To Be or Not To Be - The Classical Be Star Phenomena

摘要

Classical Be stars are fast rotators with emission lines in their spectra. We present a study of classical Be stars in the open cluster NGC 1857. With age (log age 8.25-8.45 years) and distance (2.09 ± 0.20 kpc) determinations from CCD imaging photometry, we were able to study the evolutionary status of each of the 3 Be stars found in the cluster. The previously known Be star, VES888 is found to have an unusually large amount of IR excess, yet suffers little optical extinction Given the age of the cluster, it cannot be a Herbig Be star. VES888 is on the verge to evolve off the main sequence. In contrast, other two Be stars, with much less IR excess, are at a relatively earlier evolutionary stage. Our study suggests that a classical Be star is evolving on their way to puff off its envelope to become a red giant. Because of the fast rotation, the envelope expands non-spherically in which dust grains condense to reprocess starlight to produce the infrared excess, yet with relatively little extinction, typically seen in these stars.