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1. Introduction

BN Ori (HD 245465, BD+06971) is an emission-line star about 3tex2html_wrap3674  South-East to the tex2html_wrap_inline3642 Ori cluster and probably connected to the star-formation region RSF1 in the Orion B complex by a dark lane of dust. A detailed review of the star's properties and the observational history up to 1989 has been given by Shevchenko (1989). Because red-sensitive photographic plates for astronomical use did not become widely available until the fifties, the (Htex2html_wrap_inline3644) emission character of BN Ori during the first half of the century cannot be ascertained at present. Nevertheless, before 1947 the star had several characteristics of the class which is presently known as the Herbig Ae stars: spectral type A7 (Cannon 1931) and, similar to T Ori (Beyer 1937), strong and irregular variations in brightness and colour, characteristic for the variability of circumstellar dust extinction. Around 1947 its variability character changed (Fig. 1 (click here)): the amplitude of the large brightness-variations decreased and, after a large-scale rise of tex2html_wrap_inline364605 during the next 6 months, the brightness slowly decreased until it settled down at tex2html_wrap_inline364896 around 1965. Since then the brightness remained at this level, apart from a small and short variation in 1991. The photometric history and the photometric observations made during the last decade are summarised in Sect. 2 (click here).

Table 1:   Log of photometric observations of BN Ori, NN denotes the number of nights

Shevchenko (1989) was the first to discuss the possibility to interpret the light curve of BN Ori after 1945 in terms of a process related to the large-scale light-variations of classical FUORs. The phenomenon of FUOR (Herbig 1977), although a prominent eruptive event of great intensity and duration, cannot be predicted nor explainded by the present evolution models (e.g. Palla & Stahler 1992, 1993) of pre-main-sequence stars. At the same time, more and more young non-stationary stars exhibiting high-amplitude eruptions can be considered as related to FUORs. Herbig (1977) selected a group of stars (EX Lup, UZ Tau and VY Tau) showing recurrent brightening with great intensity and duration and later labelled them "EXORs" by the name of its brightest member EX Lup. In the early sixties, DR Tau, the most active classical T Tauri star (CTTS), exhibited a progressive brightening lasting for more than 15 years (Götz 1980). Some eruption related with those of FUORs were found from the light curves and the spectra of Z CMa (Hartmann et al. 1989), V1331 Cyg and other stars which differ widely from classical FUORs.

The large-scale light curve of BN Ori as well as several spectral peculiarities indicate a certain similarity of the BN Ori properties with those of the FUORs. In this paper we first discuss the large-scale historical light curve of BN Ori (Sect. 2 (click here)).

During the last decade we have obtained several high- and low-resolution spectra of BN Ori in the visual and in the UV. A description of these spectra and a discussion of the peculiar spectral features is given in Sects. 4.2 (click here), 4.3 (click here) and 5 (click here). The analysis of the optical spectra reveals a noticeable thermal stratification of the envelope of BN Ori, which is similar to what has been deduced from the analysis of the spectra of classical FUORs on the descending branch of their light curves. In BN Ori the stratification is reflected by the different values for the full-width at half maximum (FW hereafter) for lines from different temperature regions in the envelope. This can be interpreted by differential rotation in a rotationally flattened envelope or disc. Another similarity with classical FUORs is the high wind-velocity measured from the fast variable (on time-scale of 1 day or less) P-Cygni type Htex2html_wrap_inline3660 and MgII h&k profiles of BN Ori. This type of outflow can probably only be driven by strong magnetic fields, e.g. produced in the neighbourhood of an accretion-disc boundary layer. Nevertheless, there are also differences between the spectrum of BN Ori and those of classical FUORs. Such differences may not be surprising in view of the fact that BN Ori is more massive (tex2html_wrap_inline36622tex2html_wrap_inline3664) and is rotating much faster than the classical FUORs.

We suggest that BN Ori in its present phase is the result of FUOR type outburst in a strongly variable Herbig A6- 7e star. A similar transition was observed in 1983 for the Herbig A7e star V351 Ori (HD38238), which changed from a strongly variable phase into a non-variable A7e phase (van den Ancker et al. 1996), demonstrating that such phenomena are not exceptional for Herbig Ae stars. If this also happened with BN Ori, it becomes of interest to compare its spectral features with those of a Herbig A6- 7e star as such a star might be similar to BN Ori before the FUOR outburst. We made a detailed comparison with the spectrum of HR 5999 (V856 Sco), a bright and strongly variable Herbig A5- 7e star, which has been intensively studied during the last two decades (Bessell & Eggen 1972; Tjin A Djie et al. 1989; Baade & Stahl 1989). BN Ori has most lines in common with HR 5999, although the line-widths are partly different. However, it appears that the lines from the cool outer parts of the envelope (shell) are much fainter in BN Ori than in HR 5999, as is indicated by the smaller values of the equivalent widths (EW hereafter) listed in Table 8 (click here). This is consistent with the remarkable absence of circumstellar components of the NaI D lines in the high-resolution spectra of BN Ori and the negligeable near-infrared excess (NIRE) and colour-excess in the spectral energy distribution (SED) of BN Ori. This comparison suggests that the FUOR outburst in BN Ori has removed its outer envelope almost completely, except for an optically-thin inner part where Htex2html_wrap_inline3672 is formed.

In Sect. 6 (click here) we summarise the similarities between the spectral features of BN Ori and those of the classical FUORs and the Herbig Ae stars. We also suggest a cause for the outburst in BN Ori, based on the mass-accretion rates of BN Ori, HR 5999 and BF Ori (A7e), which have been derived from the analysis of their low-resolution UV-spectra taken with IUE (Blondel & Tjin A Djie 1994, 1996).

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