2008ApJ...680L.121T

We present the results of a search for the J_K=1₀⟶0₀transition 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 PH₃frequency. 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 vibrationally excited HC₃ N (ν₇= 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, as is typical for this source, and a narrower feature approximately 4 km/s 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 H₂is estimated to be 5x10^–8 in a region with a radius of r<150R_*. If the narrower component is thermal, it implies a phosphine abundance of ∼5x10^–7 close to the stellar photosphere (r<8R_*). In CRL 2688, the PH₃ abundance is less constrained, with plausible values ranging from 3x10^ -8^ to 4x10^–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.