The FUOR characteristics of the PMS star BN Orionis^*

inferred from new spectroscopic and photometric observations

¹ Astronomical Institute of the Academy of Sciences of Uzbekistan, Astronomicheskaya 33, Tashkent 700052, Uzbekistan
² Astronomical Institute "Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Dpto. Física Teórica, C–XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, E–28049 Madrid, Spain

Send offprint request to: P.F.C. Blondel

Received: 4 June 1996
Accepted: 7 October 1996

Abstract

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. On the other hand the spectrum 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  of BN Ori is between 180 and 220 , which is typical for A-type stars of 2 - 5 . This may indicate that BN Ori is a fast rotating FUOR with an intermediate-mass precursor on which the outburst had a different effect than in the case of the classical FUORs (which are rotating slower and have a low-mass T Tauri star as precursor). In the BN Ori outburst most of the massive gas- and dust shell, characteristic for Herbig Ae stars, seems to have been removed, except for the Hα- and Mg ii-emission region close to the photosphere. The disappearance of the circumstellar 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 terms 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 was triggered by a modest increase in the accretion rate.