  Phosphine (PH3) was tentatively identified a few years ago in the carbon star
envelopes IRC+10216 and CRL2688 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 to 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 1e-8 relative to H2. The detection of PH3 challenges chemical
models, none of which offers a satisfactory formation scenario. Although PH3
locks 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 be also an important phosphorus
species in other astronomical environments. This is the first unambiguous
detection of PH3 outside the solar system, and a further step towards a better
understanding of the chemistry of phosphorus in space.
