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6. Discussion

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:

  1. The various spectral classifications of BN Ori by successive observers: A7 (Cannon 1931; Hoffmeister 1949; Payne-Gaposhkina 1952), A6- A8 (Herbig 1954, 1960a), F0 (Zajtseva 1970) and F2- F3 (Kolotilov & Zajtseva 1976) could raise the impression that BN Ori has changed to progressively later spectral types in the course of the years. However, as already suspected by Herbig (1960a), several spectral types can be present simultaneously in the spectrum of BN Ori. Our spectra (Sect. 4 (click here)) show a variety of spectral types ranging from A6- A8 from the higher Balmer terms, A8- F0 from the CaIIK line, F0- F2 from the G-band, F0- G5 from the FeI lines, F6- G0 from the OI (1)IR-triplet G5 from the CaII (2)IR-triplet until late-G to early-K from the strength of the MnI triplet around 4033Å. This suggests that the atmosphere of BN Ori is thermally stratified in the sense that it consists of an A6- A7 photosphere, surrounded by a cooler envelope. Such a thermal stratification is also a characteristic feature of FUOR atmospheres, e.g. FU Ori has a photosphere of type F2:pI-IIe (Herbig 1966), estimated from the Balmer lines and the low excitation lines of neutral and singly-ionised metals in the visual, an envelope (or disc) of type K from the CaII (2)IR-triplet\ and a type K- M from the CO bands in the NIR part of the spectrum. However, in contrast to the situation for BN Ori and for FU Ori we find only a small range of spectral types, A5- A7III (Bessel & Eggen 1972; Tjin A Djie et al. 1989), for the visual and red parts of the spectrum of the Herbig Ae star HR 5999, which suggests a much lower degree of stratification of the atmosphere of this star.
  2. Another similarity with classical FUORs is the presence of high-velocity outflow-components in the Htex2html_wrap_inline4564 P-Cygni type profiles: -680tex2html_wrap_inline4568 for BN Ori and -550tex2html_wrap_inline4572 for FU Ori. The time-scale of the shape-variability of this profile may be less than 24tex2html_wrap_inline4574 (Kolotilov & Zajtseva 1976) for BN Ori, which is comparable with those observed for FUORs. This suggests that the line is formed in a region of small extension. The large width of the emission-component profile (Fig. 7 (click here)) indicates that this region is within a distance of 1tex2html_wrap_inline4576 from the stellar surface. Similar to the fast winds from FUORs (Croswell et al. 1987), the fast outflow from BN Ori is probably driven by strong magnetic fields, originating near the accretion-disc boundary layer (Pringle 1989). In contrast with this, we find only relatively low-outflow velocity-components in the Htex2html_wrap_inline4578-profiles of HR 5999: in general tex2html_wrap_inline4580-50tex2html_wrap_inline4584 and occasionally tex2html_wrap_inline4586-120tex2html_wrap_inline4590 (Tjin A Djie et al. 1989). The time-scale of the Htex2html_wrap_inline4592 shape-variability for HR 5999 is of the same order as that of BN Ori. The profiles of MgII h&k are similar in shape to those of Htex2html_wrap_inline4594 in the three stars. The velocity of the outflow-components in the MgII profiles seems to be lower than that in Htex2html_wrap_inline4596 (tex2html_wrap_inline4598-250tex2html_wrap_inline4602 for BN Ori) but these values are rather uncertain, due to the low S/N for the neighbouring continuum. Since only one high-resolution UV-spectrum has been obtained for BN Ori and two for FU Ori, the time-scales of variability of the MgII profiles are unknown.
  3. Several prominent photospheric lines in the spectra of FU Ori and the FUOR BBW 76 (Reipurth 1990): LiI (6708Å) and BaII (6497Å) are also prominently present in the spectrum of BN Ori (Sect. 4.3 (click here)) with comparable strength. In the spectrum of HR 5999, however, the LiI line is missing.

Figure 13:   Shell regions of BN Ori, HR 5999 and FU Ori; tex2html_wrap_inline4604 vs. distance to the surface (in units of tex2html_wrap_inline4606). 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:

  1. The spectrum of BN Ori is of luminosity-class III-IV while the FUOR spectra have luminosity-classes I-II.
  2. The rotation rate tex2html_wrap_inline4608 of BN Ori is about 220tex2html_wrap_inline4610, which is close to those of several intermediate-mass Herbig Ae stars (tex2html_wrap_inline4612 = 180tex2html_wrap_inline4614 for HR 5999). In contrast with this, the rotation rates of classical FUORs are in the range of 60tex2html_wrap_inline4616 (FU Ori) which is more similar to those of their T Tauri precursors.
  3. BN Ori has a more extended shell than FU Ori. This is illustrated by the FW distribution over the excitation energies (tex2html_wrap_inline4618) of Tables 6 (click here), 11 (click here) and 8 (click here). If we limit ourselves to the unblended lines of these tables and if we can consider the FWs to be proportional to the tex2html_wrap_inline4620 of the regions of formation of these lines we can, assuming a Kepler motion, estimate mean distances of these regions to the stellar surface. The values of tex2html_wrap_inline4622 and tex2html_wrap_inline4624 needed for this transformation were taken from Table 7 (click here). The distribution of tex2html_wrap_inline4626 (of the lower-level) versus the mean Kepler distance of the line-forming regions (in tex2html_wrap_inline4628) is given in Fig. 13 (click here) for BN Ori, FU Ori and HR 5999. For comparison the thermal stratification of the FUOR V1057 Cyg is indicated by the dotted line. Welty et al. (1990) showed that this stratification originates from the differential rotation in the disk. Figure 13 (click here) demonstrates that outside the stratified envelope there is an extended shell region for BN Ori, FU Ori and HR 5999. This is especially clear from the FeII & CrII absorption lines (with lower level tex2html_wrap_inline4630 1eV) which are still formed far away from the star (tex2html_wrap_inline463240tex2html_wrap_inline4634 for FU Ori and HR 5999, and >400tex2html_wrap_inline4638 for BN Ori). The difference of the shell extension is illustrated by the least-square fits for BN Ori and HR 5999 in Fig. 13 (click here). The uncertainty for the size of the BN Ori shell is due to the fact that the UV-lines are so narrow that the widths may be dominated by Doppler broadening and therefore only give an upper limit to the distances. Although the cool shell of BN Ori extends much further out than those of FU Ori and HR 5999 we note from Table 8 (click here) that for each line: tex2html_wrap_inline4640. As the corresponding column densities follow the same order (BN:FU:HR = 1.0:4.0:8.6), this means that BN Ori has the most diluted shell of these 3 stars. This suggests that if BN Ori had a Herbig Ae star (like HR 5999) as its precursor, the thick shell has been expanded (to the shell remnant now present) as a result of a (FUOR) outburst. This expansion had a cooling influence on the extended MgII emission region and less (or no) influence on the Htex2html_wrap_inline4642 emission region which is confined to a region closer to the star. The ratios in Table 9 (click here) are consistent with this scenario.
  4. FU Ori and HR 5999 have very large excess fluxes in the NIR. These excesses are too large to be accounted for by the contribution of the accretion-discs only and therefore indicate the presence of significant quantities of radiating nearby circumstellar dust. Another indication for this are the large circumstellar colour excesses in Table 7 (click here). In contrast with this we find a very small NIRE and colour excess for BN Ori, which suggests an almost complete absence of circumstellar dust around this star.

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 Htex2html_wrap_inline4646 and MgII emission originates, the optically thin (shell) region at the outside of this envelope was expelled together with the outer part (R> 2.0tex2html_wrap_inline4650) 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 -50tex2html_wrap_inline4654 (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- 3tex2html_wrap_inline4658 pre-main-sequence stars, such as the Herbig stars, Bell (1994) predicted that this critical value is around a few times 10tex2html_wrap_inline4660tex2html_wrap_inline4662. 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 tex2html_wrap_inline4664tex2html_wrap_inline4666 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 (tex2html_wrap_inline4668 ) 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 (tex2html_wrap_inline4670 ) are also given. Identification of the lines has been made with the help of the Tables of Zaidel et al. (1969) and excitation energies (tex2html_wrap_inline4672) have been taken from Wehrse (1974). See Table 10 (click here) for the description of the used symbols

  Table 11:   Identification

Table 11: Continued

  Table 12:   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.

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