THE ASTRONOMICAL JOURNAL      VOLUME 105, NUMBER 3, PAGE 894         MARCH 1993

       STAR FORMATION IN DWARF IRREGULAR GALAXIES: DDO 210 AND NGC 3109

                                  L. GREGGIO

           Dipartimento di Astronomia, Universita di Bologna, Italy

                                  G. MARCONI

Dipartimento di Astronomia, Universita di Bologna, Italy and European Southern
                  Observatory, Garching bei Muenchen, Germany

                                    M. TOSI

                   Osservatorio Astronomico, Bologna, Italy

                                  P. FOCARDI

           Dipartimento di Astronomia, Universita di Bologna, Italy


                                   ABSTRACT

    The method proposed in Paper I [Tosi et al., AJ, 102, 951 (1991)] to
    study the star formation histories in nearby irregulars is here applied
    to the galaxies DDO 210 and NGC 3109. To this purpose we have obtained
    deep CCD photometry of two regions in DDO 210 and three regions in NGC
    3109, reaching V = 24 with the required accuracy of Sigma(V) <= 0.1
    mag. Major information on the stellar populations, star formation
    rates, and initial mass functions of these regions in the last 1 Gyr
    are derived from the comparison of the observational color-magnitude
    diagrams and luminosity functions, with the corresponding synthetic
    diagrams and luminosity functions generated by a numerical simulation
    code based on stellar evolutionary tracks. In general, for both
    galaxies we find that in the last 1 Gyr the star formation activity has
    been rather continuous, possibly occurring in long episodes of moderate
    activity separated by short quiescent periods. According to our models
    the metallicity should be fairly low (in the range 0.05 <= Z/Zsun <=
    0.5) and the initial mass function exponent in the vicinity of
    Salpeter's value, i.e., slightly flatter than in the solar
    neighborhood. The studied regions of NGC 3109 appear to contain
    different stellar populations (i.e., with slightly different
    metallicity and star formation histories), whereas those of DDO 210
    seem to have fairly similar stellar contents, although for this galaxy
    the interpretation is more uncertain. As a by-product of our method we
    find that the distance modulus to DDO 210 should be (m-M)0 approx. 28,
    which places this galaxy beyond the edge of the Local Group.