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Astron. Astrophys. 339, 773-781 (1998)

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5. Be stars in the clusters and the surrounding field

Be stars are non-supergiant B stars with variable Balmer emission and infrared excess originating in circumstellar disks. Rapid rotation and red/infrared excess of Be stars lead to redder colours and widened main sequences (see Grebel et al. 1996). The observed brightness and colour of a rotating (Be) star depend on its rotational velocity and inclination to the line of sight (rotational displacement fan, Collins & Smith 1985).

Using the [FORMULA] index to detect stars bright in [FORMULA] (R serves as continuum filter) and [FORMULA] as a temperature index we can identify Be star candidates (Fig. 8). This method was first described in Grebel et al. (1992, 1993). In Fig. 8 a pronounced clump of data points at [FORMULA] mag and [FORMULA] mag can be seen. These data points represent blue main sequence stars and blue supergiants without [FORMULA] emission. To the red side of this clump, scattered around [FORMULA] mag and extended over the whole [FORMULA] colour range, stars belonging to the RGB and red supergiants are visible in the two-colour diagram. Red giants and red supergiants can have [FORMULA] emission, and indeed some data points are located at higher [FORMULA] values (up to [FORMULA] mag). Our selection criteria for the candidate Be stars are [FORMULA] mag and [FORMULA] mag.

[FIGURE] Fig. 8. Two-colour diagram to detect Be star candidates. R serves as a continuum filter to detect stars bright in [FORMULA] ([FORMULA]), while [FORMULA] serves as a temperature index to separate the blue stars from red giants and supergiants. For more information see text

The widening of the main sequence of the CMDs due to Be stars can be clearly seen in Figs. 4 to 7, where we marked the Be star candidates with crosses. The effect is most pronounced in [FORMULA] since the I filter has the highest sensitivity to the infrared excess of Be stars.

In Fig. 1 we marked these stars with circles. Our Be star candidates are concentrated at the location of the components of the cluster pair whereas at the location of KMHK 1019 only two such stars are present.

The impression that the Be star candidates are dominantly present in SL 538 and NGC 2006 is confirmed when considering the ratio of Be stars to B stars. Since we do not have spectral classifications we simply considered the ratio of all B to Be stars within a magnitude interval of [FORMULA] to 19.1 mag. These magnitudes correspond to the mean visual magnitudes, at LMC distance, of B 0 III to B 9 V main sequence stars (Zorec & Briot 1991, Table 3). We find the following values:


[TABLE]

The errors are corresponding to [FORMULA] Gaussian errors and are calculated using the confidence limits for small number statistics from Gehrels (1986). The components of the double cluster show the same fraction of Be stars, and the sixfold amount found in the surrounding field. The difference between the ratios [FORMULA] of SL 538 and the field is 0.104 which is more than a [FORMULA]-effect. The difference between SL 538 and KMHK 1019 is 0.070 which is less than a [FORMULA]-effect according to the upper confidence limit for KMHK 1019. Thus, the Be star content of KMHK 1019 may be comparable to the cluster pair.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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