Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66

TL;DR

This study uses mid-infrared observations to analyze the chemical abundances and ionization properties of three giant HII regions, revealing homogeneous ISM conditions and elemental abundance ratios consistent with optical data.

Contribution

It provides detailed ionic and elemental abundance measurements in giant HII regions using MIR spectroscopy, highlighting the ionization structure and dust depletion effects.

Findings

MIR observations reveal higher ionization levels than optical spectra.

Ne, S, and Ar abundances scale with each other.

Small dispersion in Ne and S abundances indicates a homogeneous ISM.

Abstract

We investigate the chemical abundances of NGC3603 in the Milky Way, of 30Doradus in the Large Magellanic Cloud, and of N66 in the Small Magellanic Cloud. Mid-infrared observations with the Infrared Spectrograph onboard the Spitzer Space Telescope allow us to probe the properties of distinct physical regions within each object: the central ionizing cluster, the surrounding ionized gas, photodissociation regions, and buried stellar clusters. We detect [SIII], [SIV], [ArIII], [NeII], [NeIII], [FeII], and [FeIII] lines and derive the ionic abundances. Based on the ionic abundance ratio (NeIII/H)/(SIII/H), we find that the gas observed in the MIR is characterized by a higher degree of ionization than the gas observed in the optical spectra. We compute the elemental abundances of Ne, S, Ar, and Fe. We find that the alpha-elements Ne, S, and Ar scale with each other. Our determinations agree well with the abundances derived from the optical. The Ne/S ratio is higher than the solar value in the three giant HII regions and points toward a moderate depletion of sulfur on dust grains. We find that the neon and sulfur abundances display a remarkably small dispersion (0.11dex in 15 positions in 30Doradus), suggesting a relatively homogeneous ISM, even though small-scale mixing cannot be ruled out.