We have monitored the SiO ( v = 1 , { \mbox { J } } = 2 \rightarrow 1 ) maser polarization in 17 variable stars ( Miras , OH-IR stars , and supergiants ) to investigate the long-term persistence of masers .
The 8 epochs of observations span 2.5 years , thereby sampling multiple cycles for these stars with typical periods of \sim 1 year .
The average polarization was 23 % with a typical dispersion of 7 % , although the variability differed substantially from star to star .
In the Stokes q and u spectra of individual stars , a few strong maser features tended to dominate the polarization , with the maser features persisting for less than one stellar cycle for some stars , and for multiple cycles for a few stars .
Because individual masers are not resolved in our beam-averaged total intensity spectra , we correlated the polarization spectra between epochs to measure the characteristic lifetimes of the features , rather than attempting to trace the evolution of separate line components .
We found that individual maser feature lifetimes ranged from a few months or less to > 2 years .
These data indicate that for the sample of stars as a whole , the masers are not reset at a particular stellar phase .