CHAOS VIII: Far-Ultraviolet Spectra of M101 and The Impact of Wolf-Rayet Stars

TL;DR

This study analyzes the far-ultraviolet and optical spectra of H II regions in M101 to understand stellar populations, nebular properties, and the impact of Wolf-Rayet stars, revealing their significant role in emission features and metallicity estimates.

Contribution

It introduces new analytic functions for carbon ICF based on metallicity and highlights the importance of including Wolf-Rayet stars in stellar population models for accurate characterization.

Findings

Detection of nebular C III] emission in 7 regions

Young, hot, metal-enriched stars drive broad He II and WR features

WR stars influence metallicity measurements and stellar continuum fits

Abstract

We investigate the stellar and nebular properties of 9 H II regions in the spiral galaxy M101 with far-ultraviolet (FUV; ~900-2000 \r{A}) and optical (~3200-10000 \r{A}) spectra. We detect significant C III] 1907,1909 nebular emission in 7 regions, but O III] 1666 only in the lowest-metallicity region. We produce new analytic functions of the carbon ICF as a function of metallicity in order to perform a preliminary C/O abundance analysis. The FUV spectra also contain numerous stellar emission and P-Cygni features that we fit with luminosity-weighted combinations of single-burst Starburst99 and BPASS models. We find that the best-fit Starburst99 models closely match the observed very-high-ionization P-Cygni features, requiring very-hot, young (~< 3 Myr), metal-enriched massive stars. The youngest stellar populations are strongly correlated with broad He II emission, nitrogen Wolf-Rayet (WR) FUV and optical spectral features, and enhanced N/O gas abundances. Thus, the short-lived WR phase may be driving excess emission in several N P-Cygni wind features (955 \r{A}, 991 \r{A}, 1720 \r{A}) that bias the stellar continuum fits to higher metallicities relative to the gas-phase metallicities. Accurate characterization of these H II regions requires additional inclusion of WR stars in the stellar population synthesis models. Our FUV spectra demonstrate that the ~900-1200 \r{A} FUV can provide a strong test-bed for future WR atmosphere and evolution models.