Carbon chemistry in the envelope of VY Canis Majoris: Implications for oxygen-rich evolved stars

Observations of the carbon-bearing molecules CO, HCN, CS, HNC, CN, and HCO⁺ have been conducted toward the circumstellar envelope of the oxygen-rich red supergiant star, VY Canis Majoris (VY CMa), using the Arizona Radio Observatory (ARO). CO and HCN were also observed toward the O-rich shells of NML Cyg, TX Cam, IK Tau, and W Hya. Rotational transitions of these species at 1 mm, 0.8 mm, and 0.4 mm were measured with the ARO Submillimeter Telescope, including the J = 6→5 line of CO at 691 GHz toward TX Cam and W Hya. The ARO 12 m was used for 2 mm and 3 mm observations. Four transitions were observed for HCO⁺ in VY CMa, the first definitive identification of this ion in a circumstellar envelope. Molecular line profiles from VY CMa are complex, indicating three separate outflows: a roughly spherical flow and separate red- and blueshifted winds, as suggested by earlier observations. Spectra from the other sources appear to trace a single outflow component. The line data were modeled with a radiative transfer code to establish molecular abundances relative to H₂ and source distributions. Abundances for CO derived for these objects vary over an order of magnitude, f ∼ 0.4-5 × 10^-4, with the lower values corresponding to the supergiants. For HCN, a similar range in abundance is found (f ∼ 0.9-9 × 10^-6), with no obvious dependence on the mass-loss rate. In VY CMa, HCO⁺ is present in all three outflows with f ∼ 0.4-1.6 × 10^-8 and a spatial extent similar to that of CO. HNC is found only in the red- and blueshifted components with [HCN]/[HNC] ∼ 150-190, while [CN]/[HCN] ∼ 0.01 in the spherical flow. All three velocity components are traced in CS, which has a confined spatial distribution and f ∼ 2-6 × 10^-7. These observations suggest that carbon-bearing molecules in O-rich shells are produced by a combination of photospheric shocks and photochemistry. Shocks may play a more prominent role in the supergiants because of their macroturbulent velocities.