The ring-shaped nebulae around FU Orionis stars

All five of the known FU Orionis stars are associated with ring-shaped nebulae. Deep CCD images and spectra are presented of three of the nebulae, those associated with V1057 Cyg, V1515 Cyg, and V1735 Cyg. The spectra of all three nebulae are clearly from reflection. A crude model for the dust shells is briefly discussed, and it is argued that the shells may indicate a relatively advanced evolutionary state for the FU Orionis stars. The elongated shape of the shells may be the result of the relative motion of the star through the parent molecular cloud, in which case there is only a single cavity. The FU Orionis outburst itself may then occur when the star reaches the edge of the cavity and fresh material is accreted. Some of the more recent models for the outburst itself are discussed and some observational tests of these models suggested.

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