The Distribution of SiO in the Circumstellar Envelope around IRC +10216

Abstract

New interferometric observations of SiO J = 5 → 4 circumstellar line emission around the carbon star IRC +10216 using the Submillimeter Array are presented. Complemented by multitransition single-dish observations, including infrared observations of rovibrational transitions, detailed radiative transfer modeling suggests that the fractional abundance of SiO in the inner part of the envelope, between ≈3 and 8 stellar radii, is as high as ≈1.5 × 10-6. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (re ≈ 2.4 × 1016 cm) low fractional abundance (f0 ≈ 1.7 × 10-7) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind, while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2-5 on an angular scale corresponding to a timescale of about 200 yr. In addition, constraints on the velocity structure of the wind are obtained.