J/A+AS/124/33 J/A+AS/124/33 BN Ori UBV photometry & equivalent width V S Shevchenko O Ezhkova H R E Tjin A Djie M E van den Ancker P F C Blondel D de Winter Astron. Astrophys. Suppl. Ser. 124 33 1997 1997A&AS..124...33S Equivalent widths Photometry, UBV Stars, pre-main sequence circumstellar matter stars: emission-line, Be stars: individual (BN Ori) stars: pre-main sequence 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 1971Afz.....7..557A). 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 peculiarities. We have analysed the visual spectra obtained with the 6m BTA telescope of the SAO, the 60cm and 48cm telescopes at Mt. Maidanak, the 1.4m CAT and the 1.5m 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 vsinii of BN Ori is between 180 and 220km/s, which is typical for A-type stars of 2-5M_{sun}_. 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 H{alpha}- and MgII-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, 1994ApJ...427..987B) in its inner accretion-disc which was triggered by a modest increase in the accretion rate.

BN OriHD 24546505 36 29.4+06 50 01 HR 5999HD 14466816 08 34.2-39 06 17 FU Ori05 45 22.5+09 04 12

lambda Wavelength 0.1nm Ident Line identification --- Ex Excitation energy eV S Line source number=1 p: photospheric c: chromospheric i: interstellar s: shell, c.s. envelope d: disc --- EW1 BN Ori equivalent width 0.1nm u_EW1 Uncertainty flag on EW1 --- n_EW1 Note on EW1 number=2 b: blended vs: very strong st: strong m: medium w: weak vw: very weak EW1 data are from BTO-spectrum Nov.21, 1991 data except when n_EW1=2 where data are from Kolotilov & Zajtseva (1976Afz....12...31K) EW2 data are from Tjin A Djie et al. (1989A&AS...78....1T) EW3 data are from Herbig (1966VA......8..109H) --- FW1 BN ORI Full width at half maximum km/s u_FW1 Uncertainty flag on FW1 --- EW2 HR 5999 equivalent width 0.1nm u_EW2 Uncertainty flag on EW2 --- n_EW2 Note on EW2 number=2 b: blended vs: very strong st: strong m: medium w: weak vw: very weak EW1 data are from BTO-spectrum Nov.21, 1991 data except when n_EW1=2 where data are from Kolotilov & Zajtseva (1976Afz....12...31K) EW2 data are from Tjin A Djie et al. (1989A&AS...78....1T) EW3 data are from Herbig (1966VA......8..109H) --- FW2 HR 5999 Full width at half maximum km/s n_FW2 + when two values of FW2, : uncertainty flag --- FW2b Second FWHM value when n_FW2="+" km/s EW3 FU Ori equivalent width 0.1nm n_EW3 Note on FU Ori equivalent width number=2 b: blended vs: very strong st: strong m: medium w: weak vw: very weak EW1 data are from BTO-spectrum Nov.21, 1991 data except when n_EW1=2 where data are from Kolotilov & Zajtseva (1976Afz....12...31K) EW2 data are from Tjin A Djie et al. (1989A&AS...78....1T) EW3 data are from Herbig (1966VA......8..109H) --- HJD Heliocentric Julian date d Vmag V magnitude mag U-B U-B colour index mag B-V B-V colour index mag V-R V-R colour index mag Tel Telescope number=1 1: Zeiss 600 reflector (60cm) 3: AZT 14 reflector (48cm) --- Obs Observer number=2 C: A.V. Chernyshev G: K.N. Grankin I: M.A. Ibragimov M: S.Yu Melnikov P: D.L. Portnov S: V.S. Shevchenko Y: S.D. Yakubov --- n_Obs Note on observations number=3 Simultaneous with the high-resolution spectrum from SAO (Sect.4.3) --- tables.texLaTeX version of the tables Patricia Bauer CDS 1996Nov12P.F.C. Blondel <blondel@sara.nl> J_A+AS_124_33.xml