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The origin of thermal optical and UV emission from stellar tidal disruptionflares (TDFs) remains an open question. We present Hubble Space Telescopefar-UV (FUV) observations of eight optical/UV selected TDFs 5-10 yearspost-peak. Six sources are cleanly detected, showing point-like FUV emissionfrom the centers of their host galaxies. We discover that the light curves ofTDFs from low-mass black holes ($<10^{6.5} M_\odot$) show significant late-timeflattening. Conversely, FUV light curves from high-mass black hole TDFs aregenerally consistent with an extrapolation from the early-time light curve. Theobserved late-time emission cannot be explained by existing models forearly-time TDF light curves (i.e. reprocessing or circularization shocks), butis instead consistent with a viscously spreading, unobscured accretion disk.These disk models can only reproduce the observed FUV luminosities, however, ifthey are assumed to be thermally and viscously stable, in contrast to thesimplest predictions of alpha-disk theory. For one TDF in our sample, wemeasure an upper limit to the UV luminosity that is significantly lower thanexpectations from theoretical modeling and an extrapolation of the early-timelight curve. This dearth of late-time emission could be due to a diskinstability/state change absent in the rest of the sample. The disk models thatexplain the late-time UV detections solve the TDF "missing energy problem" byradiating a rest-mass energy of ~0.1 solar mass over a period of decades,primarily in extreme UV wavelengths.

Late-time UV observations of tidal disruption flares reveal unobscured, compact accretion disks