THE ASTRONOMICAL JOURNAL    VOLUME 109, NUMBER 6, PAGE 2503           JUNE 1995

          EMBEDDED CLUSTERS IN GIANT EXTRAGALACTIC H II REGIONS. II.
                    EVOLUTIONARY POPULATION SYNTHESIS MODEL

                                  Y. D. MAYYA

      Indian Institute of Astrophysics, Bangalore 560 034, India and the
      Infra Red Astronomy Group, Tata Institute of Fundamental Research,
                Homi Bhabha Road, Colaba, Bombay 400 005, India
                   Electronic mail: ydm@tifrvax.tifr.res.in


                                   ABSTRACT

    A stellar population synthesis model, suitable for comparison with
    Giant Extragalactic H II Regions (GEHRs), is constructed incorporating
    the recent developments in modeling stellar evolution by Maeder and
    co-workers and stellar atmospheres by Kurucz.  A number of quantities
    suitable for comparison with broadband data of GEHRs in visible and
    near-infrared parts of the spectrum are synthesized in addition to the
    hydrogen and helium ionizing photon production rates at solar
    metallicities, for three scenarios of star formation -- (i)
    instantaneous burst (IB); (ii) continuous star formation (CSF); and
    (iii) two bursts of star formation, with the older burst rich in red
    supergiants.  For the IB case, evolution of colors shows three distinct
    phases -- an initial steady blue phase, followed by a red bump (5-15
    Myr) and another steady phase with colors intermediate to the earlier
    two phases.  CSF colors asymptotically reach peak values at ~10 Myr,
    never reaching the reddest IB colors.  Ionizing photon production rate
    falls off by an order of magnitude in 6 Myr for IB, whereas it almost
    remains constant for the CSF model.  Two-burst models with burst
    separations ~10 Myr have properties of both IB and CSF, simultaneously
    producing the red IB colors and high ionizing photon rate, making such
    regions easily distinguishable using optical observations.  Flat IMFs
    result in bluest colors when the massive stars are on the main sequence
    and reddest colors during the red supergiant phase of the evolving
    massive stars.  Errors on the computed quantities due to the
    statistical uncertainties inherent in the process of star formation
    become negligible for cluster masses in excess of 10^5 Msun.  Our GEHR
    spectra in the range 200 nm to 3 um are found to be in good agreement
    with the computations of Mas-Hesse & Kunth (1991, A&AS, 88, 399).