Be Stars in the Open Cluster NGC 663

Here are some Be stars in their natural habitat - a young open cluster. As this blink comparison shows, the characteristic that distinguishes them from their neighbors is emission of deep red light in the H-alpha spectral line of hydrogen. Be stars are formally defined as non-supergiant B-type stars whose spectra have, or had at some time, one or more Balmer lines in emission.

The orange-tinted image of the cluster was taken through a narrow band filter transmitting at a wavelength slightly different from H-alpha. The red-tinted image was made by subtracting a calibrated fraction of the first image from one taken through a narrow band H-alpha filter, leaving only the pure hydrogen emission.

Both cluster images are averages of three 60 s exposures taken on November 29, 1995 with the 0.4 m telescope and Photometrics CCD camera (Thomson 512 chip) at Limber Observatory, processed with the IRAF software package. North is up, east is left, and the field of view is about 7 arcmin on a side. The colors are artificial, but they approximate the colors of light transmitted by the filters.

Above is a comparison of the optical spectra of a Be star and a normal B star, showing the H-alpha and H-beta lines in absorption for the normal B star (lower), but in emission for the Be star (upper).

The mystery of the "Be phenomenon" is that the hydrogen emission can come and go episodically on timescales as short as a few days and as long as several decades. Rapid rotation, nonradial pulsation, radiation-driven winds, flarelike magnetic activity, and binary interaction have all been proposed as factors that may cause the star to shed material from its surface, leading to the formation of an equatorially flattened disk of gas around it, where the emission would be produced.

It may be possible to learn more about the evolutionary status of Be stars by studying the statistics of their occurrence in clusters of different age. That requires a photometric color analysis or spectroscopic data in addition to the H-alpha imaging, in order to separate the Be stars from other kinds of stars that may also show H-alpha emission.

NGC 663 in Cassiopeia contains one of the largest percentages of Be stars known in any young open cluster, so there is a rich history of investigations intended to determine its age. The variety of results obtained shows clearly the practical difficulties involved with efforts to understand the origin of the Be phenomenon by applying stellar evolution theory to observational data on open clusters.