Hubble Space TelescopeObservations of the Dusty Small Magellanic Cloud HiiRegion N88A

Abstract

We present results from the analysis of Hubble Space Telescope Faint Object Spectrograph (FOS), WFPC1, IUE, and Cerro Tololo Inter-American Observatory 4 m observations of the morphology, physical conditions, and chemical abundances in the anomalous H II region N88A in the Small Magellanic Cloud. Not only is N88A unusual among SMC H II regions because it contains much dust, it also is found to have a high electron density and complex ionization structure. The derived reddening curve for the nebula is flatter in the UV than the general SMC extinction curve, suggesting the absence of small grains. A detailed abundance analysis, using both empirical emission-line diagnostics and photoionization model comparisons, indicates that carbon and silicon are enriched in the nebula, while He, O, N, Ne, and Ar are similar in abundance to other SMC H II regions. There is no evidence for the large fluctuations in temperature or density that are seen to occur in some comparably dense planetary nebulae, so we believe that our rather traditional analyses are well founded. We conclude that SMC N88A is a very young H II region forming out of a small dusty molecular cloud that is being disrupted by star formation in a larger OB association in the area. The high carbon and silicon abundances in N88A are attributed to photoevaporation of dust grains existing in the molecular cloud material—rather than being ejected from recent stellar mass loss from stars in N88A itself.