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2. Photometry

4.1. History

The variable character of BN Ori was detected by Cannon (1919) and since then the star has been monitored extensively both visually and photographically. Tsesevich made nearly 4000 tex2html_wrap_inline3676 estimates of BN Ori from sky patrol photographic plate collections of the Harvard, Odessa, Sonneberg and Dushanbe observatories and constructed a light curve of tex2html_wrap_inline3678 over the period 1898- 1964 (Dragomiretskaya 1965; Tsesevich & Dragomiretskaya 1973).

Figure 1:   Lightcurve of tex2html_wrap_inline3682 for BN Ori over the period 1898- 1964, constructed by Tsesevich & Dragomiretskaya (1973), and extended up to 1995

Table 2:   Photometric Data of BN Ori on the Walraven (WULBV) system

Table 3:   Photometric Data of BN Ori on the Johnson/Cousins tex2html_wrap_inline3732 system. The last column (N, O) gives the number of observations per night and the observatory: DASA(D), ESO(E) or Kitt Peak(K)

This light curve of BN Ori shows quiet periods with maximum brightness tex2html_wrap_inline3758 tex2html_wrap_inline3760 97, during which the brightness frequently decreases by at most 05, separated by episodes of roughly 12.5 years, during which the mean brightness dropped with 1tex2html_wrap_inline3762 or 2tex2html_wrap_inline3764 over 100- 200 days or with 3tex2html_wrap_inline3768 over 20- 50 days. Superposed on this long-term variability are rapid fluctuations with amplitude of 15 during about 10 days. It was already noted by Beyer (1937) and confirmed by Hoffmeister (1949) and Payne-Gaposhkina (1952) that the behaviour of BN Ori is reminiscent of that of the RW Aur variables T Ori and XY Per and we can extend the similarity now to other Herbig Ae stars with Algol-type minima such as BF Ori, UX Ori, WW Vul and HR 5999. In a discussion of the photometric variability of BN Ori, Dragomiretskaya (1965) concluded that the large amplitude-variations cannot be due to temperature differences on the stellar surface, since a dimming

Table 4:   NIR photometric data of BN Ori

by 15 would already require a full coverage by cool spots of the stellar surface. Dragomiretskaya also used the visual photometry by Esch (1925), Beyer (1937) and Martinov (1951), to derive the colour indices for a number of dates and the resulting colour-magnitude diagram shows that the strong variations of BN Ori follow a reddening-line similar to those followed by many Herbig Ae stars. Such a type of variation is usually ascribed to obscuration by variable amounts of circumstellar dust (Wenzel 1968; Wenzel et al. 1971). For BN Ori the slope of the reddening-line was about 3, which is close to that of interstellar dust. Tsesevich & Lange observed that BN Ori showed Algol-like variations with a period of 27 days (Martinov 1951). From an analysis of quasi-periodic features during high-activity episodes in the light curve of BN Ori, Ishchenko (1982) detected a quasi-cyclic 18- 22 day periodicity superimposed on a stochastical behaviour and attributed this periodicity to the rotation of obscuring circumstellar dust clouds.

Figure 1 (click here) shows that the large-scale structure of the light curve changed around 1940: the amplitude of the strong variations decreased and the brightness maximium increased to tex2html_wrap_inline3786 tex2html_wrap_inline3788 92 in Dec'46. Since then the variations were gradually quenched from an amplitude of 04 to about 01 in 1966, while the maximum of the brightness-variations slowly decreased to tex2html_wrap_inline3790 tex2html_wrap_inline3792 101. In Sect. 6 (click here) we shall compare this behaviour with those of classical FUORs. After 1965 all observations have been made photoelectrically. Between 1965 and 1975 Zajtseva (1968, 1970), Kolotilov & Zajtseva (1976) and Kolotilov et al. (1977) have made several observations of BN Ori on the UBV and JHKL photometric systems, but they did not detect a pronounced variation.

2.2. Photometry since 1976

The star has been observed regularly by various observers on several photometric systems (see Table 1 (click here)).

  1. Between 1976 and 1995 UBVR observations were obtained at the Mt. Maidanak observatory near Samarkand (Table 12 (click here)). This was done in the course of the ROTOR (Research of Traces of Rotation) photometric program with a single-channel photometer mounted on the 60cm Zeiss reflector and the 48cm AZTgif14 reflector (Kilyachkov & Shevchenko 1977). The data of JD2448582 are simultaneous with the high-resolution spectroscopy at SAO (Sect. 4.3 (click here))

    Figure 2:   Observed (squares) and extinction-corrected (circles) SED of BN Ori based on the data of Feb'86. The L-band observation of Cohen (1973) is indicated by the "upper" triangle (with error-bar). The L-band observation from Table 4 (click here) is indicated by the "lower" triangle (with upperlimit). The solid line is the Kurucz model of tex2html_wrap_inline3806 = 7150K, tex2html_wrap_inline3808 = 3.8 and solar abundance, normalised to the extinction-corrected V-band flux of BN Ori. The dotted lines are the observed- and extinction-corrected UV-fluxes

  2. From 1985 to 1990 BN Ori was observed on the Strömgren ubvy system by the LTPV-group (Long Term Photometry of Variables) of ESO observers, with the Strömgren Automatic Telescope (SAT; formerly known as the Danish 50cm telescope) at La Silla. This telescope is equipped with a simultaneous multicolour photometer. Any systematic effects due to non-simultaneous measurements at the different passbands are therefore avoided. Since these observations have already been published (Sterken et al. 1993, 1995), they are not listed in this paper.
  3. Photometric data on the Walraven WULBV system were obtained during 3 observing runs in 1985 and 1986 with the Dutch Light Collector at ESO, La Silla. Measuring and reduction procedures have been explained by Lub & Pel (1977). During these observations the Walraven WULBV tex2html_wrap_inline3818 intensity values were measured simultaneously in the different passbands. The results of these measurements are listed in Table 2 (click here), where the last column gives the corresponding magnitude on the Johnson system calculated with the formula suggested by Brand & Wouterloot (1988).
  4. Optical photometry of BN Ori, on the Johnson/Cousins tex2html_wrap_inline3820 system, has been obtained in 1986 and 1994 with the ESO 50cm telescope at La Silla, in Nov'86 with the KPNO 90cm telescope at Kitt Peak and on several occasions between 1984 and 1992 with the 40cm telescope at the Dutch Astronomical Station at Ausserbinn (DASA), Switzerland. The results of these measurements are collected in Table 3 (click here) and the data of JD2446747 were obtained simultaneously with the high-resolution IUE observation of BN Ori.
  5. Since 1984 BN Ori has also been observed on the tex2html_wrap_inline3822 system by W. Herbst and collaborators in the course of a monitoring program at the "Van Vleck" observatory (private communication).
  6. NIR photometric data on the ESO JHKL system (Bouchet et al. 1989, 1991), of BN Ori were obtained in Feb'86 with the ESO 1m telescope at La Silla. During these observations, the telescope was equipped with an InSb detector with a 15tex2html_wrap3938  diaphragm. Sky subtraction was achieved by chopping with a frequency of 8 Hz in the east-west direction with a throw of 30tex2html_wrap3940 amplitude. The results are given in Table 4 (click here).

After transformation of the data on the various systems to the Johnson magnitude tex2html_wrap_inline3830 and colour tex2html_wrap_inline3832 we can hardly find any change in brightness and colour over these years. The average values are: tex2html_wrap_inline3834 = 965 tex2html_wrap_inline3836 003 and tex2html_wrap_inline3838 = 047 tex2html_wrap_inline3840 002. Only on one occasion (Oct.29- Nov.2, 1991), the star seemed to dim by 05 (Kovalchuk 1991). Because there are no data available for the 34 nights preceding Oct.29 and for the 12 nights following Nov.2, we can only state that the variation was limited to at most 51 nights. The variation is along the reddening-line with slope 3, so that this event could have been due to infalling circumstellar dust which evaporated in the neighbourhood of the star.

Since 1976 we have obtained only two observations in the NIR (Table 4 (click here)), and in Fig. 2 (click here) we give a SED of BN Ori based on these data of Feb'86. After transformation of the magnitudes to fluxes and correction of these fluxes for foreground extinction with the law of Savage & Mathis (1979) and E(B-V) = 007 (Sect. 4.3 (click here)), the corrected SED could be fitted with a Kurucz (1991) model for tex2html_wrap_inline3846 = 7150K and tex2html_wrap_inline3848, corresponding to a spectral type of F0III - IV (Sect. 5 (click here)) and an E(B-V) = 017. From Fig. 2 (click here) we note that we get a good agreement in the visual and NIR wavelength ranges, but that there remains an excess in the UV. This excess will be explained (Sect. 5 (click here)) in terms of the contribution from a (hot) boundary layer between the star and accretion-disc. If we compare our NIR data of 1986 with those obtained before 1977 (Glass & Penston 1974; Kolotilov et al. 1977) we note no difference in the H and K magnitudes. However, our J-band flux is somewhat larger than that of Kolotilov et al. and our L-band flux is significantly smaller than the one Cohen (1973; tex2html_wrap_inline3860 76) measured in 1971. The earlier attempts to match the SED (Kolotilov & Zajtseva 1976; Brown et al. 1986) were based on this higher L-band flux. No IRAS point-source was detected at the position of BN Ori, probably because the far-IR fluxes of the star are not higher than the background.

From the Walraven photometry (which has two passbands in the UV: W at 3236Å and U at 3623Å), we can get a measure of the magnitude of the Balmer discontinuity (tex2html_wrap_inline3868). The mean of the 9 observations of Dec'85- Jan'86 gives tex2html_wrap_inline3872. This is a deficiency of 048 with respect to the prediction of the Kurucz (1991) model for tex2html_wrap_inline3874 = 7200K and tex2html_wrap_inline3876 which we take for the classification F0III (Sect. 5 (click here)). If this is due to the contribution of recombination of Htex2html_wrap_inline3878 free-bound transitions near the star, we can derive (Garrison 1978) that for a temperature of 10tex2html_wrap_inline3880K in this region the emission measure is 1.86 tex2html_wrap_inline3882cmtex2html_wrap_inline3884. If we assume that this ionosphere is in hydrostatic equilibrium with a scale-height of 4tex2html_wrap_inline3886 we find within 2tex2html_wrap_inline3888 an electron density tex2html_wrap_inline3890 = 3.34 tex2html_wrap_inline3892cmtex2html_wrap_inline3894.

For HR 5999 (A5- 7III) we used the Walraven photometry to derive a tex2html_wrap_inline3898 equal to 123 which results in a deficiency of 020 when compared to a Kurucz (1991) model with tex2html_wrap_inline3900 = 8000K. This gives an emission measure of 6.0 tex2html_wrap_inline3902cmtex2html_wrap_inline3904, which corresponds to tex2html_wrap_inline3906 = 5.6 tex2html_wrap_inline3908cmtex2html_wrap_inline3910 close to the star, so that tex2html_wrap_inline3912 is 2.8 times the value found for BN Ori.

Table 5:   Log of spectroscopic observations of BN Ori. Columns 8 to 10 denote the dispersion tex2html_wrap_inline3914 (tex2html_wrap_inline3916) the resolution tex2
html_wrap_inline3918 (tex2html_wrap_inline3920) and the exposure time tex2html_wrap_inline3922 (min). The IUE imagenumbers are given in Col. 5 and the first digit denotes the used camera (1 = LWP, 3 = SWP) and LAP means large aperture. Observers: AB - A. Brown, DW - D. de Winter, HC - H. Cuypers, HT - H.R.E. Tjin A Djie, MA - M.E. van den Ancker, VK - V. Kotyshev, VS - V.S. Shevchenko

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