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R.B. Phillips (MIT Haystack Observatory), D.A. Boboltz (U.S. Naval Observatory), L.A. Winter (University of Maryland), A.H. Straughn (Arizona State University)
Silicon Monoxide (SiO) masers form at a few stellar radii in the extended atmospheres of evolved stars, the innermost of the commonly seen stellar maser species. As compact, high brightness-temperature regions that lie inside the dust forming radius, SiO masers as imaged by mm-wavelength Very Long Baseline Interferometry (mm-VLBI) have frequently been cited as probes of physical conditions at the photosphere-wind boundary.
Current SiO maser studies exploit the remarkable resolution of mm-VLBI and the responsiveness and fast duty cycles of the Very Long Baseline Array (VLBA) to make images in multiple transitions of SiO. Yet VLBI leaves the observer with a disjointed set of images, with the different excitation transitions generally not registered, even within the same epoch. We have explored and applied a variety of techniques intended for unbiased extraction of physical data from SiO maser images, and explain for two red giant case studies how simple mathematical approaches can register transitions and epochs and extract apparent maser proper motions from VLBA data.
In the case of Mira (o Ceti), we fit proper motions and attempt to register the sparse shells of v=1 and v=2 7mm masers. For R Cas, we register the two bright 7mm maser shells plus the v=1 3mm maser shell, and make the case for the passage of a pulsation shock re-invigorating the 3mm masers.
This research was part of the Research Experiences for Undergraduates (REU) Program at Haystack, with additional development carried out under Haystack's mission in mm-VLBI techniques. The REU program and mm-VLBI at Haystack are sponsored by the National Science Foundation.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.