Phosphaethyne, HCP, has been detected in four successive transitions, J“ = 3 to 6. In addition, the signal-to-noise ration of the J = 2 – 1 is compatible with the intensities of the other transitions:
M. Agúndez, J. Cernicharo, and M. Guélin,
Discovery of Phosphaethyne (HCP) in Space: Phosphorus Chemistry in Circumstellar Envelopes
Astrophys. J. 662 (2007) L91−L94.
HCP is isolelectronic to HCN, and CP had long been detected toward IRC +10216. Therefore, it is somewhat surprising that the detection of this molecule has not been reported earlier. After all, it is quite abundant in this source ! Two reasons have to be considered: first, the dipole moment is fairly small, and second, it seems to be present not far out in the circumstellar envelope, where rotational temperatures are low, but rather in an intermediate region, where its lowest rotational levels are populated less than higher ones.

Contributor(s): H. S. P. Müller; 05, 2007