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Date and Time of the Query: 2019-06-18 T08:16:16 PDT
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For refcode 2004ApJ...606..237R:
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Copyright by American Astronomical Society. Reproduced by permission
2004ApJ...606..237R Missing Massive Stars in Starbursts: Stellar Temperature Diagnostics and the Initial Mass Function J. R. Rigby and G. H. Rieke Abstract. Determining the properties of starbursts requires spectral diagnostics of their ultraviolet radiation fields, to test whether very massive stars are present. We test several such diagnostics, using new models of line ratio behavior combining CLOUDY, Starburst99, and up-to-date spectral atlases. For six galaxies we obtain new measurements of He I 1.7 microns/Br10, a difficult to measure but physically simple (and therefore reliable) diagnostic. We obtain new measurements of He I 2.06 microns/Br{gamma} in five galaxies. We find that He I 2.06 microns/Br{gamma} and [O III]/H{beta} are generally unreliable diagnostics in starbursts. The heteronuclear and homonuclear mid-infrared line ratios (notably [Ne III] 15.6 microns/[Ne II] 12.8 microns) consistently agree with each other and with He I 1.7 microns/Br10; this argues that the mid-infrared line ratios are reliable diagnostics of spectral hardness. In a sample of 27 starbursts, [Ne III]/[Ne II] is significantly lower than model predictions for a Salpeter initial mass function (IMF) extending to 100 M_sun_. Plausible model alterations strengthen this conclusion. By contrast, the low-mass and low-metallicity galaxies II Zw 40 and NGC 5253 show relatively high neon line ratios, compatible with a Salpeter slope extending to at least ~40 - 60 M_sun_. One solution for the low neon line ratios in the high-metallicity starbursts would be that they are deficient in >~40 M_sun_ stars compared to a Salpeter IMF. An alternative explanation, which we prefer, is that massive stars in high-metallicity starbursts spend much of their lives embedded within ultracompact H II regions that prevent the near- and mid-infrared nebular lines from forming and escaping. This hypothesis has important consequences for starburst modeling and interpretation. Keywords: Galaxies: Starburst, Galaxies: Stellar Content, ISM: H II Regions, Infrared: Galaxies, Techniques: Spectroscopic
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