Single-Star HII Regions as a Probe of Massive Star Spectral Energy Distributions

TL;DR

This study uses single-star HII regions in the LMC to evaluate different stellar atmosphere models' ability to predict ionizing spectral energy distributions, finding WM-basic models best match observed nebular emission lines.

Contribution

It provides a quantitative comparison of atmosphere models against nebular observations, highlighting the accuracy of WM-basic and the potential for nebular data to calibrate stellar effective temperatures.

Findings

WM-basic models best reproduce observed line ratios

Systematic offset in ionizing photon rates linked to SED hardness

Effective temperatures from nebular data align with certain models

Abstract

The shape of the OB-star spectral energy distribution is a critical component in many diagnostics of the ISM and galaxy properties. We use single-star HII regions from the LMC to quantitatively examine the ionizing SEDs from widely available CoStar, TLUSTY, and WM-basic atmosphere grids. We evaluate the stellar atmosphere models by matching the emission-line spectra that they predict from CLOUDY photoionization simulations with those observed from the nebulae. The atmosphere models are able to reproduce the observed optical nebular line ratios, except at the highest energy transitions > 40 eV, assuming that the gas distribution is non-uniform. Overall we find that simulations using WM-basic produce the best agreement with the observed line ratios. The rate of ionizing photons produced by the model SEDs is consistent with the rate derived from the \Halpha\ luminosity for standard, log(g) = 4.0 models adopted from the atmosphere grids. However, there is a systematic offset between the rate of ionizing photons from different atmosphere models that is correlated with the relative hardness of the SEDs. In general WM-basic and TLUSTY atmosphere models predict similar effective temperatures, while CoStar predicts effective temperatures that are cooler by a few thousand degrees. We compare our effective temperatures, which depend on the nebular ionization balance, to conventional photospheric-based calibrations from the literature. We suggest that in the future, spectral type to effective temperature calibrations can be constructed from nebular data.