SiO maser sources in the outer disk of the Galaxy

The observations in the (SiO)-Si-28 J = 1-0 v = 1, J = 1-0 v = 2 lines and the (SiO)-Si-29 J = 1-0, v = 0 lines were simultaneously made toward 181 color-selected IRAS sources in the outer disk of the Galaxy (at longitudes between 90 degrees and 250 degrees). The (SiO)-Si-28 lines were detected in 63 sources (56 new) and the (SiO)-Si-29 line in 11 sources (9 new). Most of the detections are optically variable stars, mainly Mira-type variables and a few semiregular variables. The detection rate (35%) is much lower than that of the bulge survey performed by the same observational system-the 45 m telescope at Nobeyama Radio Observatory. The lower detection rate of SiO maser emission is partly attributed to the increase of contaminations by stars with C-rich circumstellar envelopes and young stellar objects in the outer disk sample. The kinematical implications of the observational results are also discussed. The luminosity distances to the SiO maser sources are estimated from their IRAS fluxes at 12 mu m and colors. By combining the SiO radial velocity and the luminosity distance, the rotation curve of the outer Galactic disk is found to be slightly falling within the Galactocentric distance between 8.5 and 12 kpc. The sources located within 20 degrees from the Galactic anticenter direction are used to check for a peculiar motion of the local standard of rest.