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A.H. Straughn (University of Arkansas), R.B. Phillips, C.J. Lonsdale, S.S. Doeleman (MIT Haystack Observatory)
Study of silicon monoxide (SiO) masers surrounding evolved stars provides a unique probe of extended stellar atmospheres. On April 27, 2001, simultaneous 3mm and 7mm VLBI observations were successfully made of SiO masers surrounding the long-period variable red giant R Cassiopeiae (R~Cas). Though R~Cas was reaching an SiO minimum, partial rings were detected in both the v=1, J=2 arrow 1 (3mm) and J=1 arrow 0 (7mm) transitions, as well as the v=2, J=1 arrow 0 (7mm) transition. For the first time, the bright v=1 masers of an evolved star at 3mm and 7mm were spatially registered on the sky. Spatial registration of these transitions permits direct computation of line ratios for the 3mm and 7mm v=1 masers. These line ratios vary by more than two orders of magnitude depending on location in the ring, and may prove to be a powerful constraint on the physical state and degree of mixing in the masing gas.
We discuss imaging techniques along with implications of the resulting images: comparison of the two v=1 images reveals a picture different than that predicted by models. Finally, an arc-like `cap' of masers in one transition shows highly ordered radial velocities that suggest rotation of at least a portion of the maser shell, which contrasts with recent 3mm VLBI images of R~Cas showing only localized or chaotic velocity structure.
This research was carried out under the Research Experiences for Undergraduates (REU) Program at Haystack. The REU program is sponsored by the National Science Foundation.