Astron. Astrophys. Suppl. Ser. 140, 309-326

A survey of blue - ${\rm H}\alpha$ objects in the galaxy M33

S. Fabrika - O. Sholukhova

Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 357147, Russia

Received June 18; accepted July 19, 1999

Abstract:

For the purposes of a search for SS433 type objects, LBV stars and hypergiants in M33, we have carried out a photometry of blue stars in H$\alpha$ images of this galaxy. From 2332 listed OB stars, we have isolated 549 objects, which have H$\alpha$ excess over stars of the same V magnitude. Among them 81 emission stars (s) have been selected. About 60% of isolated objects found to be extended in ${\rm H}\alpha$, among them 154 diffuse (d) and 180 bubble-type (b) nebulae. 117 objects are compact (c) and faint ${\rm H}\alpha$ sources. These groups are different in colour indices, ${\rm H}\alpha$ fluxes, surface brightness and sizes. The most probable size of the ${\rm H}\alpha$ nebulae is $FWHM\,=\,10-14$ pc. There is evidence that their properties and location in the galaxy are defined by interstellar gas pressure and related to spiral arms. The d objects are HII regions with an embedded star. The b objects are probably envelopes around WR stars or SN remnants. Among s-type objects we isolated a group of 20 brightest stars, which in their average properties fit well to the parameters of blue hypergiants or LBV-type stars. The stars of intermediate brightness follow very well to blue Ia supergiants. The interstellar absorption derived from brightest ${\rm H}\alpha$ stars is ${A_{ V}=0\hbox{$.\!\!^{\rm m}$ }93 \pm 0\hbox{$.\!\!^{\rm m}$ }05}$. We classify the faintest stars as blue Ib supergiants, their average absorption is ${ A_{ V} \approx 0\hbox{$.\!\!^{\rm m}$ }6}$. They are restricted in number by the limiting stellar magnitude in the original sample.

Key words: nearby galaxies: stellar content: individual (M33): stars: early type -- ${\rm H}\alpha$ excess