UV Spectral Classification and Opacity in Magellanic Cloud OB Stars: Implications for Modeling Young Starbursts.

Details UV Spectral Classification and Opacity in Magellanic Cloud OB Stars: Implications for Modeling Young Starbursts. UV Spectral Classification and Opacity in Magellanic Cloud OB Stars: Implications for Modeling Young Starbursts.

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.0816s&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #08.16; Bulletin of the American Astronomical Society, Vol. 31, p.1383

Astronomy and Astrophysics

Astronomy

Scientific paper

Examination of archival Hubble Space Telescope (HST) spectra shows that the UV classifications of Magellanic Cloud O and B stars are consistent with those of Neubig and Bruhweiler (1997,1999) using International Ultraviolet Explorer (IUE) data. It also reveals previously unrecognized, strong UV line-blanketing by Fe-peak elements in two spectral regions. This is most noticable in SMC O stars with low extinction. There is a definite trough, producing a saddle-shaped continuum at 1350-1500 Angstroms in the earliest O stars. This absorption arises from Fe-peak elements with a large contribution from Fe V. Strong absorption at longer wavelengths, 1600-1650 Angstroms, in later O and early B supergiants is largely due to Fe IV. This absorption has been used in earlier classification studies of IUE spectra. The broad 1350-1500 Angstroms trough was not identified in low resolution IUE spectra because of limited signal-to-noise, and because the trough occurs at an inflection point of the O star UV continuum. Consequently, using lower S/N IUE data has led to an underestimation of the line-blanketing in both the 1350-1500 Angstroms and 1600-1650 Angstroms regions and systematic errors in determining continuum levels. The strength of this opacity seems not to be reproduced in theoretical model atmospheres. If this additional blanketing is not taken into account in population spectral synthesis fitting of galaxies with very young starbursts, it leads to deduced metallicities that are systematically too low. We acknowledge support through NASA grant NAG5-3378 to CUA.

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