6 Discussion and conclusions

In Fig. 6 we have plotted the derived $v_\infty$against the stellar spectral types, together with parabolic fits to the average values quoted by Kudritzki & Puls (2000) for Galactic OB supergiants. The fits have been obtained by joining OI and BIa supergiants on the one hand and OII and BIb supergiants on the other. We have also plotted lines that indicate a 30$\%$ variation from the plotted average relations, a usual scatter range (see Fig. 4 in Kudritzki & Puls 2000).

All M 33 B-supergiants give us values that can be considered normal, except B-133. We see no difference between the stars with different suspected metallicities. This is in agreement with Puls et al. (2000) (see also Vink et al. 2000) who have argued that the terminal velocity (which depends on the slope of the line-strength distribution function, $\alpha$) is primarily controlled by the ratio of light ions vs. iron group elements, because of the different line statistics. As long as this ratio is roughly similar, the theoretical expected change of terminal velocity (due to the "indirect" $\alpha$ effect, see Puls et al. 2000, Sect. 5.2) is much smaller than if this ratio would be changed. In particular if a dense wind is present (as is the case for our supergiants) and this ratio remains unchanged (which we have to assume for the moment), the effect is expected to be very small (see Puls et al. 2000, Fig. 27), since the effective $\alpha$ then remains roughly constant. Only for thin winds and/or a significantly lower (general) metallicity the effect should become observable.

We could not find a satisfactory explanation for B-133. While its terminal velocity is well above the average for its spectral type, it could still be accepted (even with the low metallicity derived by Monteverde et al. 2000) assuming for example a bi-stability phenomenon, were it not for the anomalous red component of the Si IV doublet. Assuming on the other hand that B-133 is actually producing the red Si IV component, its velocity would be much closer to the average of its spectral type. However, we could not reproduce the observed profiles without ad hoc hypothesis, nor find conclusive evidence of a binary nature.

Although three of our stars have been analyzed by Monteverde et al. (2000) we will postpone a discussion of the terminal velocities in terms of the stellar parameters, as new analyses of their optical spectra, now including mass-loss effects, are currently under way in our group together with a set of newly observed stars.

The wind turbulent velocities derived here are larger than the typical 10$\%$ $v_\infty$ (Kudritzki & Puls 2000) or the 14$\%$ found by Herrero et al. (2001), reaching nearly 35$\%$ in the most extreme case. Including B-133 we obtain 0.25 for the mean $v_{\rm ta}/v_\infty$ratio. This result is of the same order of that found by Bresolin et al. (2002) in their analysis of M 31 B supergiants. We tentatively attribute it for the moment to the extreme character of our objects, selected among the brightest supergiants in M 33, and as a consequence are among the most luminous objects.

Finally, we have detected evidence of numerous NACs, confirming the wide presence of wind inhomogenities.

Acknowledgements

We would like to thank F. Najarro for very useful discussions during the stellar analyses. A.H. wants to acknowledge support for this work by the spanish DGI under proyect AYA2001-0436, the DGES under project PB97-1438-C02-01 and from the Gobierno Autonómico de Canarias under project PI1999/008. F.B. and J.P. acknowledge support from the German DLR, under grant RD-RX 50OR9909/2.