Infrared space observatory short wavelength spectrometer observations of H II regions in NGC 6822 and I Zw 36: Sulfur abundances and temperature fluctuations

We report Infrared Space Observatory (ISO) Short Wavelength Spectrometer infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Brα, [S III] at 18.7 and 33.5 μm, and [S IV] at 10.5 μm are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S⁺³ abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S⁺³ abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] λ9532 or λ9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method.