The FUOR characteristics of the PMS star BN Orionis

BN Ori is a young emission-line star with a peculiar light curve. During the first half of this century the star showed strong irregular brightness-variations, similar to those of Herbig Ae stars. In the current half of the century the light curve resembles that of a FU Ori-object (FUOR, after Ambartsumian 1971). It can be characterised by an initial large-scale rise in brightness followed by a gradual decay over a period of about 15 years. From various photometric patrol programmes we concluded that the star remained at the same brightness level for the last 30 years. Between 1991 and 1995 the spectrum of BN Ori was intensively observed and was found to exhibit some unique pecularities. We have analysed the visual spectra obtained with the 6 m BTA telescope of the SAO, the 60 cm and 48 cm telescopes at Mt. Maidanak, the 1.4 m CAT and the 1.5 m telescope of ESO and the UV-spectra obtained with the IUE in 1984 and 1986. The spectra show certain similarities with those of classical FUORs, such as a large-scale thermal stratification, with Balmer lines showing A6 -A7 spectral type wings, while other lines are typical for late F-type stars. However, in contrast to classical FUORs, BN Ori is not of high luminosity-class.

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