SiO isotopic maser emission from VY Canis Majoris

To investigate the peculiar line shape of the ground-state SiO emission from VY CMa, the (Si-28)O, (Si-29)O, and (Si-30)O emission profiles were observed. Strong narrow emission in the (Si-29)O spectrum was detected, twice the strength of the (Si-28)O narrow emission. No (Si-30)O emission was detected at the limit of the noise. The observations are difficult to interpret in terms of a spherically symmetric, expanding shell, but they can be explained by a model in which the inner envelope is not a shell but rather forms a rotating edge-on disk. The maser amplification would occur mainly in a radial direction since there is no radial velocity gradient. The center of the maser emission thus coincides with the velocity of the central star. The pumping radiation comes from the central star. The inversion is produced by the same eight-micron absorption mechanism as in the Robinson and Van Blerkom (1981) model, but here the reradiated photons escape in a direction perpendicular to the disk.