Ionizing photons produced by massive stars in SMC-N88a

TL;DR

This study analyzes the ionizing photon output of four massive stars in the SMC-N88a H II region, showing they can produce enough photons to sustain the nebula's ionization, with detailed modeling of dust extinction and stellar properties.

Contribution

It provides a detailed spectral energy distribution analysis of the known massive stars in SMC-N88a, confirming their sufficiency in ionizing the nebula and refining dust extinction estimates.

Findings

The four stars are compatible with O-type stars with T_eff > 40 kK.

The total ionizing photon rate is log(Q_H) > 49.6 s^{-1}.

Dust extinction varies from 0.82 to 3.84 across sources.

Abstract

The H II region N88a in the Small Magellanic Cloud (SMC) is a spherical region of $1.5~\mathrm{pc}$ diameter, with high concentration of gas and dust, and at least four massive stars within it. Previous studies suggest that the four known sources may be insufficient to ionize the region and explain the nebular emission. In this contribution we analyze the ionizing photon production of the four known sources within the H II region. We compared the available photometry in the literature with the spectral energy distribution calculated for the ``Potsdam Wolf-Rayet'' (PoWR) models of massive stars for a wide range of dust extinction coefficient (Av). In particular, we selected models of OB-type atmospheres with SMC metallicity and compared the ionizing photon flux prediction with previous estimates based on the nebular emission of the H {\sc ii} region. We found that the Av values that best reproduce the photometry of each source vary from 0.82 to 3.84, increasing toward the dust band that runs through the nebula. In addition, all four sources are compatible with O-type stars with $\mathrm{T_{eff}} > 40 ~\mathrm{kK}$ and intrinsic ratios $\mathrm{F(LyC)/F(UV)}>1$. Lastly, the total ionizing photon rate predicted by the hottest models is $\mathrm{log(Q_H)} > 49.6 ~\mathrm{ph\ s}^{-1}$, which suggests that the stars could maintain the ionization state of the nebula.