Tentative detection of phosphine in IRC +10216

Aims: The J_K = 10-00 rotational transition of phosphine (PH3) at 267 GHz has been tentatively identified with a T_MB ~ 40 mK spectral line observed with the IRAM 30-m telescope in the C-star envelope IRC +10216.
Methods: A radiative transfer model was used to fit the observed line profile.
Results: The derived PH3 abundance relative to H2 is 6 × 10^-9, although it may have a large uncertainty due to the lack of knowledge about the spatial distribution of this species. If our identification is correct, it implies that PH3 has a similar abundance to what is reported for HCP in this source and that these two molecules (HCP and PH3) together take up about 5% of phosphorus in IRC +10216. The abundance of PH3, like that of other hydrides in this source, is not well explained by conventional gas-phase LTE and non-LTE chemical models, and may imply formation on grain surfaces.

Based on observations carried out with the IRAM 30-m telescope and the Caltech Submillimeter Observatory (CSO). IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The CSO is operated by the California Institute of Technology under funding from the National Science Foundation, Grant No. AST-0540882.