Abstract
A model for the SiO maser radiation from Orion is constructed. Its main ingredients are as follows: The material around the source IRc2 is heated by turbulent motions. At a radius of approx.10/sup 14/ cm the temperature drops to approx.1500 K, and dust grains begin to condense. Radiation pressure on the newly formed grains drives the material outward, leading to a mass loss rate of approx.10/sup -3/ M/sub sun/ yr/sup -1/ at a final velocity of 18 km s/sup -1/, provided the dust is underabundant by about factor 10. The maser turns on when the expansion velocity exceeds the mean turbulent velocity, so the nature of the velocity field changes from chaotic to ordered, and the required coherence can build up. Because the dust is optically thick, the SiO vibration transitions are exposed to an undiluted blackbody radiation field, and the only way to deviate from thermal equilibrium is for the gas to be warmer than the dust. This requirement can be obeyed for a rather limited set of parameters which may explain the uniqueness of Orion as the only star formation region displaying an SiO maser.
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