The structure of stellar hydroxyl masers

Abstract

Results are presented for two spectral-line VLBI observations of 15 stellar OH masers conducted at frequencies of 1612 and 1667 MHz. The objects observed include IRC +10011, U Ori, R Aql, OH 26.5+0.6, OH 1821-12, W43 A, VX Sgr, OH 1837-05, and IRC +50137. It is found that the strongest 1612-MHz emission from Mira-type stars appears to originate from maser components with linear sizes exceeding 100 AU and brightness temperatures less than 100 million K, that the Mira OH masers have brightness temperatures about two orders of magnitude less than those of the 'supergiant' sources, and that OH masers associated with H II regions have sizes of about 10 to 1000 AU and brightness temperatures of 10 billion to 1 trillion K. The results obtained are used to evaluate models of the dynamics of the circumstellar material which involve expanding or contracting spherical shells, rotating disks, or shock-wave propagation.