Issue |
Astron. Astrophys. Suppl. Ser.
Volume 124, Number 1, July 1997
|
|
---|---|---|
Page(s) | 33 - 54 | |
DOI | https://doi.org/10.1051/aas:1997356 | |
Published online | 15 July 1997 |
The FUOR characteristics of the PMS star BN Orionis*
inferred from new spectroscopic and photometric observations
1
Astronomical Institute of the Academy of Sciences of Uzbekistan, Astronomicheskaya 33, Tashkent 700052, Uzbekistan
2
Astronomical Institute "Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3
Dpto. Física Teórica, C–XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, E–28049 Madrid, Spain
Send offprint request to: P.F.C. Blondel
Received:
4
June
1996
Accepted:
7
October
1996
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 1971).
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 pecularities. We have analysed the
visual spectra obtained with the 6 m BTA telescope of the SAO,
the 60 cm and 48 cm telescopes at Mt. Maidanak,
the 1.4 m CAT and the 1.5 m 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 of BN Ori is between 180 and
220
, which is typical for A-type stars of 2 - 5
.
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 Hα- and
Mg ii-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 1994) in its inner accretion-disc which was
triggered by a modest increase in the accretion rate.
Key words: circumstellar matter / stars: emission-line / stars: BN Ori / stars: pre-main-sequence
Based on observations collected at/with: – Mt. Maidanak Observatory, Samarkand, Uzbekistan, – Special Astrophysical Observatory, Russian Academy of Sciences, Karachai-Cherkessia, Zelenchuk region, Nizhnii-Arkhyz, – European Southern Observatory, La Silla, Chile, – Kitt Peak National Observatory, Tucson AZ, U.S.A., – Dutch Astronomical Station Ausserbinn, Switzerland, – International Ultraviolet Explorer.
© European Southern Observatory (ESO), 1997