Astronomy & Astrophysics

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Issue		A&A Volume 419, Number 1, May III 2004
Page(s)		319 - 334
Section		Stellar structure and evolution
DOI		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. Tielens^{2, 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 $\ion{i}$ $\lambda$ 4471 and $\ion{ii}$ $\lambda$ 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 [ $\ion{iii}$ ]/[ $\ion{ii}$ ] 15.5/12.8 and [ $\ion{iii}$ ]/[ $\ion{ii}$ ] 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

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