THE SENSITIVITY OF CIRCUMSTELLAR MASERS TO DUST TYPE

The incidence of masers in oxygen-rich circumstellar shells is correla ted with their IRAS low-resolution spectral type (LRS). Thus, 67% of s hells with silicate emission features, and 27% of those without, have main line OH masers when they have a known water or SiO maser. This re sult does not depend on IR color. The generality of a dependence in th e incidence of masers on dust type is tested here by compiling statist ics from extant OH, water, and SiO observations. These show that these masers each have a similar dependence on dust type that is independen t of the IR colors in thin shells. The detection rate for water and fo r OH masers from LRS 21-25 type shells is intermediate between that of ''featureless'' (In) shells and that of shells with a stronger 9.7 mu m line, When the joint occurrence of water and main line masers is co nsidered, there is a factor of 5 difference between detection rates fr om the most disparate LRS types that is not reduced much by treating a ll OH masers together, or by limiting the sample to objects from the m ore 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 typ e, which is expected when the size of dust grains about most objects w ithout silicate features is <0.02 mu m, so UV scattering becomes impor tant. More of the ITV photons are then available to degrade molecules, which reduces their ability to support masers. These changes in the i ncidence of masers are postulated to result from an increase in the nu mber 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 i s, however, an artifact of a changing proportion of the various LRS ty pes with color.