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

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

Corresponding author: M. R. Mokiem, mokiem@science.uva.nl

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 [CITE]. 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  Å is redistributed mainly within the Lyman continuum. Second, the spectral type, as defined by the ratio of the equivalent widths of  λ4471 and  λ4542, 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_eff for a given spectral type due to the inclusion of line blanketing recently found by e.g. [CITE]. 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 []/[] 15.5/12.8 and []/[] 9.0/7.0 . This dependence complicates the use of these nebular ratios as diagnostic tools for the effective temperature determination of the ionizing stars in  regions and for age dating of starburst regions in galaxies.