THE FU ORIONIS OUTBURST AS A THERMAL ACCRETION EVENT - OBSERVATIONAL CONSTRAINTS FOR PROTOSTELLAR DISK MODELS

The results of the time-dependent disk models developed in Bell and Li n are compared with observed properties of FU Orionis variables. Speci fic models are fit to the light curves of FU Ori, V1515 Cyg, and V1057 Cyg. The slow risetime of V1515 Cyg can be matched by a self-regulate d outburst model. The rapid risetimes of FU Ori and V1057 Cyg can be f itted with the application of modest perturbations to the disk surface density. Model disks display spectral features characteristic of obse rved objects. The color evolution of V1057 Cyg is naturally explained if mass flux drops in the inner disk (tau < 1/4 AU) while remaining st eady in the outer disk. The decrease in optical line width (rotational velocity) observed during the decay of V1057 Cyg may be accounted for by an outward-propagating ionization front. We predict that before fi nal decay to the quiescent phase, short-wavelength line widths (lambda < 1.5 mu m) will again increase. It is suggested that FU Orionis outb ursts primarily occur to systems during the embedded phase with ages l ess than several times 10(5) yr.