The light curve of BN Ori and its spectral type A7 (Cannon 1931) during the first half of the century suggest that in this period BN Ori was a Herbig Ae star with strong brightness-variations due to a variable circumstellar dust extinction. The recent detection of emission from a nearby reflection nebulosity (Sect. 3 (click here)) and the value of the stellar mass (Sect. 5 (click here)) supports its previous membership to this class of intermediate-mass pre-main-sequence stars, first introduced by Herbig (1960b).
Around 1947 the large-scale light curve of BN Ori changed drastically in a way which is similar to the transition in the light curves of classical FUORs. In particular the light curve of BN Ori has much in common with that of the FUOR V1515 Cyg due to its last active epoch of variations in brightness with the further rise typical for FUORs, a delay at the maximum phase and specifically a typical short minimum after the first maximum (Fig. 12 (click here)). The sequence and scale of the events in the pre-maximum and post-maximum phases of the BN Ori light curve are also consistent with those of other classical FUORs. The only possible difference with the FUOR light curves is the large amplitude of the variability in the pre-maximum epoch of BN Ori. The light-variation level of the classical FUORs was less in this phase, but for the lack of any reliable information it cannot be argued that the range of brightness-variations in the pre-flare phase of FUORs was small. Therefore, as far as the light curve is concerned, we have every reason to consider BN Ori as an object which is related to FUORs.
Figure 12: Comparison of the light curves of BN Ori (upper part; dots) and the FUOR V1515 Cyg (lower part; stars)
The spectroscopic observations of BN Ori, described in Sects. 4 (click here) and 5 (click here), show that there are also spectral similarities between BN Ori and the classical FUORs, such as FU Ori:
Figure 13: Shell regions of BN Ori, HR 5999 and FU Ori; vs. distance to the surface (in units of ). The two dashed lines are the least-square lines for BN Ori and HR 5999, using the values of the FeII and CrII lines together. The dotted line indicates the stratification of V1057 Cyg
Besides these similarities between the spectrum of BN Ori and those of the classical FUORs there are also differences:
After this comparison of the light curves and spectra of BN Ori, FU Ori and HR 5999 we propose the following scenario for the behaviour of BN Ori: The precursor of BN Ori was a Herbig A6- A7e star, similar to HR 5999 but of somewhat lower mass (comparable with BF Ori) and with a slightly higher rotation rate. Around 1947 a FUOR type outburst occurred in the star-disc system with the result that the envelope around the inner part of the accretion-disc expanded. Except for the part near the star where most of the H and MgII emission originates, the optically thin (shell) region at the outside of this envelope was expelled together with the outer part (R> 2.0) of the accretion-disc. In the spectrum of FU Ori the shell components of the lines indicate that the shell moves outward with a velocity around -50 (Herbig 1966), but for BN Ori we have no radial-velocity measurements available. Most of the circumstellar dust was probably blown away after the break-up of the outer part of the magnetic field, which confined the dust to the circumstellar region (Il'in & Voshchinnikov 1993) before the outburst. The disappearance of this dust (or the smoothing of its distribution), may explain the levelling-off of the light curve after the outburst.
This scenario raises several questions, the most important being whether a FUOR outburst can occur in a Herbig Ae star. Recently Bell et al. (1995) have shown that recurrent FUOR outburst can occur in young stars with disc accretion as a result of a thermal-runaway in the inner part of the accretion-disc when the mass-accretion rate rises above a certain critical value. For 2- 3 pre-main-sequence stars, such as the Herbig stars, Bell (1994) predicted that this critical value is around a few times 10. It may be interesting to note that from matching the low-dispersion UV-spectra of BN Ori, BF Ori and HR 5999 we found mass-accretion rates of 0.18, 3.2 and 5.4 respectively. The latter two values are close to the critical value of the mass-accretion rate, while the low rate for BN Ori (compared e.g. with that of FU Ori) may be due to the present lack of supply of inflow from the circumstellar shell region.
If we accept the FUOR character of BN Ori we are faced with the question why BN Ori is different from the classical FUORs. Two main reasons for differences may be proposed: First of all there is the difference in mass and rotation rate between Herbig Ae and T Tauri precursors which could influence the relative violence of the outbursts. The present low accretion rate of BN Ori (corresponding to an optically thin disc) compared to that of FU Ori may permit a more rapid expansion of its outer shell, which could explain the difference mentioned in point 3 discussed above. Secondly, successive outbursts in a recurrent FUOR have differences in their initial conditions and may therefore show differences in their post-flare appearence. Recurrent outbursts in BN Ori would facilitate a gradual exhaustion of the shell supply, as well as the accumulative production of sizable amounts of LiI at the surface of BN Ori, e.g. by spallation reactions induced by high fluxes of energetic protons in the FUOR flares.
Table 10: Description of the symbols and references used in Tables 6 (click here), 11 (click here) and 8 (click here) and source ( ) of lines in the high-resolution visual spectra of BN Ori, HR 5999 and FU Ori. The equivalent widths (EW) in (Å) and full widths at half maximum (FW) in ( ) are also given. Identification of the lines has been made with the help of the Tables of Zaidel et al. (1969) and excitation energies () have been taken from Wehrse (1974). See Table 10 (click here) for the description of the used symbols
Table 11: Continued
Johnson (UBVR) Photometric Data of BN Ori
Table 12: continued
Table 12: continued
The authors would like to thank Dr. A. Brown (JILA) for collaborating in the IUE observations and reductions of BN Ori. Furthermore, we are indebted to all the observers mentioned in Tables 1 (click here), 5 (click here) and 12 (click here) and to Dr. J.R.W. Heintze (RUU) for his efforts to develop and maintain the photometric facilities at DASA. This research has made use of the Simbad data base operated at CDS, Strasbourg, France and of the IUE archives operated by ESA at Villafranca del Castillo, Spain.