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

Photoionization models of the giant HII region (GHIIR) 30 Doradus are built and confronted to available ultraviolet (UV), optical, infrared (IR) [Infrared Space Observatory (ISO)] and radio spectra, under blackbody or CoStar spectral energy distributions 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 hydrogen-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 ~2per cent of the total gaseous mass, are 2-3 times cooler and denser than the ambient nebula. Their O/H abundance ratio is ~0.9dex 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 neon, such as sulphur and argon, are quite possibly enhanced in proportions similar to oxygen, as indicated by the most satisfactory dual-abundance model obtained. This suggests that the posited hydrogen-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal Large Magellanic Cloud composition. The average gaseous abundances of the chemically inhomogeneous models are ~0.08dex higher than those of the homogeneous models, yet they are lower by a similar ~0.08dex than those derived from standard empirical methods (fully corrected for inaccuracies in ionization correction factors and differences in atomic data) which postulate temperature fluctuations in a chemically homogeneous medium. Attention is drawn to a bias in the determination of HII region (HIIR) helium abundances in the presence of hydrogen-deficient inclusions. It is argued that these results provide evidence for incomplete small-scale mixing of the interstellar medium (ISM). The case for the existence of abundance inhomogeneities in HIIRs is examined in the light of current theoretical considerations regarding the process of chemical homogenization in the ISM.