Tentative detection of phosphine in IRC+10216

Aims. The JK = 10−0_0 rotational transition of phosphine (PH_3) at 267 GHz has been tentatively identified with a TMB ~ 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 PH_3 abundance relative to H_2 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 PH_3) together take up about 5% of phosphorus in IRC +10216. The abundance of PH_3, 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