Be Stars in Young Magellanic Cloud Clusters

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Session 17 -- Stellar Spectroscopy and Photometry: Surveys and General results
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[17.06] Be Stars in Young Magellanic Cloud Clusters

W. J.\ Roberts (CSC at STScI), E.\ K.\ Grebel (ESO \& Universit\"at Bonn)

We present a photometric method to identify Be stars and calibrate their H$\alpha$ fluxes using imaging in only two broad-band filters and one H$\alpha$ filter.

Very little is known about the Be star content in the Magellanic Clouds. Young clusters and associations may be expected to have Be stars among their B stars, and they form a coherent population. We obtained CCD frames of a number of young SMC and LMC clusters and searched for Be stars there using our photometric method.

We find a pronounced difference between the Be star content in MC clusters and in the Milky Way. The young SMC cluster NGC 330 is outstanding in its high Be star content. The highest fraction can be found among the earliest B type stars. In the LMC, we find lower Be star fractions which turn out to be double-peaked. The first and strongest frequency peak can be found at B0, and a second peak is located at B6.

In contrast, in the Galaxy we find the highest Be star frequency at B2 and faint indications for a second peak at B6. Both peaks are at most half as high than what we find in the MCs.

The ages of the clusters investigated constrain at the same time ages and evolutionary status of the Be stars found. The majority of our Be stars belongs to the main sequence. In NGC 330, a number of Be stars were found to be $\lambda$ Eri variables. Not enough data are yet available, though, to decide whether this is a general property of Be stars in the Magellanic Clouds. Furthermore, we may expect to find high rotational velocities a general property of the investigated clusters. However, we do not yet have an explanation for either the high Be star fractions in the MCs nor for the different frequency peaks.

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