Properties of the Hα-emitting circumstellar regions of Be stars

Long-baseline interferometric observations obtained with the Navy Prototype Optical Interferometer of the H alpha-emitting envelopes of the Be stars eta Tau and beta CMi are presented. For compatibility with the previously published interferometric results in the literature of other Be stars, circularly symmetric and elliptical Gaussian models were fitted to the calibrated H alpha observations. The models are adequate for characterizing the angular distribution of the H alpha-emitting circumstellar material associated with these Be stars. To study the correlations between the various model parameters and the stellar properties, the model parameters for eta Tau and beta CMi were combined with data for other Be stars from the literature. After accounting for the different distances to the sources and stellar continuum flux levels, it was possible to study the relationship between the net H alpha emission and the physical extent of the H alpha-emitting circumstellar region. A clear dependence of the net H alpha emission on the linear size of the emitting region is demonstrated, and these results are consistent with an optically thick line emission that is directly proportional to the effective area of the emitting disk. Within the small sample of stars considered in this analysis, no clear dependence on the spectral type or stellar rotation is found, although the results do suggest that hotter stars might have more extended H alpha-emitting regions.