SIMBAD references

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 Observatory (CAO), the European Southern Observatory (ESO) and the National Solar Observatory (NSO). In August-September, 1992, a very deep ({DELTA}V=2.5mag), long lasting minimum of the light of the isolated Herbig Ae star UX Ori occurred. At this event the star was observed photometrically (UBVRI) and polarimetrically at the CAO, and spectroscopically (high resolution: Hα and NaiD) at the ESO. The spectroscopic observations were continued at the NSO with the McMath solar/stellar telescope in October-December, 1992, when the star returned to maximum brightness and again at the ESO in July and October, 1993, when the star was bright. The main results of our observations can be briefly summarized as follows: 1) The photometric and polarimetric results are in agreement 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 Hα profile observed at maximum light changed to single-peaked at deep minimum; obscuration of a part of the circumstellar gas by an optically thick dust cloud is causing this variation, 3) The inverse P Cygni profiles and variable redshifted absorption components have been observed in the NaiD lines indicating the infall of cool gas onto the star. We believe that, such as in the case of the star β Pictoris, violent comet-like activity takes place in the young protoplanetary disk of UX Ori which causes the observed variability.