TESTING THE CLUMP MODEL OF SIO MASER EMISSION

Building on the detection of the J=7-6 SiO maser emission in both the nu=1 and nu=2 vibrational states towards the symbiotic Mira R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes i n the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86 GH z, represent a test of the popular thermal-instability clump models of SiO masers. The 'clump' model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surrou ndings in the values of one or more physical variables (SiO abundance, for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J= 7-6 transitions coincides with an outward-moving shock impinging on the i nner edge of the maser zone.