4 Analysis of the resonance lines

We use the method described by Haser (Haser 1995, see also Lamers et al. 1999) to analyse the UV P Cygni lines in order to derive wind terminal velocities. We have also derived H I  column densities towards the M 33 stars by fitting the IS L$\alpha$ line, in the same way as in Herrero et al. (2001) (see also Jenkins 1970; Bohlin 1975). The values quoted in Table 2 for these column densities are similar to those obtained for stars in our Galaxy (see Fig. 2 of Shull & Van Steenberg 1985).

After continuum rectification, we extract small regions of each spectrum centered in our strategics doublets: N V $\lambda \lambda$ 1238.819, 1242.798; Si IV $\lambda \lambda$ 1393.73, 1402.73 and C IV $\lambda \lambda$ 1548.191, 1550.761. In case of discrepancies or difficulties, we give more weight to the Si IV and C IV lines than to the N V one.

To get the wind terminal velocities we have to correct for the underlying photospheric components, which we do in an approximate way, by using IUE spectra of hot stars with weak winds (and projected rotational velocities as low as possible) as templates. These templates have been convolved with appropriate rotational profiles, to account for the individual stellar rotational speeds. We excluded N V from this procedure, as the corresponding spectra are noisy and do not show any features.

We selected a sample of Milky Way dwarfs with spectral types from O9V to B3V taken from the INES database. This covers the spectral type range of our M 33 stars. Selection of photospheric template and continuum rectification has a big impact when fitting emission peaks, but has little effect on the determined terminal velocities. The IUE spectra have been corrected for the relative velocity between stars and observer, in the same way as for M 33 stars. The stars used for the photospheric templates are specified in Table 2. Note that the profiles of interest are very weak in these stars, but even so the different metallicity may be a small source of error.

The velocity stratification is parameterized as a usual $\beta$-law, and we account for the wind turbulent velocity in the same manner as described in (Haser 1995) and Herrero et al. (2001). The indetermination in the exponent of the velocity field, $\beta$, produces an additional uncertainty in the terminal velocities.

Table 2 gives the fit results with respect to the wind and turbulent velocities. The final fit to each line is shown in Figs. 4 and 5.

Figure 4: Final fits for the three earliest stars of our sample. Lines are plotted from left to right (N V, Si IV, C IV) and stars from top to bottom in the same order as they are listed in the tables.

Figure 5: As Fig. 4, however for the three last stars in Table 1.