A&A 459, L21-L24 (2006)
DOI: 10.1051/0004-6361:20066392
LETTER TO THE EDITOR
R. S. Schnerr1 - H. F. Henrichs1 - R. D. Oudmaijer2 - J. H. Telting3
1 - Astronomical Institute "Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 - School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK
3 - Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
Received 13 September 2006 / Accepted 20 September 2006
Abstract
Context. Be stars, which are characterised by intermittent emission in their hydrogen lines, are known to be fast rotators. This fast rotation is a requirement for the formation of a Keplerian disk, which in turn gives rise to the emission. However, the pulsating, magnetic B1IV star
Cephei is a very slow rotator that still shows H
emission episodes like in other Be stars, contradicting current theories.
Aims. We investigate the hypothesis that the H emission stems from the spectroscopically unresolved companion of
Cep.
Methods. Spectra of the two unresolved components have been separated in the 6350-6850 Å range with spectro-astrometric techniques, using 11 longslit spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma.
Results. We find that the H emission is not related to the primary in
Cep, but is due to its 3.4 mag fainter companion. This companion has been resolved by speckle techniques, but it remains unresolved by traditional spectroscopy. The emission extends from about -400 to +400 km s-1. The companion star in its 90-year orbit is likely to be a classical Be star with a spectral type around B6-8.
Conclusions. By identifying its Be-star companion as the origin of the H
emission behaviour, the enigma behind the Be status of the slow rotator
Cep has been resolved.
Key words: stars: emission-line, Be - stars: individual:
Cep - stars: magnetic fields - stars: early-type - stars: activity - binaries: close
This behaviour is typical of Be stars. The enigma is that nearly
all Be stars are rapid rotators with equatorial rotation rates of
typically 70-80% of the critical rotation velocity
(e.g. Porter & Rivinius 2003), or perhaps even higher
(Townsend et al. 2004). However,
Cep is a very slow rotator
with
km s-1 and has a very well-determined rotation period of 12.00 days (Henrichs et al. 1993), much
longer than the inferred rotation periods of other Be stars.
Interestingly, the star was discovered to be an oblique magnetic
rotator (Henrichs et al. 2000) with a polar field of
360 G
(see also Donati et al. 2001), which strongly modulates the outflowing stellar wind with the rotation period. This has been very clearly observed in the UV resonance lines of C IV, Si IV, and N V with the
satellite over more than 15 years. This spectral line modulation could be modelled reasonably well as being due to the interaction of the magnetic field with the stellar wind (Schnerr et al. 2006), similar to the rotationally modulated winds of the magnetic Bp stars
(e.g. Townsend et al. 2005), which also show H
emission.
The serious problem, however, is that every model so far predicts
that this 12-day rotation period of Cep should also be
clearly visible in the H
emission (probing the outflow near
the stellar surface), whereas no sign of any 12-day modulation could be found
in more than 300 high-resolution H
profiles taken over 6 years (Henrichs et al. 2006). This discrepancy seriously hampers our
understanding of the Be phenomenon: if
Cep really belongs to the (phenomenologically defined) class of Be stars, rapid rotation would not be required for the explanation of the Be phenomenon, opposed to all existing models. In addition, the origin of the unmodulated
H
emission would remain a mystery. Current modelling efforts would clearly benefit from resolving this critical issue.
The aim of this study is to investigate the hypothesis that the source of the H
emission is not
Cep itself, but its nearby companion, which has been resolved by speckle techniques. This suggestion has already been put forward by Tarasov (see Henrichs et al. 2003), which was at that time, ironically, rejected by one of the current authors. If this close companion were to turn out to be a Be
star, this would clearly mean a major step forwards in understanding the
Cep system, and also remove the unfulfillable constraint on Be star models it poses now.
As the target is very bright and the approximate orbit is known, the technique of spectro-astrometry is particularly well-suited to resolving the question of the origin of the H
emission. Spectro-astrometry measures the relative spatial position of spectral
features from a long-slit spectrum (see Bailey 1998a; Porter et al. 2004, and references therein). If one star in an otherwise unresolved binary has, for
example, H
emission, the photocentre across the line perpendicular to the dispersion direction will shift towards that star.
So far the technique has mainly been used to detect close binary companions (e.g. Baines et al. 2006; Bailey 1998a), but also the individual spectra of binaries with a separation down to tens of milliarcseconds (mas) can be obtained.
A total of 11 spectra were obtained on 28 August 2006, between 5:40 and 5:53 UT (HJD 2453975.74), with exposure times between 2 and 5 s. The star was positioned at three different locations on the slit, to check for possible instrumental effects. The angle of the slit on the sky was set to 42
(NE), which was confirmed by images obtained without the slit, leaving the orientation of the sky unchanged with this instrument.
Data were reduced using the IRAF software package. The CCD-frames were corrected for the bias level and divided by a normalised flatfield. Scattered light was subtracted. Wavelength calibration spectra were obtained using an Ne lamp. The resulting two-dimensional spectra were fitted by a Gaussian profile in the spatial direction at each wavelength step with the fitprofs routine, using a 5-point running average in the dispersion direction (comparable to the spectral resolution). We have checked that similar results were obtained when no correction for scattered light was applied, or when Voigt instead of Gaussian profiles were used. Further consistency checks were carried out by comparing the results for all individual spectra taken at different slit positions. All traces were similar to each other, strongly suggesting that instrumental artefacts are not present.
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Figure 1:
Average H![]() |
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The resulting spectra are shown in Fig. 3. The results for three possible separations are shown, and apart from the strength of H
the results are qualitatively similar. The conclusion that the NE component is the source of the H
emission is confirmed when the spectra are split. We find that the secondary has a double-peaked emission profile, characteristic of a classical Be star.
In the He I line the signal is also in the direction of the companion, but it has the same width as the absorption line in the total intensity spectrum. In the separated spectra it can be seen that this line is present only in the primary and not in the secondary, as expected for its later spectral type.
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Figure 2:
Spectro-astrometric observations of H![]() ![]() ![]() ![]() |
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Figure 3:
The results of the separation of the spectra of the primary and secondary components. We show the normalised intensity line profile ( top) and the separated line profiles of the primary ( middle) and the secondary ( bottom). For the splitting of the spectra we have assumed a separation of 0.07
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We find that the H
emission extends from about -400 to +400 km s-1, in agreement with the results from Hadrava & Harmanec (1996) and Pan'ko & Tarasov (1997). This is independently confirmed by the extent of the variability shown in Fig. 1. The large width of the H
emission suggests a relatively high value for
,
which points to a high inclination angle. With the orbital inclination angle of 87
(Pigulski & Boratyn 1992) and the high inclination angle of
Cep itself (>60
,
Donati et al. 2001; Telting et al. 1997) this means that the spin and orbital angular momentum vectors could well be aligned. An interesting question is how such a binary system with one, presumably spun-down, magnetic B star and a Be star may have evolved.
Our result implies that the observed H
emission is not related to the magnetic field of the primary star. This agrees with models explaining the variability observed in the UV wind-lines as due to the rotation of the magnetic field.
New spectro-astrometric observations to obtain a wider spectral coverage are being planned and will allow us to further constrain the
and spectral type of the secondary star.
Acknowledgements
Based on observations obtained with the Nordic Optical Telescope, which is operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The data presented here were taken using ALFOSC, which is owned by the Instituto de Astrofisica de Andalucia (IAA) and operated at the Nordic Optical Telescope under agreement between IAA and the NBIfAFG of the Astronomical Observatory of Copenhagen. RS and HFH thank F. Leone for useful discussions.