Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Identification of Be stars in the Magellanic Clouds

Dirk Hoffmann

Astronomisches Institut der Ruhr-Universität Bochum

Received 18th February 1998

1. Aims of this work

In this work I want to determine the Be to B star ratio in as many clusters of the Magellanic Clouds (MCs) as possible. With this database in comparison with similar data from galactic clusters in the literature I want to draw conclusions for the influence of metallicity to the Be to B ratio.

Up to now investigations of Magellanic Cloud clusters have shown Be to B star ratios comparable with the highest ratios measured in galactic clusters. But these data are not sufficient to find a general solution of this question, because they deal only with very few selected clusters of the MCs. With a larger database I want to reduce the influence of special clusters, because it is known that this ratio is not homogeneous, even not in open galactic clusters of the same age.

A second idea is to have a look at the dependency of the ratio on cluster age. For this aim I need a sample of rich young clusters. In our Galaxy we find only a few clusters with more than 4 Be stars, so that the statistical error of the single Be to B ratio prevents us from finding this ratio in our own Galaxy.

2. Identification method

To detect Be stars I use narrow band Halpha photometry compared with Johnson-Cousins R photometry. Be stars are defined by their Halpha emission, so we can detect them by their higher Halpha luminosity. To separate B stars from red giants and supergiants I use the B-R colour of the stars. This method was first used by E.K. Grebel (1997) for the 3 MC clusters NGC 330, NGC 1818 and NGC 2004.

3. Observations

The data I present here I got from an observing run in November at the 1 m telescope of the South African Astronomical Observatory (SAAO) within a collaboration with L. Balona, using an Halpha filter with an equivalent width of 50Å.

For final results in this project an analysis of LMC cluster data obtained at the 61 cm Bochum telescope at La Silla will follow.

4. Detection of emission stars

Because of non photometric weather condition a zero point calibration of the B and R data is impossible. So the B-R axis given has only value as separation criteria between red and blue stars, the given values are no real B-R colours.

Up to now there are only results of 4 young SMC clusters (see Fig. 1). The numbers of detected red giant or supergiant stars, non-emission B stars and possible Be stars are given in Table 1. Additionally there is given the fraction of Halpha emission stars of all detected blue stars and the average R-Halpha value for all blue emission stars.

Table 1. The numbers of detected red giant or supergiant stars, non-emission B stars and possible Be stars in the four SMC clusters
Cluster red stars B Be Be-fraction [%] R-Halpha [mag]
IC 1655   9 38 22 36 0.33
IC 1662 12 28   8 22 0.33
NGC 220 13 36 26 41 0.47
NGC 222 13 34 22 39 0.51

[Click here to see Fig. 1!]

The Be star fraction shows a variability between 20 and 40% and the average emission strength of the different clusters also shows differences. Moreover we see no hint for a correlation between Be fraction and emission strength. To draw statistical conclusions we have to wait for further results from Magellanic Cloud clusters.

References

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First version: 09thMarch,1998
Last update: 08thOctober,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)