6 Discussion

In our previous work (Janot-Pacheco et al. 1999), a signal with 0.78 c/d was detected and interpreted as due probably to a NRP mode. Keeping in mind the mechanism of mass ejection based on prograde modes proposed by Osaki (1986), such a signal is not expected to produce mass ejection, as it is retrograde. A similar signal was not detected in this work, but another was found with 0.6 c/d. According to the fundamental stellar parameters presented in Table 5, the frequency $\nu_1 = 0.6$ c/d can also be interpreted as being due to an orbiting perturbation either at $R/R_* \simeq 2.0$ for a Keplerian velocity law or at $R/R_* \simeq 1.3$ if an angular momentum conservation law is prevailing.

For comparison, we give in Table 6 the semi-separation of emission peaks $V_{\rm P}$ (in km s^-1) in the H$\alpha$ line profiles and the radius $(R/R_*)_{\rm K}$ associated with $V_{\rm P}$ assuming the CE is in Keplerian rotation. We see that $R/R_* \sim$ $(1.7\pm0.3)(R/R_*)_{\rm K}$ while $V_{\Omega} \sim$ $(1.5\pm0.3)V_{\rm P}$ ( $V_{\Omega}$ and $V_{\rm P}$ are both aspect angle projected velocities). This peculiar behavior is due to the CE opacity effect (Hummel 1994; Chauville et al. 2001), according to which for line profiles of 1996-1998 type our simulations show that keeping the parameters ( $V_{\Omega},R,H$) unchanged, the line emission peak separation widens as $\tau_{\rm o}$ increases from $\tau_{\rm o} = 0$ to 1.0. It stretches down again for still higher values of $\tau_o$. In line profiles of 2000-2001 type, the separation of the emission peaks reduces for increasing values of $\tau_{\rm o}$. Due to this effect, it is difficult to infer from the present numerical simulation the characteristics of the CE rotational law. Such a deduction is further hindered by the fact that $V_{\Omega}$ is an average rotational velocity that probably resumes the kinematic characteristics of the whole CE from the star up to an external radius $R_{\rm E}/R_* \gg$ R/R_*. Let us still note that for $\tau_{\rm o} \gg 1$, because of the $\tau_{\rm o}^{1/2}$-dependence of the source function, our model line profiles become bottle-shaped.

Noting that for the sake of a simple order of magnitude estimation we can write $\tau_{\rm o} \propto$ $\int N_{\rm e}^2{\rm d}R \sim ({\cal N}_{\rm e}/\pi R^2H)^2R$, where  ${\cal N}_{\rm e}$ is the total electron number in the CE region studied, we can draw the following conclusion concerning the CE evolution as derived from the H$\alpha$ line emission changes: from 1996 to 1997 these changes implied an increase of the emitting region by about 40% and an increase of ${\cal N}_{\rm e}$ by 60%; the 1997-2000 transition is characterized by an increase of the emitting region extent by a factor 5.5 and ${\cal N}_{\rm e}$ by a factor 2.2; the 2000-2001 transition is almost passive as the emitting region shrank by about 60%, while ${\cal N}_{\rm e}$ changed very little. The high NRP activity noticed in this work in the 2000 epoch may then correspond to an effective CE replenishing phase. Moreover, it is worth noticing that the nearly spectroscopic "shell" aspects of the H$\alpha$line in the 1996 and 1997 epochs that could be interpreted at first glance as due to an extended, highly absorbing disc, are actually produced by a CE region which accretes the ejected mass from the star in such a way that the region is rather close to the central star and that the extent of the region changes little.

Figure 15: Periodograms and confidence level diagrams for epochs 1996, 1997/98 and 2000 of periodicities found in the wings of the He  I 6678 line beyond $v \sin i$. The red and blue sides of line profiles were folded into a unique positive velocity scale.

Figure 16: Fits of observed H$\alpha$ line emission profiles. The photospheric absorption component is also shown.