Synthesis model in HII regions. II.

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 color indexes shows three distinct phases - an initial steady blue phase, followed by a red bump (5-15Myr) and another steady phase with color indexes intermediate to the earlier two phases. CSF color indexes asymptotically reach peak values at ~10Myr, never reaching the reddest IB color indexes. Ionizing photon production rate falls off by an order of magnitude in 6Myr for IB, whereas it almost remains constant for the CSF model.