1999AJ....118.1043H

UBVR photometric monitoring of Herbig Ae/Be stars and some related objects has been carried out at Maidanak Observatory in Uzbekistan since 1983. More than 71,000 observations of about 230 stars have been obtained and are made available for anonymous ftp. Virtually all Herbig Ae/Be stars observed are irregular variables (called ``UXors'' after UX Ori), but there is a wide range of amplitudes from barely detectable to more than 4 mag in V. Our data confirm the results of previous studies, which indicate that large-amplitude variability is confined to stars with spectral types later than B8. The distribution of variability ranges is quite similar to what is seen in classical T Tauri stars. A careful search has failed to reveal any evidence for periodic variations up to 30 days, which can be interpreted as rotation periods. This is a clear distinction between the light variations of low-mass and high-mass pre-main-sequence stars. The Herbig Ae/Be stars evidently do not possess either the large, stable cool spots or persistent hot spots associated with strong surface magnetic fields and magnetically funneled accretion in classical T Tauri stars. A wide variety of shapes, timescales, and amplitudes exists, but the most common behavior is well illustrated by the light curve of LkHα 234. There are two principal components: (1) irregular variations on timescales of days around a mean brightness level that changes on a much longer timescale (typically years), sometimes in a quasi-cyclic fashion, and (2) occasional episodes of deep minima, occurring at irregular intervals but more frequently near the low points of the brightness cycles. Our data suggest that many T Tauri stars of K0 and earlier spectral type share the same variability characteristics as Herbig Ae/Be stars and should be regarded as UXors. Two FU Orionis stars (``FUors''), FU Ori and V1515 Cyg, also have recent light curves that are similar, in some respects, to UXors. The most developed model to account for the variations of some large-amplitude UXors involves variable obscuration by circumstellar dust clumps orbiting the star in a disk viewed nearly edge-on. However, there are problems in extending this model to the entire class, which lead us to propose an alternative mechanism, i.e., unsteady accretion. Evidence favoring the accretion model over the obscuration model is presented. It is suggested that the thermal instability mechanism responsible for outbursts in interacting binary system disks, and possibly FUors, may be the cause of the deep minima in UXors.