A search for phosphine in circumstellar envelopes: PH₃ in IRC +10216 and CRL 2688?

We present the results of a search for the J_K = 1 _o→0_o transition of PH₃ (phosphine) at 267 GHz toward several circumstellar envelopes using the Arizona Radio Observatory 10 m Submillimeter Telescope (SMT). In the carbon-rich shells of IRC +10216 and CRL 2688, we have detected emission lines exactly at the PH₃ frequency. Toward the oxygen-rich supergiant VY Canis Majoris, only an upper limit was obtained, while in the evolved carbon-rich proto-planetary nebula CRL 618, the transition is contaminated by vibrational excited HC₃N (V₇ = 4). The line shape in IRC +10216 appears to consist of two distinct components: a flat-topped profile with a width of ∼28 km s _-1, as is typical for this source, and a narrower feature approximately 4 km s″1 wide. The narrow component likely arises from the inner envelope (r < 8R*) where the gas has not reached the terminal expansion velocity, or it is nonthermal emission. Based on the broader component, the abundance of PH₃ with respect to H₂ is estimated to be 5 x 10^-8 in a region with a radius of r< 150R*. If the narrower component is thermal, it implies a phosphine abundance of ~5 x 10^-7 close to the stellar photosphere (r< 8R*). In CRL 2688, the PH₃ abundance is less constrained, with plausible values ranging from 3 x 10 ^-8 to 4 x 10^-7, assuming a spherical distribution. Phosphine appears to be present in large concentrations in the inner envelope of C-rich AGB stars, and thus may function as a parent molecule for other phosphorus species.