Physical Properties of Be Stars in Open Clusters

In spite of the large efforts spent so far and the large body of observational data available, no satisfactory explanation has been found for the origin of the envelopes around Be type stars. This is due, among other reasons, to the fact that the spectrum of the central star is contaminated by the radiation of the envelope. Therefore, it is mandatory to separate the combined spectrum into its two components: central star + envelope. Moreover, as these objects show large photometric and spectroscopic variability, the combination of data from different epochs leads to uncertain conclusions. With this ideas in mind, we have developed an observational program where simultaneous uvbybeta photometry and Balmer line spectroscopy of Be stars in open clusters are combined to yield general properties of Be stars, as a class, and physical properties of their circumstellar envelopes, by means of a new method to decouple the combined spectrum. The main results of this research can be summarized as follows: A uvbybeta calibration has been developed to obtain the intrinsic radiative parameters of the underlying B stars. Linear relationships between the equivalent widths of the Balmer emission lines and the photometric anomalies are presented. It is independently confirmed that, on the average, Be stars are 0.3 mag brighter than B stars of the same spectral type due to the circumstellar contribution to MV. In the (b-y)0-MV plane, Be stars deviate to redder positions with respect to the non-emission B stars due to additional reddening caused by hydrogen free-free recombination in the envelope. In the c0-MV plane we have found two different behaviors: Be stars earlier than B5 V show smaller (bluer) c0 values due to free-bound transitions in the envelope, which reduce the Balmer jump, while late-type Be stars show normal or larger c0 values (in spite of the emission levels at Hα ) indicating absorption at the Balmer discontinuity of circumstellar origin. Balmer-line spectroscopy supports the picture of an envelope with axial symmetry, probably of disk-like geometry, with a quasi-Keplerian rotation field, already deduced for Be field stars. I have found evidence however, that the bulk of the Balmer line emission arises in a relatively small fraction of the envelope. Average physical properties of the envelopes are inferred from a study of Balmer decrements and can be resumed in the following table: The spread in the Balmer decrements can be ascribed partially to different temperatures of the circumstellar material which may range between Te ~ (2.0-1.0)x 104 K. For the latest spectral types, the temperature of the envelope could be slightly lower. I find that this temperature is not clearly correlated with the temperature of the central star but with the size of the envelope. I conclude that, on the average, the smallest envelopes are the hotter, due to luminosity conservation. Finally some clues on the evolutionary status of Be stars are presented. I conclude that a Be star cannot be a very young object. My results would point towards the interpretation of Be objects as the post mass-transfer phase of close binary systems.