A photoionization modeling study of 30 Doradus: the case for small-scale chemical inhomogeneity

Photoionization models of the giant HII region 30 Doradus are built and confronted to available UV, optical, IR (ISO) and radio spectra, under black-body or CoStar SEDs for the primary source and various density distributions for the nebular gas. Chemically homogeneous models show very small rms electron temperature fluctuations and fail to reproduce the heavy element optical recombination line (ORL) spectrum of the nebula. Dual abundance models incorporating small-scale chemical inhomogeneities in the form of H-deficient inclusions which are in pressure balance with the normal composition ambient gas, provide a better fit to the observed heavy element ORLs and other nebular lines, while most spectral features are satisfactorily accounted for. The inclusions, whose mass is ~2 per cent of the total gaseous mass, are 2-3 times cooler and denser than the ambient nebula. Their O/H abundance ratio is ~0.9 dex larger than in the normal composition gas and have typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found to be about as deficient as hydrogen in the inclusions, while elements heavier than Ne, such as S and Ar, are quite possibly enhanced in proportions similar to O. This suggests that the posited H-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal LMC composition. Attention is drawn to a bias in the determination of HII region helium abundances in the presence of H-deficient inclusions. It is argued that these results provide evidence for incomplete small-scale mixing of the ISM. The case for the existence of abundance inhomogeneities in HII regions is examined in the light of current theoretical considerations regarding the process of homogenization in the ISM.