Phosphorus-bearing molecules in solar-type star forming regions: First PO detection

As part of the Large Program ASAI (Astrochemical Surveys At IRAM), we have used the IRAM 30m telescope to lead a systematic search for the emission of rotational transitions of P-bearing species between 80 and 350 GHz towards L1157-B1, a shock position in the solar-type star forming region L1157. We report the detection of several transitions of PN and, for the first time, of prebiotic molecule PO. None of these species are detected towards the driving protostar of the outflow L1157-mm. Analysis of the line profiles shows that PN arises from the outflow cavity, where SiO, a strong shock tracer, is produced. Radiative transfer analysis yields an abundance of 2.5e-9 and 0.9e-9 for PO and PN, respectively. These results imply a strong depletion (approx 100) of Phosphorus in the quiescent cloud gas. Shock modelling shows that atomic N plays a major role in the chemistry of PO and PN. The relative abundance of PO and PN brings constraints both on the duration of the pre-shock phase, which has to be about 1 Myr, and on the shock parameters. The maximum temperature in the shock has to be larger than 4000K, which implies a shock velocity of 40 km/s.