4. Towards a model of the variable wind of HD 4004
Of many models invoked in the literature so far to explain variability in similar objects the two-component wind with magnetic, equatorial disc of rotating star (Bjorkman & Cassinelli 1993, Ignace, Cassinelli, Bjorkman 1998) seems to be most adequate for HD 4004. However, to consider this model seriously one should address rotation, magnetic field and polarization of this object.
The role of rotation for stars evolving with mass-loss has recently been discussed by Maeder (1999), who finds the possibility of a strong enhancement of rotation velocity in stellar evolution towards the Wolf-Rayet stars through strong wind asymmetry (polar wind enhancement causing small momentum losses via stellar wind). The rotation velocity of any WR stars including WR 1 is not available but since the rotation velocity of WR progenitors, O stars is of the order of Vsin(i) 100-300 km s-1 and it may rise during evolution it is likely that WR stars are rotating with velocities which may be important for the shape of their winds. In the case of WR 1 the observed LPVs recurrence period is probably of the order of several days and may appear as reflection of rotation on non-uniform disc formed at the distance of several stellar radii.
The strengths of magnetic fields in case of WR stars are not known at all. The only indication of them might be the non-thermal character of radio emission. In the particular case of WR 1 the non-thermal character of radio emission was found by Altenhoff et al. (1994). This makes our considerations more likely.
Another factor supporting the two-component wind in case of HD 4004 would be any indication of non-spherical wind. That is very difficult since no polarization studies are available for WR 1 at all. The only support for such supposition may come form the IR excess reported by Hackwell (1974) which according to Coté & Waters (1987) can suggest optical polarization.
In such a two-component wind the source of LPV is located in the base of the slow wind, where probably a minor part of observed profiles is created. This is in agreement with an observational fact that only a small amount of a line profile varies. Most of line emissions comes from a more stable area. In the base of the slow wind a non-stable disk structure may temporally appear (Bjorkmann & Cassinelli 1993). Irregularities connected with the slow wind might travel outwards the envelope with velocity law different from that of fast wind where most of emission is created. The sudden appearance or disappearance of the unstable inner disc may result in photometric brightening as observed by Morel el al. (1999). Between large wind instabilities connected with violent disc creation/destruction the general brightness of WR 1 is relatively constant and only minor variations are observed as LPVs. The interactions of two wind components may however result in the observed, random X-ray variability (Wessolowski & Niedzielski 1996) of WR 1.
Two-component winds may change our understanding of the WR stars. Already Ignace, Cassinelli and Bjorkman (1998) noted that in such a case the momentum problem becomes much less severe. Another interesting consequence of the two-component winds may be a geometric differentiation of WR stars depending on which (fast or slow) wind we mostly observe.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100