THE BETA-PICTORIS PHENOMENON AMONG YOUNG STARS .1. THE CASE OF THE HERBIG AE STAR UX ORIONIS

In this paper we present the first results of the study of very young stars having non-periodic Algol type brightness minima. It is based on cooperative observations between the Crimean Astrophysical Observator y (CAO), the European Southern Observatory (ESO) and the National Sola r Observatory (NSO). In August-September, 1992, a very deep (Delta V = 2.(m)5), long lasting minimum of the light of the isolated Herbig Ae star UX Ori occurred. At this event the star was observed photometrica lly (UBVRI) and polarimetrically at the CAO, and spectroscopically (hi gh resolution: H alpha and NaI D) at the ESO. The spectroscopic observ ations were continued at the NSO with the McMath solar/stellar telesco pe in October-December, 1992, when the star returned to maximum bright ness and again at the ESO in July and October, 1993, when the star was bright. The main results of our observations can be briefly summarize d as follows: 1) The photometric and polarimetric results are in agree ment with the model according to which UX Ori is surrounded by an edge -on circumstellar disk-like envelope, and its variability is caused by variable obscuration of the star by opaque circumstellar dust clouds. 2) The double-peaked Ha profile observed at maximum light changed to single-peaked at deep minimum; obscuration of a part of the circumstel lar gas by an optically thick dust cloud is causing this variation, 3) The inverse P Cygni profiles and variable redshifted absorption compo nents have been observed in the NaI D lines indicating the infall of c ool gas onto the star. We believe that, such as in the case of the sta r beta Pictoris, violent comet-like activity takes place in the young protoplanetary disk of UX Ori which causes the observed variability.