Statistical studies based on simultaneous SiO and H2 O maser surveys toward evolved stars.
KIM J., CHO S.-H. and KIM S.J.
Abstract (from CDS):
Based on the simultaneous observational results of SiO and H2 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 H2O masers in both Mira variables and OH/IR stars at most optical phases. However, the relative intensity ratios of H2O to SiO masers in OH/IR stars are larger than those in Mira variables. Moreover, the intensity ratios of H2O to SiO masers are found to be increased around the optical phases from 0-0.4. The H2O photon luminosities also tend to be more dependent on the optical phase compared with those of SiO masers. These facts suggest that H2O masers are more sensitive to expanding the motion of circumstellar envelopes and also shock waves arising from the pulsations of the central star compared with SiO masers. This result may also be related to the differences in the maser location and pumping mechanism between H2O and SiO masers. The full width at zero power of SiO masers in Mira variables shows similar values to those of OH/IR stars, while those of H2 O masers in OH/IR stars show larger values than those of Mira variables. These differences may originate from the different mass-loss rates and the different location of the two masers. The mean velocity shift of SiO and H2O masers with respect to the stellar velocity was investigated as a function of optical phase. The velocity shift of the H2O 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. These features show an associated pattern with the CO ΔV = 3 radial velocity curve, which exhibits a typical pulsation motion. On the other hand, the velocity shift of the SiO v = 2 maser shows slightly similar features to the H2O maser, while that of SiO v = 1 does not show these similar features. Finally, 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.