-B1, based on spectral surveys performed with the Herschel-HIFI and IRAM 30 m telescopes. The L1157 outflow is driven by a low-mass Class 0 protostar and is considered the prototype of the so-called chemically active outflows. The young (2000 yr), bright blueshifted bow shock, B1, is an ideal laboratory for studying the gas chemically enriched by the release of dust mantles due to the passage of a shock. A total of 12 emission lines (up to E _u = 63 K) of CH₂DOH, HDCO, and DCN are detected. In addition, two lines of NH₂D and HDO are tentatively reported. To estimate the deuteration, we also extracted from our spectral survey emission lines of non-deuterated isotopologues (¹³CH₃OH, H₂ ¹³CO, H¹³CN, H₂ ¹³CO, and NH₃). We infer higher deuteration fractions for CH₃OH (D/H = 0.2-2 × 10^-2) and H₂CO (5-8 × 10^-3) than for H₂O (0.4-2 × 10^-3), HCN (~10^-3), and ammonia (<=3 × 10^-2). The measurement of deuteration of water, formaldehyde, and methanol in L1157-B1 provides a fossil record of the gas before it was shocked by the jet driven by the protostar. A comparison with gas-grain models indicates that the gas passed through a low-density (<=10³ cm^-3) phase, during which the bulk of water ices formed, followed by a phase of increasing density, up to 3 × 10⁴ cm^-3, during which formaldehyde and methanol ices formed.