Sulfur chemistry in the envelope of VY Canis Majoris: Detailed analysis of so and SO₂ emission

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 SO₂ 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. SO₂ 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 H₂, of both molecules is f ∼ 2.5 × 10^-7 at r ∼ 25 R _*, and steadily decreases outward. SO₂ appears to be a "parent" molecule, formed near the stellar photosphere. In the asymmetric outflows, both SO and SO₂ are more prominent at large stellar radii in dense (10⁶-10⁷ 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.