CONFIRMATION OF CIRCUMSTELLAR PHOSPHINE

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 PH3 in 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 H-2. The detection of PH3 challenges chemical models, none of which offer a satisfactory formation scenario. Although PH3 holds 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.