6. The nature of the pulsations
6.1. Strange-mode instabilities?
Kiriakidis et al. (1993) have suggested that the variability of LBVs could be due to strange-mode instabilities. They have calculated the location in the HR-diagram where this instability occurs using the OPAL opacities for various metallicities: , 0.02 and 0.03. The location depends very strongly on the metallicity. This is shown in Fig. 5, where we plot the regions where the strange-mode instability occurs for two values of the metallicity: Z=0.004 and 0.03. The location of our program stars is also indicated. The LMC stars are compared with the prediction for (although this metallicity is lower than the LMC value of ) and the Galactic stars are compared with the predictions. The values of of LBVs is varying. In this figure we used the mean value of log at the epoch where we measured the microvariations.
The temperature range in the HRD where the strange-mode instability occurs is very sensitive to the metallicity. The LMC stars are outside the instability region for . However, the instability region for (not predicted) might cover the location of the LBVs of the LMC. Of the Galactic stars, AG Car is in the predicted instability region, but the other two program stars are outside the region. They are too faint by about 0.2 dex. However, given the uncertainty in the stellar parameters and in the location of the edges of the strange mode instability region, the disagreement between predictions and observations might not be significant.
It is very unlikely that the strange-mode instability is responsible for the microvariations of most of the LBVs. The reason is simple: the microvariations of LBVs are very similar to those of normal supergiants (where it is sometimes called Cyg variability). The only difference is that the amplitudes and periods of the LBVs are larger than for normal supergiants by about a factor two. All early type supergiants, from types O through A, show these microvariations. This is not in agreement with the predicted limited location of the strange-mode instability in the HRD.
We now investigate if the microvariations of the two LBVs for wich we found a much larger Q-value, i.e. S Dor and AG Car, could be due to strange modes.
We conclude that the microvariations of the LBVs cannot be explained by strange-mode instabilities.
6.2. The similarity to Slowly Pulsating B-stars: non-radial g-modes?
We compare the microvariations of the LBVs with those of the Slowly Pulsating B-stars (hereafter: SPBs, Waelkens 1991). Pamyatnykh (1998) recently predicted a continuous extension of gravity mode instability due to the -mechanism from the SPB- domain in the HR-diagram towards large stellar masses. The observational findings of Waelkens et al. (1998) with respect to the position in the HR diagram of variable supergiants discovered by Hipparcos are fully compatible with Pamyatnykh's theoretical prediction. The main period of the variation of the supergiants derived from the Hipparcos photometry also suggests an extension of the SPB instability domain. This can be seen on Fig. 3 of Waelkens et al. (1998): the position of all the supergiants suggests that they exhibit g-modes similar to those excited in SPBs.
Since the microvariations of the LBVs are very similar to those of the normal B-type supergiants (with the exception of AG Car and S Dor, see Sect. 5.2) this suggests that the microvariations of the LBVs are also due to pulsation in g-modes.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998