2014ApJ...790L..27A

Phosphine (PH₃) was tentatively identified a few years ago in the carbon star envelopes IRC +10216 and CRL 2688 from observations of an emission line at 266.9 GHz attributable to the J = 1-0 rotational transition. We report the detection of the J = 2-1 rotational transition of PH₃ in IRC +10216 using the HIFI instrument on board Herschel, which definitively confirms the identification of PH₃. Radiative transfer calculations indicate that infrared pumping in excited vibrational states plays an important role in the excitation of PH₃in the envelope of IRC +10216, and that the observed lines are consistent with phosphine being formed anywhere between the star and 100 R_*from the star, with an abundance of 10^–8 relative to H₂. The detection of PH₃challenges chemical models, none of which offer a satisfactory formation scenario. Although PH₃holds just 2% of the total available phosphorus in IRC +10216, it is, together with HCP, one of the major gas phase carriers of phosphorus in the inner circumstellar layers, suggesting that it could also be an important phosphorus species in other astronomical environments. This is the first unambiguous detection of PH₃ outside the solar system, and is a further step toward a better understanding of the chemistry of phosphorus in space.