THE EXCESS INFRARED-EMISSION OF HERBIG AE BE STARS - DISKS OR ENVELOPES/

We suggest that the near-infrared emission of many Herbig Ae/Be stars arises in surrounding dusty envelopes, rather than circumstellar disks . Hillenbrand et al. and Lada & Adams showed that circumstellar disk m odels which reproduce the approximately 3 mum peaks in the near-infrar ed spectral energy distributions of Ae/Be stars must have high accreti on rates, and must either be transparent in their inner regions, or ha ve physical inner disk ''holes.'' However, we show that disks around A e/Be stars are likely to remain optically thick at the required accret ion rates. Alternatively, the assumption of a physical hole in the dis k implies either that large amounts of material pile up at approximate ly 10 stellar radii or that approximately 90% of the accretion luminos ity escapes detection. To avoid these difficulties we propose that the infrared excesses of many Ae/Be stars originate in surrounding dust n ebulae instead of circumstellar disks. These dust envelopes could be a ssociated with the primary star or a nearby companion star. One pictur e supposes that the near-infrared emission of the envelope is enhanced by the same processes that produce anomalously strong continuum emiss ion at temperatures approximately 1000 K in reflection nebulae surroun ding hot stars. This near-infrared emission could be due to small grai ns transiently heated by ultraviolet photons. Some Ae/Be stars show ev idence for the 3.3-3.6 mum emission features seen in reflection nebula e around hot stars, which lends further support to this suggestion. Gi ven the difficulties of applying circumstellar disk models to Ae/Be st ars, dusty nebula hypotheses deserve further consideration.