Confirmation of circumstellar phosphine

TL;DR

This paper confirms the presence of phosphine (PH3) in the circumstellar envelope of IRC+10216 using Herschel observations, revealing its significant abundance and challenging existing chemical formation models.

Contribution

First definitive detection of PH3 outside the solar system, providing new insights into phosphorus chemistry in space and its formation in circumstellar environments.

Findings

PH3 detected via J=2-1 transition in IRC+10216

Infrared pumping influences PH3 excitation

PH3 accounts for 2% of phosphorus in the envelope

Abstract

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.