The ring-shaped nebulae around FU Orionis stars

Abstract

All five of the known FU Orionis stars are associated with ring-shapednebulae. Deep CCD images and spectra are presented of three of thenebulae, those associated with V1057 Cyg, V1515 Cyg, and V1735 Cyg.The spectra of all three nebulae are clearly from reflection. A crudemodel for the dust shells is briefly discussed, and it is arguedthat the shells may indicate a relatively advanced evolutionary statefor the FU Orionis stars. The elongated shape of the shells may bethe result of the relative motion of the star through the parentmolecular cloud, in which case there is only a single cavity. TheFU Orionis outburst itself may then occur when the star reaches theedge of the cavity and fresh material is accreted. Some of the morerecent models for the outburst itself are discussed and some observationaltests of these models suggested.