Metallicity effects in the spectral classification of O-type stars. Theoretical

Nevena Markova$^1$, Luciana Bianchi$^2$, Boryana Efremova$^2$ and Joachim Puls$^3$

1-Institute of Astronomy, NAO, BAS,P.O.Box 136, 4700 Smolyan, Bulgaria;
2-Deptartment of Phys.& Astron., JHU,3400 N.Charles St., Baltimore, MD21218;
3-Universit"{a}ts-Sternwarte, Scheinerstrasse 1, D-81679 M"unchen, Germany

Based on an exteded grid of NLTE, line blanketed model atmospheres with stellar winds as calculated by means of FASTWIND, we have investigated the change in the strengths of strategic Helium transitions in the optical as caused by a 0.3 decrease in metallicity with respect to solar abundances. Our calculations predict that only part of the observed increase in Teff of O-type dwarfs could be explained by metallicity effects on the spectral type indicators, while the rest must be attributed to other reasons (e.g., different stellar structures as a function of metallicity or differences between observed and theoretical wind parameters etc.). In addition, we found that using the He~II~4686 line to classify stars in low metallicity environments (Z$le$ 0.3 solar) might artificially increase the number of low luminosity (dwarfs and giants) O-stars, on the expense of the number of O-supergiants.

Reference: To appear in the Bulgarian Astronomical Journal as part of the Proceeding of the 4th Conference of the Bulgarian Astronomical Society
Status: Conference proceedings