Astronomy and Astrophysics – Astronomy
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..786l&link_type=abstract
Astrophysical Journal v.462, p.786
Astronomy and Astrophysics
Astronomy
7
Stars: Circumstellar Matter, Ism: Dust, Extinction, Infrared: Ism: Continuum, Masers
Scientific paper
The incidence of masers in oxygen-rich circumstellar shells is correlated with their IRAS low-resolution spectral type (LRS). Thus, 67% of shells with silicate emission features, and 27% of those without, have main line OH masers when they have a known water or SiO maser. This result does not depend on IR color. The generality of a dependence in the incidence of masers on dust type is tested here by compiling statistics from extant OH, water, and SiO observations. These show that these masers each have a similar dependence on dust type that is independent of the IR colors in thin shells. The detection rate for water and for OH masers from LRS 21-25 type shells is intermediate between that of "featureless" (1n) shells and that of shells with a stronger 9.7 μm line. When the joint occurrence of water and main line masers is considered, there is a factor of 5 difference between detection rates from the most disparate LRS types that is not reduced much by treating all OH masers together, or by limiting the sample to objects from the more sensitive OH searches of the Arecibo sky.
This LRS-type dependence of masers is caused by a change in the UV extinction of dust with type, which is expected when the size of dust grains about most objects without silicate features is <0.02 μm, so UV scattering becomes important. More of the UV photons are then available to degrade molecules, which reduces their ability to support masers. These changes in the incidence of masers are postulated to result from an increase in the number of large absorptive rather than scattering grains as the silicate feature strengthens: there is also some evidence for a UV wavelength dependence to the dust-grain extinction. The previously noted blue IR color sensitivity of both the water and OH main line detection rates is, however, an artifact of a changing proportion of the various LRS types with color.
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