5 The Population I Wolf-Rayet nature of the primary light source in WR46

5.1 Wind versus disc(-wind) model

As mentioned in Paper I (Sect. 2), WR46 has recently been grouped together with three other objects (Steiner & Diaz 1998). Their group of so-called V Sge stars shows highly ionized ions like N V and O VI. Apart from the short-term variability, the group has been defined by the intensity of the He II 4686 emission line being larger than H$\beta$. The objects are suggested to be related to the Super Soft X-ray Sources (SSS) found in the Magellanic Clouds. The latter type of systems, all binaries, are considered to burn hydrogen steadily on the surface of a white dwarf, which is fuelled from an accretion disc at a near-Eddington rate (reviewed by Kahabka & van den Heuvel 1997). Association of WR46 with SSS had been suggested earlier by Niemela et al. (1995). Those authors argued that the emission-line spectrum is formed in a luminous accretion disc in an evolved binary system.

However, there is strong observational evidence that a strong wind is the dominant feature of WR46. First, two UV lines show a P-Cygni profile (Fig. 18, see also CSH). Second, our observations provide a wealth of time-delay effects, which are difficult to comprehend within the framework of a luminous disc, since the highest velocities would have to be produced at the inner edge of the disc. In contrast, the time-delays are easily reconciled as a result of a stratified atmosphere, where the pattern of variability travels outwards in an accelarating wind (Sect. 4.1). The apparent direct link between the continuum- and the line flux also indicates a common source of origin, as in the case of an optically thick wind.

In order for the disc-model to be consistent with these observations, it would be required to assume an optically thick wind from the disc. Such disc-winds are observed in the case of cataclysmic variables during nova-type outburst and thought to be driven by hydrogen surface burning (Kato & Hachisu 1994). Since WR46 does not show hydrogen (CSH; Hamann et al. 1995a; Massey & Conti 1981; note to the table by Smith et al. 1996 on marginal hydrogen presence can probably be disregarded, Smith private comunication), the donor star feeding the disc has to be a He-star, but without a WR wind. We consider this idea rather contrived, and conclude that the dominant light source in WR46 is a Population I WR star blowing its own stellar wind. Also CSH and MAB reached this conclusion before. Possible binary companions of the WR star will be discussed in Paper III.

5.2 Variability of WR46 within the WR Standard Model and the oxygen lines

CSH successfully applied their so-called "WR standard model'' to WR46. They were able to explain the whole UV to IR spectrum as a result of a rather low-density, spherically-symmetric, optically-thick WR stellar wind from a hot evolved star. We list the resulting fundamental parameters by CSH in Table4 together with the results of a similar study by Hamann & Koesterke (1998; hereafter HK). Finally, the high luminosity of WR46 is supported by the analysis by CSH of its interstellar lines. They derived a distance of $4\, \pm 1.5$ kpc. For a more elaborate discussion on the distance to the system, the reader is referred to Veen & Wieringa (2000).

The colour behaviour of the system (Paper I: red when bright and blue when faint) is related to its high temperature. The spectral energy distribution peaks in the ultraviolet. Calculations using the "standard model'' show that, if the temperature increases, at fixed bolometric luminosity, the visual continuum flux will decrease, as more flux will emerge in the EUV. Thus, the "peculiar'' colour behaviour is completely understandable.

The variability of the N V 4604/20 line is much stronger than that of the neighbouring He II 4686 line. This provides a nice illustration of its WR nature since the two fits by CSH (see their Fig.6) show that N V 4604/20 is more susceptible for changes in temperature and mass-loss rates. We conclude that the short-term variability of the photometry and the line fluxes indicates a varying local density distribution in the wind.

So far, we ignored one peculiarity that emerged from the analysis by CSH. In their modelling the oxygen abundance appears three times larger than for the two other weak-lined WNE stars (WR128 and WR152) in their study. CSH argued that the oxygen content is not due to imperfections of the model, but truly anomolous, since the stellar parameters of WR3 (Hamann et al. 1995a) are nearly identical to those of WR46 and it does not show the O VI3811/34 line. However, analyses of other stars by one of us (PAC) have shown that the O VI 3811/34-doublet is more susceptible to low wind-density. Thus, a more proper abundance indicator is the ratio of O VI 5290 to He II 5411. Figure 19 shows these emission lines of WR3 (grey) scaled to those of WR46 with a factor of 2.5. Clearly, the ratio of these emission lines is very similar, thus the peculiar oxygen abundance of WR46 is probably not genuine.

As to the long-term variability, we can ascribe the brightening to the WR star itself, since the amplitude of the short-period photometric and line-flux variability increased despite the additional light. This is in accordance with the absence of any spectral feature from a companion. The increase in mean continuum- and mean line flux indicates an increase of the radius of the emission forming layers, which suggests a higher wind density. Since the P-Cygni absorption trough of O V 1371 became deeper without changing the edge-velocity (Fig.18), and the emission lines do not change their width, we assume that the velocity law remained constant and conclude that the mass-loss rate increased. Thus, the long-term variation of the photometry and the line fluxes indicate a varying global density of the wind, as already noted by MAB.

This correlation between brightness and mass-loss rate is in agreement with the relation found by Smith & Maeder (1998) from their analysis of the whole group of WNE stars (note, however, Howarth & Schmutz 1992). Schaerer & Maeder (1992), using a mass-radius relation, derive a luminosity-radius relation. Application of this relation translates the brightness increase of 12% between early 1989 and early 1991 into a radius increase of 4%. This suggestion is in line with the notion that the simultaneous increase of the amplitude of the variability (and small reddening) is interpreted as a radius increase of the various emission forming layers in the wind. Thus, this indicates that the period decrease and brightening was accompanied by a radius increase; an intriguing clue.

Figure 19: The emission lines of WR3 (grey) scaled by a factor of 2.5 to the spectrum of WR46 (black). Note that the difference with the value of 2.0 as determined by MAB can be attributed to the variability of WR46. Despite the absence of the O VI 3811/34 line from the spectrum of WR3, the comparable ratio of O VI 5290 over He II 5411 for both stars indicates a similar O/He abundance (Sect. 5.2) (spectra by courtesy of Dr. W.-R. Hamann).