STATISTICAL STUDIES BASED ON SIMULTANEOUS SiO AND H sub(2)O MASER SURVEYS TOWARD EVOLVED STARS

Based on the simultaneous observational results of SiO and H sub(2)O masers toward 401 evolved stars, we have performed statistical analyses. We find that the peak and integrated intensities of SiO masers are stronger than those of H sub(2)O masers in both Mira variables and OH/IR stars at most optical phases. This result may also be related to the differences in the maser location and pumping mechanism between H sub(2)O and SiO masers. The mean velocity shift of SiO and H sub(2)O masers with respect to the stellar velocity was investigated as a function of optical phase. The velocity shift of the H sub(2)O masers shows that the redshifted emission dominates during the phases from 0.3-0.6, while the blueshifted emission appears at phase 0.6 and coexists with the redshifted emission during other phases. The distributions of all 401 observed sources are investigated in the IRAS two-color diagram in relation to the late evolutionary processes of asymptotic giant branch stars.