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Issue A&A
Volume 419, Number 1, May III 2004
Page(s) 319 - 334
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20040074



A&A 419, 319-334 (2004)
DOI: 10.1051/0004-6361:20040074

Metallicity and the spectral energy distribution and spectral types of dwarf O-stars

M. R. Mokiem1, N. L. Martín-Hernández2, A. Lenorzer1, A. de Koter1 and  A. G. G. M. Tielens2, 3

1  Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2  Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3  SRON, National Institute for Space Reasearch, PO Box 800, 9700 AV Groningen, The Netherlands

(Received 18 November 2002 / Accepted 11 February 2004 )

Abstract
We present a systematic study of the effect of metallicity on the stellar spectral energy distribution (SED) of O main sequence (dwarf) stars, focussing on the hydrogen and helium ionizing continua, and on the optical and near-IR lines used for spectral classification. The spectra are based on non-LTE line blanketed atmosphere models with stellar winds calculated using the CMFGEN code of Hillier & Miller (1998). We draw the following conclusions. First, we find that the total number of Lyman photons emitted is almost independent of line blanketing effects and metallicity for a given effective temperature. This is because the flux that is blocked by the forest of metal lines at $\lambda < 600$ Å is redistributed mainly within the Lyman continuum. Second, the spectral type, as defined by the ratio of the equivalent widths of  $\ion{He}{i}$  $\lambda$4471 and  $\ion{He}{ii}$  $\lambda$4542, is shown to depend noticeably on the microturbulent velocity in the atmosphere, on metallicity and, within the luminosity class of dwarfs, on gravity. Third, we confirm the decrease in $T_{\rm eff}$ for a given spectral type due to the inclusion of line blanketing recently found by e.g. Martins et al. (2002). Finally, we find that the SED below ~ 450 Å is highly dependent on metallicity. This is reflected in the behaviour of nebular fine-structure line ratios such as [ $\ion{Ne}{iii}$]/[ $\ion{Ne}{ii}$] 15.5/12.8 and [ $\ion{Ar}{iii}$]/[ $\ion{Ar}{ii}$] 9.0/7.0  ${\rm\, \mu m}$. This dependence complicates the use of these nebular ratios as diagnostic tools for the effective temperature determination of the ionizing stars in $\ion{H}{ii}$ regions and for age dating of starburst regions in galaxies.


Key words: stars: atmospheres -- stars: early-type -- stars: fundamental parameters -- stars: abundances -- ISM: HII regions -- ISM: planetary nebulae: general

Offprint request: M. R. Mokiem, mokiem@science.uva.nl




© ESO 2004


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