4 Discussion and conclusion

In the present paper, we have seen that the star HD 826 shows temporal changes in the shapes of its C III$\lambda$5696 and C IV $\lambda \lambda$5801/12+C III$\lambda$5826 emission lines. The amplitudes of the variations range up to 25-30% of the adjacent continuum flux, over timescales of hours. The variabilities of both lines are quite well correlated, although they are somewhat weaker for the C IV complex. On the whole, the 1998 January run appears as a particular epoch of high activity for this central star.

As was already noticed for massive WR stars, the blue-shifted absorption component of the lines exhibiting P-Cygni profiles in HD 826 is significantly more variable than the emission component. This is likely mainly due to the small volume of matter in front of the stellar "disk'', a large fraction of which is subject to a higher level of coherent variability. This suggests linear sizes for the blobs of $\sim$1 $R_{\ast }$.

The subpeaks show large measurable velocity shifts in HD 826 during their lifetime. Subpeaks (or gaps) on the top of the C III line generally move from about the line center towards line edges in a symmetric fashion. This is consistent with wind features accelerated outward along radial trajectories. Since the lifetime of the subpeaks is a few hours, they cross, at speed $\stackrel{<}{\approx}$1000 km s^-1, a zone limited to about a few tenths or less of the line formation region in radial extension. Thus the wind of HD 826 is highly variable on a very short time-scale, which supports a turbulent origin.

The kinematics of 120 structures on top of the C III$\lambda$5696 line of HD 826 have been measured. Adopting $v_{\infty} = 1000$ km s^-1 and $R_{\ast}=0.33$ $R_{\odot}$ the kinematics are well reproduced by a $\beta$-velocity law with $\beta\approx 10$, in contrast with the value $\beta=1$ adopted in the atmosphere model. The line formation region is evaluated to span radial distances 10-100 $R_{\ast }$ from the central star. Within the accuracy of our acceleration measurements, $R_{\ast}=0.33$ $R_{\odot}$ and $\beta =10$ (hence $\beta R_{\ast}\approx 3.3$) seem to be good estimates of the stellar radius and of the $\beta$ parameter. Keeping $\beta=1$ and fitting the observed maximum acceleration with the theoretical $(a_{\rm R},\,v_{\rm R})$-relation would require a downward revision of the ratio $v^2_\infty/R_\ast$ by a factor $\approx$10. Because of the quite good reliability of the terminal velocity estimate ($\approx$1000 km s^-1), keeping $\beta=1$ would therefore imply rather high, unrealistic values of the stellar radius (3.3-3.6 $R_{\odot}$). Therefore we reject a $\beta$value of 1 in favor of $\beta\approx 10$, which is consistent with previous estimates of $\beta$ for massive WR stars given in Moffat (1996).

The line variability in HD 826 is somewhat similar to that observed in the massive WC 8 star WR 135 (see Robert 1992; Lépine & Moffat 1999; Lépine et al. 2000). Thus, on the whole, the wind fragmentation process appears to be a purely atmospheric phenomenon, despite the strong differences between both types of underlying hot star. However, some differences exist: i) in the complex C IV $\lambda \lambda$5801/12+C III$\lambda$5826 emission line originating in HD 826, moving subpeaks appear with adjacent ghost images. This is likely due to line blending. Unfortunately, this blending prevented us from clearly identifying moving features. This was not the case for WR 135 (see Lépine et al. 2000); ii) in addition, Lépine et al. (2000) find lifetimes for C III$\lambda$5696 blobs in the wind of WR 135 to be of the order of the crossing time in the C III$\lambda$5696 formation zone, thus implying relatively long-lasting blobs, compared to those observed in HD 826.

Combining the results of Paper I with those of the present paper we find that [WC 9] and [WC 8] central stars exhibit similar changes in their C III$\lambda$5696 and C IV $\lambda \lambda$5801/12+C III$\lambda$5826 emissions. We therefore suspect that, if more data had been secured for a larger sample of late subtype [WC] stars, the phenomenon of emission line variability in central stars might have revealed itself to be more common and universal. However, the details differ. For example, accelerations exhibited by the clumps originating in HD 826 are often significantly larger than those in BD +30$^\circ$3639 or massive WR stars. This difference could be understood by the smaller hydrostatic radius of HD 826. However, it will be important in the near future to test whether the data indicate any true correlation of the observed variations of the emission lines with radius or any other fundamental stellar parameters, for a larger sample of stars. In the case of pop. I WR stars, no such correlation is seen (Lépine & Moffat 1999).

Finally, high resolution ($\le$0.5 Å), high S/N ratio, temporally resolved, optical spectra of HD 826 are needed in order to investigate the appearance and dynamics of subpeaks observed so far at inferior resolutions. In particular, the possible hierarchy of subpeaks within each individual subpeak (such a fractal-like structure is expected in the context of supersonic, compressible turbulence) could be tested in this way. For that purpose, the need for large, 10 m-class telescopes is critical.

Acknowledgements

YG acknowledges financial aid from the French Ministry of Foreign Affairs. AFJM is grateful to NSERC (Canada) and FCAR (Québec) for financial support. AFJM acknowledges the award of a Killam Fellowship from the Canada Council for the Arts. We thank Thomas Eversberg for his help in the data acquisition (OMM data).