THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 6, PAGE 2480           JUNE 1995

           LATE-TYPE STARS IN M31. I. PHOTOMETRIC STUDY OF AGB STARS
                           AND METALLICITY GRADIENTS

                     JAMES P. BREWER AND HARVEY B. RICHER

    Department of Geophysics and Astronomy, University of British Columbia,
             129-2219 Main Mall, Vancouver, B.C., V6T 1Z4, Canada
           Electronic mail: brewer@astro.ubc.ca, richer@astro.ubc.ca

                              DENNIS R. CRABTREE

    Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics,
               National Research Council, 5071 W. Saanich Road,
                         Victoria B.C. V8X 4M6, Canada
                     Electronic mail: crabtree@dao.nrc.ca


                                   ABSTRACT

    We have imaged five 7'x7' fields in M31 spanning galactocentric radii
    from 4 to 32 kpc along the SW semi-major axis.  The fields were
    observed through two broadband (V and I) and two narrowband (CN and
    TiO) filters.  The broadband data were used to construct I, (V-I)
    color-magnitude diagrams (CMDs) and, in some of our fields, we found
    significant numbers of stars in the Cepheid instability strip.  A
    distance modulus for the Cepheids in the middle field was found that
    agreed well with other values in the literature.  The width of the
    giant branch (GB) in the I, (V-I) CMD of all five fields was
    investigated, and we show that in four of the fields a likely
    explanation for the GB width is a combination of both metallicity and
    mass variations.  Using the broadband data, the asymptotic giant branch
    (AGB) luminosity functions (LFs) were measured in the five fields, and
    we show that differences exist between these LFs.  We speculate on how
    the different star-forming histories in the fields may lead to the
    observed AGB LFs and GB widths.  Using the narrowband data along with
    the broadband data we separated the AGB stars into carbon-rich (C) and
    oxygen-rich (M) types.  The carbon stars LFs were used to obtain an
    estimate for the distance modulus of M31 which agrees with the value
    derived from Cepheids.  The ratio of C- to M-stars (C/M) is believed
    to be an indicator of gaseous chemical abundance at the time of
    formation of these stars.  We show that the C/M ratio increases
    smoothly with galactocentric distance, suggesting an inverse
    correlation with metallicity.  This is the first demonstration of this
    effect within a single extragalactic system.  We find that differences
    in the width of the GB and the AGB LFs do not significantly affect the
    C/M ratio.  We consider the effect of the increasing C/M ratio on the
    ISM in M31, and cite evidence in favor of a model where the grain
    composition in M31 is a function of galactocentric distance.