Calibrating Nebular Diagnostics of T and Abundance

Abstract

We obtained nebular spectroscopy of LMC H II regions having classified stellar populations, thereby strongly constraining the ionization input parameters. Using photoionization models, we then evaluate the performance of nebular diagnostics of T* and abundance. We introduce [Ne III]/Hβ as a nebular diagnostic of the ionizing stellar T*. In contrast to the widely-used η1 parameter, [Ne III]/Hβ has greater sensitivity to mid and early O-stars, and is robust to nebular morphology and the presence of shocks. We present a preliminary calibration of both T* diagnostics for LMC metallicity. We also introduce S234 ≡ ([S II] + [S III] + [S IV])/Hβ as a diagnostic of S abundance. S234 is much less sensitive to the nebular ionization parameter than is S 23 or R23. The intensity of [S IV] 10.5 μm is easily estimated from the optical and near-IR line ratios. We present calibrations of S23 and S234 that are reliable at metallicities Z ≲ 0:5Z⊙. Show more