On the similarity of FU Orionis stars to class I protostars: evidence from the submillimeter

On the basis of analysis of new submillimeter maps of a sample of most known FU Orionis stars, we confirm that these stars are extremely young, more similar to Class I protostars than to Class II T Tauri stars. The submillimeter maps reveal that FU Orionis stars have accretion disks that are larger and contain an order of magnitude more mass than those of T Tauri stars but are comparable in mass to those seen around Class I sources. The disks around the FU Orionis stars are sufficiently massive to act as reservoirs of material for replenishing the mass of the disks after eruptions, in which large amounts of material are believed to fall onto the underlying stars. We have identified a number of FU Orionis stars that are surrounded by extended cloud emission, and in several cases we find nearby submillimeter sources that have no optical or near-infrared counterparts. These sources most likely are young protostellar Class I or Class 0 sources. In at least in one case, V1735 Cyg, the dominant submillimeter source is the protostar, not the optically visible FU Orionis star.