Laboratory and Astronomical Detection of the SiP Radical (X²Π_i ): More Circumstellar Phosphorus

The millimeter-wave spectrum of the SiP radical (X²Π_i) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH₄ in argon in an AC discharge. Fifteen rotational transitions (J + 1 ← J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151-533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J = 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J = 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300 R _*, and an abundance, relative to H₂, of f ∼ 2 × 10⁻⁹. From additional modeling, abundances of 7 × 10⁻⁹ and 9 × 10⁻¹⁰ were determined for CP and PN, respectively, both located in shells at 550-650 R _*. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.