PopStar Evolutionary Synthesis Models II: Optical emission-line spectra from Giant H{\sc ii} regions

TL;DR

This paper presents synthetic optical emission-line spectra from the PopStar models for HII regions, covering various ages, masses, and metallicities, including very low metallicity, and compares results with observations and previous models.

Contribution

It introduces new synthetic spectra for HII regions with a broader metallicity range and a physical model for region size, improving metallicity diagnostics.

Findings

[OIII] lines are weak in low metallicity nebulae, similar to high metallicity regions.

Oxygen lines alone cannot reliably distinguish metallicity levels.

Additional tracers like [SIII] lines are necessary for accurate metallicity assessment.

Abstract

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 H{\sc ii} regions photoionized by young star clusters, for seven values of cluster masses and for ages between 0.1 and 5.2 Myr. The ionizing Spectral Energy Distributions (SEDs) are those obtained by the PopStar code \citep*{mgb09} 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 H{\sc ii} 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 and data set of giant H{\sc ii} regions for which abundances have been derived in a homogeneous manner. The values of the [OIII] lines (at $\lambda\lambda$ 4363, 4959, 5007\AA) in the lowest metallicity nebulae are found to be very weak and similar to those coming from very high metallicity regions (solar or over-solar). 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-\textit{IR}.