Predictions of the extent of self-enrichment in oxygen of giant metal-poor HII regions

TL;DR

This study models the potential self-enrichment of oxygen in metal-poor HII regions by Wolf-Rayet star ejecta, finding that the maximum enrichment is too small to be observed, explaining the lack of clear signs of self-enrichment.

Contribution

The paper introduces a detailed simulation of oxygen self-enrichment in metal-poor HII regions considering stellar feedback and superbubble dynamics, revealing minimal observable enrichment.

Findings

Maximum oxygen enrichment of 0.025 dex at 4 Myr with no observable effect.

Self-enrichment levels are too small to detect in typical observations.

Modeling explains the absence of clear self-enrichment signs in HII regions.

Abstract

In general, HII regions do not show clear signs of self-enrichment in products from massive stars (M > 8 M_sun). In order to explore why, I modeled the contamination with Wolf-Rayet star ejecta of metal-poor (Z=0.001) HII regions, ionised either by a 10^6 M_sun cluster of coeval stars (cluster 1), or a cluster resulting from continuous star formation at a rate of 1 M_sun yr^-1 (cluster 2). The clusters have Z=0.001 and a Salpeter initial mass function (IMF) from 0.1 to 120 M_sun. Independent one dimensional constant density simulations of the emission-line spectra of unenriched HII regions were computed at the discrete ages 1, 2, 3, 4, and 5 Myr, with the photoionisation code CLOUDY, using as input, radiative and mechanical stellar feedbacks predicted by the evolutionary synthesis code STARBURST99. Each HII region was placed at the outer radius of the adiabatically expanding superbubble of Mac Low and McCray (1988). For models with thermal and ionisation balance time-scales of less than 1 Myr, and with oxygen emission-line ratios in agreement with observations, the interior of the superbubble and the HII region were uniformly and instantaneously polluted with stellar ejecta predicted by STARBURST99. I obtained a maximum oxygen abundance enhancement of 0.025 dex, with cluster 1, at 4 Myr. It would be unobservable.