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Astron. Astrophys. 337, 39-42 (1998)

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1. Introduction

The gas component and the stellar distribution of M 33 have been well studied. Catalogues of HII regions have been published by Boulesteix et al. (1974), Viallefond et al. (1986) and Courtes et al. (1987). CCD surveys of this galaxy have been undertaken by Freedman (1984), Wilson et al. (1990), Wilson (1991) and Regan and Wilson (1993). Extensive photographic catalogues of blue and red stars in M 33 have been published by Humphreys and Sandage (1980) and Ivanov et al. (1993) (hereafter IFM). A list of the Wolf-Rayet stars has been presented by Massey and Conti (1983) and Massey et al. (1987). Massey et al. (1995) have found seven new WN stars and later Massey et al. (1996) (hereafter MBHS96) published a list of previously unknown 28 WR stars. Nowadays the number of known WR stars in M 33 amounts to 168. In the present study we combine those surveys to compare the distributions of massive stars in M 33. We also discuss the ratios of the number of OB star to red-supergiant candidates (OB/R) as a function of distance from the centre of M 33. Maeder et al. (1980) (hereafter MLA) explained the gradient in these ratios as a consequence of the chemical gradient. However serious doubts exist about the presence of the gradient of the ratio (OB/R) in M 33 because of measurement errors (Freedman, 1984). The main goal of this paper is to find observational evidences for a radial gradient of the ratio (OB/R). First, by applying an improved, more realistic technique to determine the error counts, the gradient becomes pronounced. Second, we propose a new correlation technique for comparison of the stellar populations in M 33 which supports the existence of a radial gradient in the ratio OB/R.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998