3. Correlation between stellar populations in M 33
3.1. Correlation between OB stars and other stellar populations
Using UBV-photometry and spectral determinations, MBHS96 have selected 89 OB stars. This is probably only a small fraction of all OB stars in M 33. On the basis of spectroscopy the membership in M 33 of these stars with masses is confirmed. Compared with the CCD photometry of Regan and Wilson (1993), the photometry of MBHS96 is more suitable for applying the present method and nearly covers the whole area of M 33. The data presented in Table 1 can be interpreted as an evidence of tight correlations between OB stars, WC stars and the bright HII regions (BR). About 60% of the OB stars are associated with bright HII regions and WC stars. The first correlation is expected since the source for ionization of the gas in a bright HII region are stars earlier than B2. Hence the OB stars selected by CCD photometry should correlate with bright HII regions. The tight correlation between OB stars and bright HII regions is a check of the present criterion is suitable for studying the correlation between stellar populations in galaxies. A correlation between OB stars and WC also exists. The ratios given by Eq. 2for WC stars and given by Eq. 3are high. This can be expected because the progenitors of WC stars have masses , and massive OB stars as well as WC stars must originate from the same sites of star formation.
Table 1. Correlation parameters between OB stars of MBHS96 and other stellar populations in M 33
3.2. Photographic UBV-photometry
Ivanov, Freedman and Madore (1993) (IFM) presented a UBV-photometry of the blue and red stars in M 33 based on plates obtained with the 3.6 m Canada-France-Hawaii Telescope and the 2.0 m telescope of the Bulgarian National Observatory in the Rhodopa Mountains. The completeness limit of this catalogue reaches V = 19.5 mag. IFM have selected 389 red stars with . Judging from colour criteria, most of them are probable member of M 33 and are red-supergiant candidates (RSG). Some results based on these data have already been published (Freedman 1985a, 1985b; Ivanov 1991; Georgiev and Ivanov 1997). The photographic UBV-photometry of IFM is not very suitable for selecting massive stars due to its poor accuracy for determining . However it allows to study the correlation between OB stars and red supergiants for different galactocentric distance. This can be seen from the data in Table 2. The photographic data also include a large number OB star-candidates and therefore enable the study of the distribution of probabilities . Following Massey et al. (1987), FitzGerald (1970) and Flower (1977), a reddening-free parameter for OB stars is
Using Eqs. 4we have selected 206 stars with , , , and . Hereafter we call them OB star candidates whereas the stars selected from the CCD UBV photometry are called simply OB stars. The distributions of OB star candidates, OB stars and other stellar populations as a function of the galactocentric distance are given in Table 2.
Table 2. The number of stars for different galactocentric distances R
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998