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 tow ard 181 color-selected IRAS sources in the outer disk of the Galaxy (a t longitudes between 90 degrees and 250 degrees). The (SiO)-Si-28 line s 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 detect ion rate (35%) is much lower than that of the bulge survey performed b y the same observational system-the 45 m telescope at Nobeyama Radio O bservatory. The lower detection rate of SiO maser emission is partly a ttributed to the increase of contaminations by stars with C-rich circu mstellar envelopes and young stellar objects in the outer disk sample. The kinematical implications of the observational results are also di scussed. The luminosity distances to the SiO maser sources are estimat ed 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 Galact ocentric distance between 8.5 and 12 kpc. The sources located within 2 0 degrees from the Galactic anticenter direction are used to check for a peculiar motion of the local standard of rest.