Embedded Clusters in Giant Extragalactic H II Regions. II. Evolutionary Population Synthesis Model

Abstract

A stellar population synthesis model, suitable for comparison with Giant Extragalactic HII Regions (GEHRs), is constructed incorporating the recent developments in modelling stellar evolution by Maeder and co-workers and stellar atmospheres by Kurucz. A number of quantities suitable for comparison with broad band 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 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 $\sim 10$~Myr, never reaching the reddest IB colors. Ionizing photon production rate falls off by an order of magnitude in 6~Myr for IB, where as it almost remains constant for CSF model. Two-burst models with burst separations $\sim 10$~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.