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R. Fourre (University of Colorado at Boulder, CO), P.R. Jewell (NRAO, Greenbank, WV), J. Glenn (University of Colorado at Boulder, CO)
Miras, supergiants, and OH-IR stars are long-period (80-1500 days) variable stars on the Asymptotic Giant Branch. They have carbon cores, helium and hydrogen burning shells, and highly convective atmospheres. In these extended atmospheres (~1 AU), conditions are often favorable for silicon monoxide (SiO) masing in rotational transitions, the microwave equivalent of lasing. Population inversion is maintained by a combination of infrared and collisional excitations of vibrational states. VLBA observations have shown that masing occurs in cells forming around the star.
It has been proposed that the masers are created anew each atmospheric pulsation cycle, persisting for one cycle. To investigate the relationship between maser variability and stellar cycle, the 86 GHz SiO (\(v=1\), \(J=2arrow1\)) transition in 17 evolved stars was observed during eight epochs over three years. The observations were made with a dual beam polarimeter and receiver (configured for linear polarimetry) at the NRAO 12 Meter Telescope on Kitt Peak. Each polarized beam was analyzed by a filter bank for Doppler shift measurements.
For each star and each epoch, the average fractional polarization and polarization position angle distribution were measured for each emission frequency. These spectra allowed the determination, for each source, of an average gas velocity dispersion (line width) and an average polarization feature lifetime, presumed to represent the average masing cell lifetime. Average gas velocity dispersions ranged from 0.33 to 0.67 km \(\textup{s}-1\), remarkably similar for all stars, and practically independent of stellar phase. Polarizations as high as 50% were observed, with substantial average polarization variability from star to star (4 to 40%). By modeling the polarization variability of discrete spectral features, lifetimes ranging from 30 to 430 days were derived. The maser lifetimes showed no correlation to the stellar periods or phases, indicating that maser generation is not related to atmospheric pulsation in a simple way.