AAS 201st Meeting, January, 2003
Session 115. Cool Star Atmospheres and Envelopes
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[115.10U] SiO masers: Three-Transition Movie for a Red Giant Atmosphere

L.M. Winter (Villanova University), R.B. Phillips, R.J. Crowley (MIT Haystack Observatory)

Silicon monoxide (SiO) masers are powerful tools in probing common yet unknown processes around red giant stars. R Cassiopeiae is a bright (mv 4.7 to 13.5) Mira-type long period variable, and one of the nearest (107 pc) to show strong SiO masers. It is unclear how stellar pulsations affect the SiO masers, which have variously been attributed to radiative, collisional, or combination pumping mechanisms.

We present simultaneous 3mm and 7mm VLBI observations of R Cas, taking advantage of a predicted SiO maximum. This work follows upon inaugural April 2001 simultaneous 3mm and 7mm VLBI observations of the 3mm and 7mm v=1 shells during stellar SiO minimum (Straughn et al. 2001). The new observations, consisting of three epochs spaced between late February and March 2002, provide a new, dynamic picture of the extended atmosphere of R Cas. Imaging of the three epochs, spaced approximately ten days apart, revealed rings in the v=1 and v=2, J=1 arrow 0 (7mm) transitions and partial rings in the v=1, J=2 arrow 1 (3mm) transition.

For the first time, all three bright transitions have been registered spatially, using an original algorithm, based on individual maser component correlations within velocity planes. This allowed for comparisons of changing behavior among transitions from epoch to epoch. Maser components towards the west end of the rings overlap in all transitions, but on the eastward side of the ring the masers come from concentric-appearing arcs in the sequence 7mm v=2, 7mm v=1 next, and 3mm v=1 outermost. The results are in partial agreement with the pulsation-driven model of Humphreys et al (2002), and show the potential actual VLBI measurements have as model inputs.

This research was supported by the NSF Research Experiences for Undergraduates Program at Haystack.

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