[Previous] | [Session 73] | [Next]
W. Herbst (Wesleyan Univ.), V. S. Shevchenko (Tashkent Astronomical Inst.)
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 magnitudes in V. Our data confirm the results of previous studies which indicate that large amplitude variablitiy 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. It suggests that magnetically funneled accretion which results in hot spots on classical T Tauri stars is not operative in Herbig Ae/Be stars, nor are there any large cool spots on the surfaces of these stars. A wide variety of light curve shapes, time scales and amplitudes exists, but the most common behavior is illustrated in relatively clean fashion by the example of LkH-alpha 234. It has two principal components: 1) irregular variations on timescales of days around a mean brightness level which changes on much longer timescale (typically years) sometimes in quasi-cyclic fashion, and 2) occasional episodes of deep minima, occuring 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 FUors, FU Ori and V1515 Cyg, also have recent light curves which 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 - variable accretion. Evidence favoring the accretion model over the obscuration model is discussed.
The author(s) of this abstract have provided an email address for comments about the abstract: wherbst@wesleyan.edu