8 Conclusion

In this paper identification and photometry of blue stellar objects in H$\alpha$ images of the galaxy M33 have been performed. Out of 2332 blue stars of the cataloque by IFM we have managed to identify and measure 1619 objects in H$\alpha$. On the H$\alpha$ flux -- V magnitude diagrams the stars without noticeable emission form a nonemission sequence above which emission line objects are located. Their H$\alpha$ emission may be both the intrinsic radiation of the star's envelope and the radiation of a compact HII region around the star. All in all 549 objects have been isolated by the criteria of the H$\alpha$ excess. Based on the relationships between a size and H$\alpha$ line flux, a size and ${\rm H}\alpha$ surface brightness, it has been found that 334 or 60$\%$ of the candidate stars are extended objects in ${\rm H}\alpha$ having the characteristics of diffuse nebulae or bubbles. There have also been isolated 81 stars and 117 compact star-like emission sources.

The objects selected could be considered as candidates for supergiants, hypergiants, WN and SS433 type stars. All of them are brightest stars in M33 and they are mainly star-like objects in U, B and V bands (IFM). The isolated ${\rm H}\alpha$ morphological groups differ in a wide variety of characteristics: luminosities, colours, fluxes and suzes in ${\rm H}\alpha$, and distribution over M33. There is evidence that the location and properties of the ${\rm H}\alpha$ nebulae depend on the density and pressure of interstellar gas and are related with spiral arms.

The diffuse nebulae are HII regions with an exciting star. Hydrogen emission is likely to be dominant in their spectra and nebula lines are weak, their colours may be defined by the star. The bubbles are probably composed of high excitation gas and nebula lines must predominate in their spectra. The contribution of the central star to the total flux of these objects may be insignificant. By their properties, we identify these objects with envelopes round WR stars or supernova remnants. The group of common objects comprises objects of all types, but, basically, these are stars and compact diffuse nebulae. The diffuse and large bubble-type nebulae in our list are likely to belong to the same assemblies of nebulae listed by Courtes et al. (1987), but they are small-sized, mainly 7-20 pc.

Among the selected emission stars we isolate a group of the 20 brightest stars which, in their average characteristics, conform to the blue hypergiants or LBV type objects. The stars of intermediate luminosity comply with the bright supergiants of average spectral class B1Ia. The interstellar absorption for these two types of stars is A_V = ${\rm0{}^{\rm m}\!\!\!.\,93 \pm 0{}^{\rm m}\!\!\!.\,05}$. The faintest stars have in main to be blue Ib supergiants. The mean absorption of these stars is ${A_V=0{}^{\rm m}\!\!\!.\,6}$, they are not observable across the whole disk but only in its nearest to the observer part. The number of the latter stars is restricted by the limiting stellar magnitude in the original list.

Acknowledgements

The authors thank V.V. Vlasyuk for providing us with program packages for processing images, G.R. Ivanov for providing the catalogue prior to publication, S.N. Dodonov for H$\alpha$ photographs made available, to N.A. Tikhonov, A.I.Zakharov and T.B. Georgiev for helpful discussions. This work has been supported by a grant N3-13 of the Program "Astronomy'' RAS, a grant A-02-021 of the ESO C&EE Program and the RFBR grant 97-02-16423.