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Circumstellar envelope structure of late-type stars
The arcsecond structure of OH maser clouds surrounding 20 Mira variablesand late-type supergiants has been measured using the VLA in a spectralline mode at 1612 MHz. Angular radii of the maser clouds range from<0arcsec.5 to 4arcsec and have been combined with phase lagmeasurements of the shell size to estimate distances for some of thestars. The outer radii range from 100 AU to 104AU, and theauthors estimate a ratio of outer to inner radii ⪉4 for the maserregions. The previously discovered correlation between the mass lossrate and the outer radius of the maser region is confirmed and used todetermine the mass loss rates for eight stars. The results of this workare compared with current theoretical models of circumstellar shells andthe implications for mass loss mechanisms and the evolutionary fate ofthese stars are considered. It is suggested that the rare cases ofnonspherical shells are indicating the presence of close binarycompanions.
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