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

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.

💡 Research Summary

The paper addresses a long‑standing puzzle in extragalactic astronomy: why giant, metal‑poor H II regions rarely exhibit observable signatures of self‑enrichment from the ejecta of massive stars (M > 8 M☉). To investigate this, the author constructs two idealised star‑forming environments at a metallicity of Z = 0.001. The first (Cluster 1) is a coeval population containing 10⁶ M☉ of stars, while the second (Cluster 2) represents continuous star formation at a rate of 1 M☉ yr⁻¹. Both adopt a Salpeter initial mass function from 0.1 to 120 M☉.

For each cluster, the evolutionary synthesis code STARBURST99 provides age‑dependent ionising photon rates, mechanical luminosities, and the yields of Wolf‑Rayet (WR) winds and core‑collapse supernovae. The author selects discrete ages of 1, 2, 3, 4, and 5 Myr and feeds the resulting stellar feedback into the photo‑ionisation code CLOUDY. The H II region is placed at the outer radius of an adiabatically expanding superbubble, following the analytical solution of Mac Low & McCray (1988). The superbubble interior is assumed to be hot, low‑density gas, while the surrounding shell hosts a constant‑density (≈100 cm⁻³), 10⁴ K ionised nebula.

Only models that satisfy two criteria are retained: (i) the thermal and ionisation balance timescales must be shorter than 1 Myr, ensuring the nebular conditions are quasi‑steady, and (ii) the predicted strong‑line oxygen ratios (e.g.,