Sulfur Chemistry in the Envelope of Vy Canis Majoris: Detailed Analysis of So and So2emission

Detailed radiative transfer modeling has been carried out for SO 2 and SO originating in the envelope of the O-rich supergiant star VY Canis Majoris (VY CMa). A total of 27 transitions of SO 2 and 7 transitions of SO lying in the energy range 3.0–138.2 cm −1 were analyzed using a new non-LTE radiative transfer code that incorporates non-spherical geometries. The spectra were primarily obtained from the Arizona Radio Observatory (ARO) 1 mm spectral survey of VY CMa, conducted with the Submillimeter Telescope; additional lines were measured with the ARO 12 m antenna at 2 and 3 mm. SO 2 and SO were found to arise from five distinct outflows within the envelope, four which are asymmetric with respect to the star. Three flows arise from high-velocity red-shifted material, one from a blue-shifted wind, and the final from a classic "spherical" expansion. In the spherical component, the peak fractional abundance, relative to H 2 , of both molecules is f ∼ 2.5 × 10 −7 at r ∼ 25 R * , and steadily decreases outward. SO 2 appears to be a "parent" molecule, formed near the stellar photosphere. In the asymmetric outflows, both SO and SO 2 are more prominent at large stellar radii in dense (10 6 –10 7 cm −3 ), clumpy material, achieving their maximum abundance between 200 and 600 R * with f ∼ 3.0 × 10 −8 –1.5 × 10 −7 . These results suggest that in the collimated outflows, both species are either produced by shock chemistry or are remnant inner shell material swept up in the high-velocity winds.