Modeling Ionized Gas in the Small Magellanic Cloud: The Wolf–Rayet Nebula N76

Abstract

We present Cloudy modeling of infrared emission lines in the Wolf–Rayet (WR) nebula N76 caused by one of the most luminous and hottest WR stars in the low metallicity Small Magellanic Cloud. We use spatially resolved mid-infrared Spitzer/InfRared Spectrograph and far-infrared Herschel/PACS spectroscopy to establish the physical conditions of the ionized gas. The spatially resolved distribution of the emission allows us to constrain properties much more accurately than using spatially integrated quantities. We construct models with a range of constant hydrogen densities between n _H = 4–10 cm ^−3 and a stellar wind-blown cavity of 10 pc, which reproduces the intensity and shape of most ionized gas emission lines, including the high ionization lines [O iv ] and [Ne v ], as well as [S iii ], [S iv ], [O iii ], and [Ne iii ]. Our models suggest that the majority of [Si ii ] emission (91%) is produced at the edge of the H ii region around the transition between ionized and atomic gas while very little of the [C ii ] (<5%) is associated with the ionized gas. The physical conditions of N76 are characterized by a hot HII region with a maximum electron temperature of T _e ∼ 24,000 K, electron densities that range from n _e ∼ 4 to 12 cm ^−3 , and high ionization parameters of $\mathrm{log}(U)\sim -1.15\ \mathrm{to}-\,1.77.$ By analyzing a low-metallicity WR nebula with a single ionization source, this work gives valuable insights into the impact WR stars have on the galaxy-integrated ionized gas properties in nearby dwarf galaxies.

Keywords

• Interstellar medium
• H II regions
• Small Magellanic Cloud
• Wolf-Rayet stars
• Photoionization