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Session 114 - Circumstellar Material & Nebulae.
Oral session, Saturday, January 10
We present an analysis of emission-line images of giant HII regions (GHRs) in the nearby Sc galaxy M33. The observations, obtained at the Apache Point Observatory 3.5m telescope, include H\alpha and H\beta as well as [SII]6717,6731 and [OIII]5007 images of seven GHRs and Pa\beta(1.28\mum), Pa\gamma(1.09\mum), and Br\gamma(2.17\mum) near-infrared hydrogen line images of the two largest GHRs, NGC 604 and NGC 595.
Various indices of stellar poplation within the GHRs can be measured with the present data. Hydrogen line luminosities and equivalent widths indicate the number of Lyman continuum photons the young stars are producing; [OIII]/H\beta estimates the excitation of the gas and thus the effective temperature of the central stars; and [SII]/H\alpha assesses the importance of shock excitation across the nebulae. In addition, ratios of near-infrared to optical hydrogen line intensities provide maps of differential reddening.
The structure of the GHRs of M33 can be seen in the present data down to a linear resolution of about 6 pc, allowing a study of the spatial variation of the population indices as well as integrated properties. Because the target GHRs span a wide range of metallicities, the effects of metal abundance on the population indices can be examined. In the GHRs where the IMF is known (NGC 604, NGC 595, and CC93; Hunter et al. (1996, ApJ, 456,174), Malumuth et al. (1996, AJ, 111, 1128), and Malumuth et al. (1996, The Interplay between Massive Star Formation, the ISM, and Galaxy Evolution, p. 485), respectively), the population indices can be correlated with the IMF of the central ionizing stars.
Program listing for Saturday