A photometric catalog of Herbig Ae/Be stars and discussion of the nature and cause of the variations of UX Orionis stars

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 mad e available for anonymous ftp. Virtually all Herbig Ae/Be stars observed ar e irregular variables (called "UXors" after UX Ori), but there is a wide ra nge of amplitudes from barely detectable to more than 4 mag in V. Our data confirm the results of previous studies, which indicate that large-amplitud e variability is confined to stars with spectral types later than B8. The d istribution of variability ranges is quite similar to what is seen in class ical 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 per iods. 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 ass ociated with strong surface magnetic fields and magnetically funneled accre tion 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 LkH alpha 234. There are two principal components: (1) irreg ular variations on timescales of days around a mean brightness level that c hanges on a much longer timescale (typically years), sometimes in a quasi-c yclic fashion, and (2) occasional episodes of deep minima, occurring at irr egular 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 V15 15 Cyg, also have recent light curves that are similar, in some respects, t o UXors. The most developed model to account for the variations of some lar ge-amplitude UXors involves variable obscuration by circumstellar dust clum ps orbiting the star in a disk viewed nearly edge-on. However, there are pr oblems in extending this model to the entire class, which lead us to propos e an alternative mechanism, i.e., unsteady accretion. Evidence favoring the accretion model over the obscuration model is presented. It is suggested t hat the thermal instability mechanism responsible for outbursts in interact ing binary system disks, and possibly FUors, may be the cause of the deep m inima in UXors.