3 Spectral classification

It is perhaps worthwhile to recall that spectral types have been, and are still, assigned to ${\rm W}-{\rm R}$ stars exclusively on the basis of the morphological properties of their emission line spectra, according to the classification scheme originally introduced by Beals (1938). Following this scheme, the ${\rm W}-{\rm R}$ stars are arranged into two parallel sequences: one in which the helium and nitrogen lines dominate the optical spectrum, the WN sequence; and another one in which, besides the helium lines, strong carbon and oxygen emissions are observed, the WC sequence. In the scheme adopted by the IAU Commission 29 (Beals 1938), ranging from the highest to the lowest excitation, only the subtypes WN5 to WN8 and WC6 to WC8 were considered. Subsequent systematic investigations of the spectra of ${\rm W}-{\rm R}$ stars in the Galaxy and the Magellanic Clouds (Hiltner & Schild 1966; Smith 1968a; Walborn 1974; Breysacher 1981; van der Hucht et al. 1981; Massey & Conti 1983b; Torres et al. 1986; van der Hucht et al. 1988; Conti & Massey 1989; Pakull 1991; Smith et al. 1994; Smith et al. 1996) led to a refinement of the ${\rm W}-{\rm R}$classification and to the inclusion of additional subtypes in both the WN and WC sequences which now extend from WN1 to WN11 and WC4 to WC11 respectively.

In order to distinguish, in the WC sequence, those of the ${\rm W}-{\rm R}$ stars which exhibit unusually strong oxygen lines, Barlow & Hummer (1982) have proposed to have a third sequence called WO. According to these authors, WO stars can be divided into four subtypes, WO1 to WO4, mainly defined by the relative strengths of the OIV $\lambda$3400, OV $\lambda$5590 and OVI $\lambda\lambda$3811, 3834 lines, the lowest excitation lines of oxygen being observed in the spectra of WO4 $\equiv$ WC4 stars. Rustamov & Cherepashchuk (1987) introduced later on the subtype WO5.

In the LMC, there are stars for which neither an Of- nor a WN-type unambiguously account for the observed spectral features. A fourth generic category designated as "Of/WN'' allows to separate these objects from the classical WN stars. The spectral type Ofpe/WN9, interpreted as an extension to later types of the WN sequence, was originally assigned by Bohannan & Walborn (1989) to ten LMC stars. Most of them have now been re-classified WN10 or WN11 by Crowther & Smith (1997). These stars are believed to represent the minimum or quiescent state of Luminous Blue Variables (Smith et al. 1998), hereafter LBVs. A few other LMC stars classified O3If$^\ast$/WN6-A (Walborn 1982a, 1986; Walborn & Blades 1997; Massey & Hunter 1998) also enter the Of/WN category.

With the advent of linear area detectors the quantification of the ${\rm W}-{\rm R}$classification system has been greatly facilited, allowing the determination of objective spectral types for WN2-8 stars (Smith et al. 1996), WN9-11 stars (Smith et al. 1994), WC4-9 stars (Smith et al. 1990b), WO1-5 stars (Kingsburgh et al. 1995), WC4-11 and WO1-4 (Crowther et al. 1998). The spectral types adopted in the present catalogue originate primarily from such quantitative studies; the most recent works being given preference. The three-dimensional scheme for WN stars introduced by Smith et al. (1996), and currently employed by others (e.g. Crowther & Smith 1997; Morgan 1999), is largely used. For those stars which are not yet classified according to these quantitative schemes, we have selected in the literature the best possible determined spectral types, or at least, estimated as such. Whenever available, other classification work references are provided in the catalogue.

In the classification of W - R stars, the question of companions still remains a delicate issue. Although the only secure method to establish the binary nature of a ${\rm W}-{\rm R}$ star is to search for radial velocity variations and/or eclipses, two other criteria, namely "small ratio of emission-line to continuum intensities'' and "presence of absorption lines'' are sometimes used too. Several detailed investigations of galactic ${\rm W}-{\rm R}$ stars (see Fitzpatrick 1982, and references quoted therein) having shown that these spectral characteristics are also exhibited by single ${\rm W}-{\rm R}$ stars, it is a bit hazardous to assume that ${\rm W}-{\rm R}$ stars of similar types necessarily have similar spectral appearance in the absence of a companion. Binarity deduced from the use of these criteria only is therefore doubtful. Although the LMC is a system seen nearly "face on'', and not much extended in depth (de Vaucouleurs & Freeman 1972), it is risky as well, to conclude that a ${\rm W}-{\rm R}$ star is a binary exclusively on the basis of its high luminosity, in view of the absolute-magnitude distribution obtained for the single WN- and WC-type stars in the LMC (Breysacher 1986).

In Fig. 1, we present spectra of LMC ${\rm W}-{\rm R}$ stars representative of the WNE, WNL (cf. Vanbeveren & Conti 1980) and WCE (cf. Torres et al. 1986) subclasses. The spectrograms were obtained, in 1980, with the Boller & Chivens spectrograph equipped with an Image Dissector Scanner, at the Cassegrain focus of the ESO 3.6 m telescope, at La Silla. The reciprocal dispersion used was 172 Å mm^-1. The main emission lines present in the observed spectral range ($4000-7500~{\mbox \AA}$) are identified.