THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 106:463-488, 1996 October

               SiO MASER SOURCES IN THE OUTER DISK OF THE GALAXY

                                  B. W. JIANG
   Department of Astronomical Science, The Graduate University for Advanced
    Studies, Nobeyama Radio Observatory, National Astronomical Observatory,
                 Minamimaki, Minamisaku, Nagano 384-13, Japan

                                  S. DEGUCHI
        Nobeyama Radio Observatory, National Astronomical Observatory,
                 Minamimaki, Minamisaku, Nagano 384-13, Japan

                                  I. YAMAMURA
    Department of Astronomy, University of Tokyo, Bunkyo, Tokyo 113, Japan

                                   Y. NAKADA
         Kiso Observatory, Institute of Astronomy, Faculty of Science,
            University of Tokyo, Mitake, Kiso, Nagano 397-01, Japan

                                   S. H. CHO
  Korea Astronomy Observatory, San 36-1, Whaam, Yusong, Daejon 305-348, Korea

                                      AND

                                  T. YAMAGATA
  Elementary and Secondary Education Bureau, Ministry of Education, Science,
          and Culture, 3-2-2, Kasumigaseki, Chiyoda, Tokyo 100, Japan


                                    ABSTRACT

        The observations in the 28SiO J = 1-0 v = 1, J = 1-0 v = 2 lines
     and the 29SiO 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 deg and 250 deg).  The 28SiO lines were detected
     in 63 sources (56 new) and the 29SiO 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 um 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 deg from the Galactic anticenter direction
     are used to check for a peculiar motion of the local standard of rest.

     Subject headings: Galaxy: kinematics and dynamics -- ISM: molecules --
                       masers -- stars: variables: other (long-period
                       variables)