3 Description of the spectra

After all the above corrections have been performed, we finally obtained the spectra displayed in Fig. 1.

Figure 1: The observed rectified UV spectra. From top to bottom: M 33-0900, M 33-110-A, M 33-B-38, M 33-B-133, M 33-B-526 and M 33-1137. Relative fluxes have been arbitrarily displaced in ordinates for the sake of clarity. The main IS lines are marked at the top, and below we have indicated the rest wavelengths of the most important stellar lines.

One of the difficulties of stellar analyses in M 33 is the large possible variation in metallicity, that complicates the interpretation of the spectrum. Thus in Fig. 2 we show a comparison of the N V, Si IV and C IV lines of the M 33 stars with Galactic stars of similar spectral classification. Their identifications and spectral types are given in Table 2.

The Galactic UV spectra have been taken from the INES IUE archive, and have been degraded from the original high resolution to the lower resolution of our M 33 HST spectra. We emphasize that we have taken the rectification points of the M 33 stars as a guide to rectify the IUE spectra in the same way. Therefore, the extra absorption seen redwards of the Si IV and C IV profiles in some Galactic stars are probably an effect of metallicity. Furthermore these extra absorptions are consistent with the profiles indicating a lower metallicity in the M 33 stars. The N V comparison is strongly affected by the different L$\alpha$ absorptions.

Figure 2: Comparison of the N V (left), Si IV (middle) and C IV (right) lines in the M 33 (full line) and Galactic (dashed line) stars. From top to bottom: M 33-0900 and HD 154090; M 33-110-A and HD 148688; M 33-B-38 and HD 148688; M 33-B-133 and HD 38711; M 33-B-526 and HD 198487; M 33-1137 and HD 51309. Spectral clssifications are given in Table 2. Relative fluxes have been arbitrarily displaced in ordinates for the sake of clarity. Galactic spectra have been degraded to the M 33 HST resolution.

M 33-0900 has Si IV and C IV profiles that are very similar to those of HD 154090. Although N V is stronger in M 33-0900, the star looks normal for its spectral type.

This is not the case for M 33-110-A, which in comparison to HD 148688 displays very weak Si and C lines. Moreover considering the luminosity effect we would expect for Si IV, the difference is even larger. This is consistent with the low Si and O abundances derived by Monteverde et al. (2000). Thus we can expect a low metallicity for 110-A, also consistent with the higher continuum level redwards from Si IV and C IV. The N V profile is again stronger than in the comparison star. The slopes of the blue wings of both the Si and C lines are smaller than in HD 148688. This will be interpreted during the fit procedure as a larger dispersion velocity. Finally we shall mention the broad absorption in the C IV blue wing that becomes more apparent due to the low absorption in this line. This additional absorption is present in both individual M 33-110-A spectra and can be clearly indentified in Fig. 3 where we show a comparison of the C IV line in 110-A and in B-38. We have used this figure as a guide in fitting the C IV line in 110-A.

Figure 3: Comparison of the C IV line in M 33-110-A (solid line) and M 33-B-38 (dashed line).

M 33-B-38, for which Monteverde et al. (2000) again derive low O and Si abundances, has also been compared with HD 148688. Similar comments as for 110-A are applicable, although effects are less extreme and the N V profile in this case is marginal (as in the Galactic star).

This has previously been observed and analized by Bianchi et al. (1996) and was later reanalyzed by Prinja & Crowther (1998). Both used the same HST GHRS spectrum, which looks like ours, including the lower slope in the blue wings as compared to Galactic stars (see Fig. 4 from Bianchi et al. 1996). Bianchi et al. (1996) find a narrow absorption feature (narrow absorption component, NAC) at a blueshifted velocity of $\sim$1250 km s^-1, which is close to the edge velocity determined by Prinja & Crowther (1998), 1225 km s^-1. Interestingly, we also find evidence of a NAC at the same velocity, which may indicate that this is a more permanent feature.

M 33-B-133 shows the most puzzling spectra. The red component of the Si IV doublet is deeper than the blue one, which cannot be explained by the theory. The effect is due neither to noise nor to variability, as it is present in the same form in both individual spectra of M 33-B-133. We see numerous features in the C IV profile that could be interpreted as NACs. From them, only the absorptions at 250, 585 and 1990 km s^-1 show both components. None of these NACs, however, seems strong enough to explain the red component of Si IV.

This is not the only remarkable feature in the spectrum of M 33-B-133. This object has been classified by Monteverde et al. (2000) as B1.5 Ia, and is compared with HD 152236 (B1.5 Ia+) in Fig. 2. We see that the profiles are quite different. In fact, the spectrum of B-133 resembles more that of $\kappa$ Ori (HD 38771), a B0.5 Ia star, although a comparison of the optical spectra excludes such an early spectral type for M 33-B-133.

Another possibility is a blend with the spectrum of a hotter star. This is supported by the resemblance of the spectra of B-133 and $\kappa$ Ori, and by the data in Table 1. Note that there B-133 has a larger S/N than would be expected from a comparison of its visual magnitude and spectral type with those of the other stars. Thus, although a particular set of circumstances is required because the B1.5 supergiant dominates the blue spectrum, it is not impossible that a hotter companion of type O9.5-B0.5 is dominating the UV spectrum. If this is the case the apparent strong red component of the Si IV doublet would be the main contribution of the B1.5 star in the UV spectrum.

However, we could not reproduce the observed spectrum by combining IUE spectra of different spectral types without extra ad hoc assumptions (as arbitrary relative displacements), nor can we detect any clear indication of binarity in the HST WFPC2 images of B-133, or other features indicative of a companion in the blue and UV spectrum, and thus the suspected binary nature of B-133 remains unconfirmed.

M 33-B-526 displays lines very similar to those of HD 198487, in spite of its suspected binary nature from its appearance in the Keck I screen (McCarthy, private communication).

The C IV profile displays several absorption features redwards of the rest wavelength that could be interpreted as contributions from a companion star. However, their positions do not correspond to those of the doublet separation Thus we expect this to be the spectrum of a single star.

M 33-1137 has Si IV profiles that are clearly weaker than those of the selected Galactic counterpart, HD 51309. The C IV profile in M 33-1137 is clearly broader than in HD 51309, with a shallow slope in the blue wing. It displays additional absorptions that agree with the positions of the C IV components at -75 km s^-1, thus clearly pointing to wind inhomogeneities. Weaker absorptions are compatible with components at -550 km s^-1. Other absorptions cannot be clearly assigned to the C IV doublet, specially considering the low SNR. The N V feature is more evident in M 33-1137 than in the Galactic star. The observed spectrum points again to a lower metallicity in the M 33 star than in the Galactic comparison star.

Therefore the UV spectral morphology of the observed M 33 stars separates them in three groups: a group (particularly evident in Si IV) with lines weaker than their Galactic comparison stars (M 33-110-A, M 33-B-38 and M 33-1137), a second group with lines similar to their Galactic counterparts (M 33-0900 and M 33-B-526) and one star (M 33-B-133) showing peculiar profiles for its spectral type. In addition, the M 33 stars seem to have lower slopes in the blue wings, pointing to larger dispersion velocities. This is particularly clear in the first group of stars.