THE FUOR CHARACTERISTICS OF THE PMS STAR BN ORIONIS INFERRED FROM NEWSPECTROSCOPIC AND PHOTOMETRIC-OBSERVATIONS

BN Ori is a young emission-line star with a peculiar light curve. Duri ng the first half of this century the star showed strong irregular bri ghtness- variations, similar to those of Herbig Ae stars. In the curre nt half of the century the light curve resembles that of a FU Ori-obje ct (FUOR, after Ambartsumian 1971). It can be characterised by an init ial large-scale rise in brightness followed by a gradual decay over a period of about 15 Sears. From various photometric patrol programmes w e concluded that the star remained at the same brightness level for th e last 30 years. Between 1991 and 1995 the spectrum of BN Ori was inte nsively observed and was found to exhibit some unique pecularities. We have analysed the visual spectra obtained with the 6m BTA telescope o f the SAG, the 60cm and 48cm telescopes at Mt. Maidanak, the 1.4m CAT and the 1.5 In telescope of ESO and the UV-spectra obtained with the I UE 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 a re typical for late F-type stars. However, in contrast to classical FU ORs, BN Ori is not of high luminosity-class. On the other hand the spe ctrum also shows similarities with those of Herbig A7e stars, although these stars have much stronger absorption lines in the UV from their outer shell and their circumstellar dust excesses are much larger than for BN Ori. Also, the rotation rate nu sin i of BN Ori is between 180 and 220 km s(-1), which is typical for A-type stars of 2 - 5 M-circle dot. This may indicate that BN Ori is a fast rotating FUOR with an in termediate-mass precursor on which the outburst had a different effect than in the case of the classical FUORs (which are rotating slower an d have a low-mass T Tauri star as precursor). In the BN Ori outburst m ost of the massive gas- and dust shell, characteristic for Herbig Ae s tars, seems to have been removed, except for the H alpha- and Mg II-em ission region close to the photosphere. The disappearance of the circu mstellar dust shell may explain the drastic variability-change in the light curve of BN Ori. The mass-accretion rates of BN Ori and several Herbig A7e stars (derived from the analysis of their UV-spectra in ter ms of disc-accretion) suggest that the FUOR outburst in BN Ori was due to a thermal runaway (Bell 1994) in its inner accretion-disc which wa s triggered by a modest increase in the accretion rate.