SiO masers in stars in the inner and outer galactic disk

Abstract

Observations in the J=1--0, v=1 and v=2 ^{{28}} SiO and v=0 ^{{29}} SiO maser lines were made towards 97 outer- and 19 inner-disk IRAS sources with typical IRAS colors of AGB stars. 21 new ^{{28}} SiO and 1 new ^{{29}} SiO maser sources were detected above the 5 sigma level of ~ 1 Jy with the Nobeyama 45 m telescope. Collecting all of the observational data in SiO J=1--0 maser lines taken with a 45 m telescope, a comparison is made between the outer-disk, inner-disk, and bulge samples. The samples, themselves, align a sequence of mid-infrared color, flux at 12 mu m and IRAS variability index. The detection rates are 66%, 51%, and 31%, respectively, in the bulge, inner disk, and outer disk. This fact is consistent with the tendency of increasing proportion of C-rich stars in the IRAS sample and metallicity gradient with the galactocentric distance.