Letter to the Editor

Tentative detection of phosphine in IRC +10216^*

¹ Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain e-mail: [marce;cerni;pardo]@damir.iem.csic.es
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d'Hères, and LERMA/École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France e-mail: guelin@iram.fr
³ California Institute of Technology, Downs Laboratory of Physics 320-47, Pasadena, CA 91125, USA e-mail: tgp@submm.caltech.edu

Received: 14 May 2008
Accepted: 27 May 2008

Abstract

Aims. The J_K = 1₀-0₀ rotational transition of phosphine (PH₃) at 267 GHz has been tentatively identified with a  ~ 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.