Dust Formation around Main Sequence Be Stars with Large Infrared Excess

We present a class of Be stars which have prominent infrared excess emission, yet are not associated with any current or recent star-forming regions. Most of these stars show forbidden lines in the spectra and are among the strongest Balmer emitters. Their infrared excess, extending from near- to mid- or even to far-infrared wavelengths, cannot be explained by free-free emission alone, and must be accounted for by dust thermal emission. A comprehensive "neighborhood census" concludes that these Be stars must have evolved beyond the pre-main sequence stage. The circumstellar dust therefore should be produced in situ by condensation in their expanding envelopes. The freshly made grains could be very small in size, and distributed in a compact disk configuration, in contrast to the case in pre-main sequence Herbig Ae/Be stars that inherit dust grains grown since in parental molecular clouds. This class of main sequence stars hence serves as an additional source of cosmic dust production to the usual post-main sequence stars.