Galactic Abundance Gradients from Infrared Fine‐Structure Lines in Compact HiiRegions

Abstract

We present new observations of the [S III] 19 μm, [O III] 52 and 88 μm, and [N III] 57 μm lines toward 18 compact and ultracompact (UC) H II regions. These data were combined with data from the literature and high-resolution radio continuum maps to construct detailed statistical equilibrium and ionization equilibrium models of 34 compact H II regions located at galactocentric distances (D_G) 0-12 kpc. Our models simultaneously fitted the observed IR fine-structure lines and high-resolution radio continuum maps. Abundance gradients are found of the form [S/H] = (-4.45 ± 0.04) - (0.063 ± 0.006) D_G (kpc), [N/H] = (-3.58 ± 0.04) - (0.072 ± 0.006) D_G (kpc), and [O/H] = (-2.85 ± 0.06) - (0.064 ± 0.009) D_G (kpc), and we derive T_e = (4560 ± 220) + (390 ± 40) D_G (kpc). The T_e gradient is consistent with the T_e gradient determined independently via radio recombination lines (Afflerbach et al.). We observe no dependence of S/O, N/O, or T_eff on D_G. Gradients in N⁺⁺/O⁺⁺ and O⁺⁺/S⁺⁺ are observed in the sense of increasing ionization with increasing D_G. This is entirely consistent with the decreased line blanketing with increasing D_G required by the above abundance gradients. All three gradients are best fitted by a linear dependence on D_G. The abundances are consistent with production of sulfur, nitrogen, and oxygen by primary nucleosynthesis. Comparison with abundances in other galaxies implies a Hubble type between Sab and Sb for our Galaxy and an unbarred or mixed galactic structure (Vila-Costas & Edmunds). Our derived T_eff is 2000-10,000 K lower than T_eff expected from ZAMS stars of the same Lyman continuum flux (Panagia; Vacca et al.), probably owing to uncertainties in the UV flux of stellar models for E ≥ 35.1 eV, uncertainties in the luminosity-T_eff calibration, and/or ionization of H II regions by multiple stars in some sources.