6 Summary and discussion

First of all, we note that WR46 obviously varies on a variety of time scales.The available spectroscopic monitoring data are hardly sufficient to investigate its behaviour in detail. Nevertheless, the variability can be summarized as follows:

1.

The line-flux variability mimics the continuum flux variability on the short-term time scale (see also MAB).

2.

The line-flux variability has larger relative amplitude than the continuum flux variability.

3.

The line-flux variability confirms the large photometric period change between 1989 and 1991.

4.

The radial velocity shows (i) obvious variation on a time scale of the photometric double-wave, and (ii) so-called stand-stills (see also MAB).

5.

Time-delays are observed from line-to-line in the case of (i) the line-flux variability, (ii) the radial-velocity variation and (iii) the entry of a stand-still, and also (iv) within a single emission line, depending on the height above the continuum. The higher (part of) an emission line is formed in the atmosphere, the larger the time-delay, which is in accordance with the stratified atmosphere of a WR star.

6.

He II 4686 shows, in addition to line-flux and radial-velocity variation, significant line-profile variability on a time scale of the same order as the identified periods.

7.

In a total of 17 hours of monitoring, we observed one flare-like event, i.e., in two He II emission lines a "bump'' occurred lasting at most 5 min. And

8.

On the long-term time scale of years the line flux follows the photometric behaviour from faint to bright states, which implies that the WR star itself is brightening instead of light from a possible companion.

We conclude that both the short-term and the long-term variabilities identify WR46 as a Population I WR star, for which the peculiar oxygen abundance is probably not genuine. The short-term variation of the photometry and the line fluxes indicate a varying local density distribution, while the long-term variation is ascribed to a varying global density distribution due to changing mass-loss rate. Thus, despite the coarse resemblance of spectral features and short-term variability, WR46 has wrongly been suggested to be related to Super Soft X-ray Sources. The following discussion of various aspects of the observed variability may help to understand the cause of the variability.

Time-delays are intrinsic to spectroscopically variable WR stars and studied best in the case of binarity but not limited to binaries alone. An example is provided by the eclipsing WR binary WR151 (CX Cep, WN4+O5V, P=2.13 d). Lewis et al. (1993) showed that the outer wind lines trail the orbital motion of the inner wind lines. In addition, these authors found a phase-delay up to 0.04 phase (=0.08 d) between the centroid measurements of nearly the whole line relative to the peak of the line. No explanation was put forward. We, here, suggest that the wings of the emission lines are formed more outwards in the wind, and, therefore, trail the central parts of the lines, originating in the inner wind.

Since we cannot determine the period from the radial-velocity measurements, its period may be different from that controlling the photometry, and, probably, the line fluxes. Of course, this would explain the apparent "phase-delays'', simply as different phases of different variations, but not the stand-stills. We proposed that the photometric double-wave period controls the light-, the colour-, the line-flux, and the radial-velocity curves, while the last is sometimes disturbed beyond recognition (stand-still) due to the large time-delays. Support was provided by the single observation of the start of a stand still. It was preceded by a radial motion half a period out of phase with respect to the presumed motion of the stellar core. We assume that the phase-delay between the stellar core and the line-emitting regions can grow to be of the order of $\Delta\phi =0.5$, and that the emission lines then lose the information on the radial velocity of the central object.

We investigate the latter assumption. The time scale of the wind-flow needs to be of the same order as the orbital period. Using the canonical value of $\beta=1$ in the so-called $\beta$-velocity law, the wind flows more than 10 $R_{\star}$ in one third of the double-wave period. Clearly, this would not fit the assumption above. However, lower acceleration has been suggested for WR stars (e.g., Schmutz 1997; Moffat 1996). According to CSH, a slower velocity law ( $\beta \approx 2$) is indicated by the absorption trough of O V 1371, which is formed in the inner wind. For a $\beta = 2$-law the wind travels less than two stellar radii within one third of the period. However, the He II lines, formed further away from the star, do not support such a modification. In this respect it is interesting to note that Hillier & Miller (1999) propose, albeit for much denser WR winds than WR46 has, a bimodal beta law (inner wind $\beta=1$ and outer wind $\beta = 50$). Anyway, we suppose that the loss of coherency leading to a stand-still within one or a few orbital revolutions is quite well possible.

Another type of variability of WR46 is the flare-like behaviour displayed in Fig.16. Photometric variability on such a time scale for WR stars has been reported by Matthews et al. (1992) in the case of WR6. These authors found a possible brightning of 1% lasting about ten minutes. Furthermore, Zhilyaev et al. (1991) monitored WR134 with a time-resolution of 1 s simultaneously in two narrow-band filters tuned to the continuum and to He II 4859, respectively. They observed "saw tooth'' flares with a quasi-period of 250 s in the emission line with an amplitude of 0 $.\!\!^{\rm m}$3, along with short-lived flares of 5-7 s. As such intense monitoring with high-time resolution has been performed only occasionally and only for known variable WR stars, we cannot address here the significance of the flare-like event in WR46.

The interpretation of the short- and long-term photometric and spectroscopic variability in terms of pulsation and binarity is deferred to Paper III, where the variability of WR46 is also compared to that of other WR stars.

Acknowledgements

We are grateful to the colleagues who provided us with spectra of WR46: Werner Schmutz, Virpi Niemela, Sergey Marchenko. The WN atlas (Hamann et al. 1995b) can presently be found at: ftp.astro.physik.uni-potsdam.de in the directory /pub/hamann/WNatlas. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.