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Astron. Astrophys. 334, 845-856 (1998)

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1. Introduction

In a previous paper on WC stars (Smith & Maeder 1991, Paper I), we have shown that the abundance ratio (C+O)/He is the primary parameter defining the classification of WC subtypes. On this basis it was possible to explain the luminosity-subclass relationship and the number statistics of WC subclasses in galaxies and regions of different metallicities.

Recently a three-dimensional spectroscopic classification for WN stars has been proposed by Smith et al. (1996, henceforward SSM96). Our aim is to determine the relationship between the three parameters of this classification and the theoretical quantities that describe stellar models. While stellar models lead to the expectation that a WN star is fully defined by its mass and remaining hydrogen content, the reality appears rather more complicated and we shall, in this paper, discuss in detail only the WN stars with no remaining hydrogen. Such stars are essentially helium-stars, with a few thousandths of a percent of nitrogen and a core containing a mixture of helium, carbon and oxygen. The models suggest that such stars form an essentially one-parameter family and that the present stellar mass is the key parameter (cf. Maeder 1983; Langer 1989a; Schaerer & Maeder 1992).

Sect. 2 compares the spectroscopic classification parameters with corresponding model parameters. Sect. 3 considers the effect of hydrogen content on [FORMULA] and [FORMULA] and identifies the WN b stars as the group that are entirely hydrogen free. Sect. 4 considers the relationships between EW 5411, FWHM 4686 and [FORMULA]. We discuss the degree to which the WN b stars appear to be a one-parameter family and the apparent need for a second parameter. Sect. 5 compares atmosphere and evolution models. Sect. 6 considers possible "second parameters" that might affect the terminal velocity of the wind. Sect. 7 presents our conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998