Phosphine ( PH _ { 3 } ) 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 PH _ { 3 } in IRC +10216 using the HIFI instrument on board H erschel Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA . , which definitively confirms the identification of PH _ { 3 } .
Radiative transfer calculations indicate that infrared pumping to excited vibrational states plays an important role in the excitation of PH _ { 3 } 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 PH _ { 3 } challenges chemical models , none of which offers a satisfactory formation scenario .
Although PH _ { 3 } 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 PH _ { 3 } outside the solar system , and a further step towards a better understanding of the chemistry of phosphorus in space .