2010MNRAS.403.2012M

This is the second paper of a series reporting the results from the popstar evolutionary synthesis models. Here, we present synthetic emission-line spectra of HII regions photoionized by young star clusters, for seven values of cluster masses and for ages between 0.1 and 5.2Myr. The ionizing spectral energy distributions (SEDs) are those obtained by the popstar code for six different metallicities, with a very low-metallicity set, Z = 0.0001, not included in previous similar works. We assume that the radius of the HII region is the distance at which the ionized gas is deposited by the action of the mechanical energy of the winds and supernovae from the central ionizing young cluster. In this way, the ionization parameter is eliminated as free argument, since now its value is obtained from the cluster physical properties (mass, age and metallicity) and from the gaseous medium characteristics (density and abundances). We discuss our results and compare them with those from previous models and also with a large data set of giant HII regions for which abundances have been derived in a homogeneous manner. The values of the [Oiii] lines (at λλ 4363, 4959, 5007Å) in the lowest metallicity nebulae are found to be very weak and similar to those coming from very high-metallicity regions (solar or oversolar). Thus, the sole use of the oxygen lines is not enough to distinguish between very low and very high metallicity regions. In these cases, we emphasize the need of the additional support of alternative metallicity tracers, like the [Siii] lines in the near-infrared.