A SURVEY OF (SIO)-SI-30 EMISSION FROM EVOLVED STARS

We present the first detection of (SiO)-Si-30 upsilon = 1, J = 1-0 mas er emission toward the Mira variable TX Cam among 18 surveyed evolved stars. The line width of this maser is much narrower than that of (SiO )-Si-30 upsilon = 0, J = 1-0 masers and the intensity is also weaker, indicating that it is unsaturated. The detection of the (SiO)-Si-30 ma ser emission in the lowest rotational transition of the first vibratio nally excited state, together with nearly simultaneous observations of the large number of (SiO)-Si-29 and (SiO)-Si-28 masers, provides good constraints for establishing the SiO maser pumping model of TX Cam. T he global features of (SiO)-Si-30 and (SiO)-Si-29 upsilon = 1, J = 1-0 lines are similar, but detailed characteristics, such as their halfwi dths, their peak intensity ratios of the upsilon = 1, J = 1-0 line to the upsilon = 0, J = 1-0 line, and their intensity variation with time , are different. New detection of the (SiO)-Si-30 upsilon = 0, J = 1-0 and J = 2-1 lines is also reported in six evolved stars: TX Cam, R Ga s, chi Cyg, W Hya, U Lyn, and WX Psc. They show a different masing cha racteristics of the (SiO)-Si-30 that depends on both sources and trans itions. This is probably related to a critical condition for masing.