Reevaluation of ALMA detection of circumstellar PH3 in the AGB envelope IRC+10216: evidence for misidentification with HCN

TL;DR

This paper reexamines ALMA data claiming to detect phosphine (PH3) in IRC+10216, demonstrating that the observed spectral feature is actually due to vibrationally excited HCN, thus questioning the previous PH3 detection.

Contribution

The study corrects a misidentification in prior work by showing the spectral feature attributed to PH3 is actually from vibrationally excited HCN, reaffirming the PH3 detection with independent observations.

Findings

The spectral feature is from vibrationally excited HCN, not PH3.

Previous PH3 detection is confirmed by single-dish telescope data.

Misidentification can occur in spectral line analysis of complex sources.

Abstract

The article "Confirmation of interstellar phosphine towards asymptotic giant branch star IRC+10216" by A. Manna and S. Pal uses ALMA data of the C-star envelope IRC+10216 to claim a confirmation of the detection of PH3 in this source. The article however incorrectly assign an emission feature observed in the ALMA spectrum of IRC+10216 to PH3, while we find that it arises from a highly vibrationally excited state of HCN. Concretely the feature can be confidently assigned to the J=3-2 l=0 transition of HCN in the v1+4v2 vibrational state based on the observation of the l=+2 and l=-2 components of the same rotational transition, J=3-2, with the observed relative intensities in agreement with the relative line strengths. The detection of PH3 in IRC+10216 remains confirmed based on the observation of the J=1-0 and J=2-1 lines with the single-dish telescopes IRAM-30m, ARO SMT-10m, and Herschel (Agundez et al. 2008, 2014; Tenenbaum et al. 2008).