5 Individual comments

We briefly comment here on individual aspects of the analysis that could be of interest.

5.1 M 33-0900

We used HD 22591 (B0.5 V) for the photospheric profiles. The N V profile shows a large emission peak that we cannot reproduce completely (see Sect. 3). The $v_{\rm ta}/v_\infty$ ratio is 0.18, larger than in Galactic stars.

5.2 M 33-110-A

We used HD 39777 (B1.5 V) for the underlying photospheric profiles. We find the same problems as for M 33-0900: a large red emission peak in N V, and a large ratio $v_{\rm ta}/v_\infty$, which with a value of 0.25 is much larger than in Galactic B-supergiants. Furthermore, the additional absorption contaminating the blue side of C IV makes the fit a bit more difficult, affecting mainly the determination of  $v_{\rm ta}$. This does not seriously affect the uncertainty in the adopted values for $\beta$, $v_\infty$ and $v_{\rm ta}$.

5.3 M 33-B-38

The terminal velocity of this star has also been determined by Bianchi et al. (1996) and Prinja & Crowther (1998). The first authors use an analysis method similar to the one employed here, and thus it is not surprising that our value agrees with their. Prinja & Crowther (1998) determine the terminal velocities of their stellar sample from the violet edges of the profiles and the NACs. The velocity they obtain for this star is much larger (1225 km s^-1) than ours (730 km s^-1) or the one by Bianchi et al. (about 700 km s^-1). Our velocity would support the interpretation that what we have seen at 1250 km s^-1 is actually not a NAC.

Bianchi et al. (1996) do not give the turbulent velocity of their stars. We derive a very large value, 250 km s^-1, nearly 35$\%$ of $v_\infty$.

We adopted a large $\beta$-value, both to improve the consistency between the Si IV and C IV fits, and to improve the fit of the blue side of the red emission peak.

 

 

Table 2: Results obtained for the observed stars. $v_\infty$ and $v_{\rm ta}$ (turbulence velocity in outer wind) and the uncertainty of their sum, $\Delta v$, are given in km s^-1.

Ident	Spectral	Galactic	Spectral	log	$v_\infty$	$v_{\rm ta}$	$\Delta v$	$\beta$
	Type	Star	Type	N(H I)
M 33-0900	B0-B1 I	HD 154090	B0.7 Ia	20.85	950	170	50	1.0
M 33-110-A	B1 Ia+	HD 148688	B1 Ia	21.15	800	200	50	1.0
M 33-B-38	B1 Ia	HD 148688	B1 Ia	20.70	730	250	50	2.0
M 33-B-133	B1.5 Ia	HD 152236	B1.5 Ia+	20.97	2050	150	100	1.0
M 33-B-526	B2.5 I	HD 198487	B2.5Ia	21.10	380	120	75	2.0
M 33-B-1137	B3 Ia	HD 51309	B3Ib	20.85	750	250	100	2.0

5.4 M 33-B-133

We have used HD 39777 for phostospheric profiles, as in the case of M 33-110-A and M 33-B-38.

In spite of the low O and Si abundances derived by Monteverde et al. (2000) for this star, its UV spectrum shows comparatively strong C IV and Si IV P-Cygni profiles, indicating a strong and fast wind (see Sect. 3). The terminal velocity reaches 2050 km s^-1. The turbulent velocity is 150 km s^-1, a modest 7$\%$. This is the only star for which we obtain a terminal velocity clearly above the galactic average for its spectral type (taken from Kudritzki & Puls 2000), thus challenging the low abundances or the spectral classification (or both!). The spectral type, however, should change from B1.5 to O9 in order to have a terminal velocity close to the spectral type average. This is completely ruled out from inspection of the optical spectrum.

The fits displayed in Fig. 5 also show a behaviour different from those of the other stars. N V is well fitted. The fit of the C IV profile is good in the bluest part of the wind absorption profile but is bad in the rest of the profile. Fortunately, the first one is the important part for determining the terminal velocity. The unsatisfactory fit at low wind velocities is cosmetically very dependent on the underlying photospheric profile, and thus does not mean very much by itself, but it would be consistent with the presence of additional C IV absorption at low wind velocities. We keep the low $\beta$ value as there is no need to increase $\beta$ to improve the consistency between C IV and Si IV. However, the anomaly in Si IV indicated in the preceding section, i.e., a stronger red component, can also not be fitted and indicates an extra absorption at low wind velocities. This uncertainty does not affect the determination of the terminal and turbulent wind velocities, that have an accuracy of $\pm$100 km s^-1.

This does not solve the problem referred to above. The star has a large $v_\infty$ for its spectral classification and a Si IV red component stronger than the blue one. The first might be attributed to a case of bi-stability (Pauldrach & Puls 1990), similar to that of P-Cygni in our Galaxy, where the ratio of terminal velocity to escape velocity is larger. This however does not explain the red Si IV component. One possible solution to this puzzle is that we are looking at a composite spectrum, B-133 being the star contributing to the red component of Si IV (see Sect. 3). If this is the case, we estimate its terminal wind velocity to be of the order of 450 km s^-1, but would expect to see extra absorption in Si IV at low wind velocities.

Figure 6: The $v_\infty$ obtained for the M 33 stars. Solid lines represent the fits to OI + BIa (lower curve) and to OII + BIb supergiants (upper curve) of the Kudritzki & Puls (2000) Galactic data. Dashed lines are $\pm$30$\%$ variations of these curves. The solid dot indicates the approximate position of B-133 if derive it from fitting the red Si IV component (see text for details).

5.5 M 33-B-526

It has been classified as B2.5I (Monteverde et al. 1996), from its optical spectrum. We used a B2.5V galactic star for the photospheric profiles (HD 44402, Z CMa).

The derived terminal velocity is the lowest in our sample and the UV spectrum shows only weak signs of mass loss. However, the turbulence velocity is relatively high, with a ratio $v_{\rm ta}/v_\infty$ in excess of 0.3. This large value is the result of a compromise between the fit of C IV and that of Si IV. The first would allow a lower turbulence, but then we cannot fit the second one. Again, the value of $\beta$ has to be large to favour consistency between both doublets and a better fit to C IV. However, the terminal velocity is not seriously affected. The uncertainty of both velocities is different now, being that of $v_{\rm ta}$$\pm$100 km s^-1 and that of $v_\infty$ $\pm$50 km s^-1. For the sum of both we have adopted $\pm$75 km s^-1.

5.6 M 33-1137

This is the coolest star of our sample, with spectral type B3Ia. We take HD 32630 (B3V) for the photospheric profiles.

The Si IV doublet is mainly photospheric or in any case the wind has a small contribution. The fit to N V is poor.

The C IV is the main profile for deriving the parameters of the velocity field. The low SNR of the spectra and the additional absorptions described in the preceding section make the fit difficult. We find the best fit at 750 km s^-1, 250 km s^-1 and 2.0 for $v_\infty$, $v_{\rm ta}$ and $\beta$. Again, $v_\infty$ is larger than the Galactic average from Kudritzki & Puls (2000), but now much more moderately than for B-133. The uncertainties are slightly larger than those of the other fits, as already expected from the described difficulties. We adopt $\pm$100 km s^-1 for $v_\infty$, $v_{\rm ta}$and its sum.