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 $\beta$ Cephei is a very slow
rotator that still shows H$\alpha$ emission episodes like in other Be stars,
contradicting current theories. We investigate the hypothesis that the
H$\alpha$ emission stems from the spectroscopically unresolved companion of
$\beta$ Cep. Spectra of the two unresolved components have been separated in
the 6350-6850\AA range with spectro-astrometric techniques, using 11 longslit
spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma. We find
that the H$\alpha$ emission is not related to the primary in $\beta$ Cep, but
is due to its 3.4 magnitudes 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}$. The
companion star in its 90-year orbit is likely to be a classical Be star with a
spectral type around B6-8. By identifying its Be-star companion as the origin
of the H$\alpha$ emission behaviour, the enigma behind the Be status of the
slow rotator $\beta$ Cep has been resolved.Comment: 4 pages, 3 figures. Accepted by A&A Letter

Bailey

Baines

Donati

Gezari

H. F. Henrichs

Hadrava

J. H. Telting

Neiner

Pigulski

Porter

R. D. Oudmaijer

R. S. Schnerr

Takami

Tarasov

Telting

Townsend

English

arXiv

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.</p

Schnerr, R.S.

Henrichs, H.F.

Oudmaijer, R.D.

Telting, J.H.

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)On the Hα emission from the β Cephei systemSchnerr, R.S.; Henrichs, H.F.; Oudmaijer, R.D.; Telting, J.H.DOI10.1051/0004-6361:20066392Publication date2006Document VersionFinal published versionPublished inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Schnerr, R. S., Henrichs, H. F., Oudmaijer, R. D., & Telting, J. H. (2006). On the Hα emissionfrom the β Cephei system. Astronomy & Astrophysics, 459(1), L21-L24.https://doi.org/10.1051/0004-6361:20066392General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:09 Mar 2023A&A 459, L21–L24 (2006)DOI: 10.1051/0004-6361:20066392c© ESO 2006Astronomy&AstrophysicsLetter to the EditorOn the Hα emission from the β Cephei systemR. S. Schnerr1, H. F. Henrichs1, R. D. Oudmaijer2, and J. H. Telting31 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlandse-mail: rschnerr@science.uva.nl2 School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK3 Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, SpainReceived 13 September 2006 / Accepted 20 September 2006ABSTRACTContext. Be stars, which are characterised by intermittent emission in their hydrogen lines, are known to be fast rotators. This fastrotation is a requirement for the formation of a Keplerian disk, which in turn gives rise to the emission. However, the pulsating,magnetic B1IV star β Cephei is a very slow rotator that still shows Hα emission episodes like in other Be stars, contradicting currenttheories.Aims. We investigate the hypothesis that the Hα emission stems from the spectroscopically unresolved companion of β Cep.Methods. Spectra of the two unresolved components have been separated in the 6350–6850 Å range with spectro-astrometric tech-niques, using 11 longslit spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma.Results. We find that the Hα emission is not related to the primary in β Cep, but is due to its 3.4 mag fainter companion. This com-panion has been resolved by speckle techniques, but it remains unresolved by traditional spectroscopy. The emission extends fromabout −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 Hα emission behaviour, the enigma behind the Be status ofthe slow rotator β Cep has been resolved.Key words. stars: emission-line, Be – stars: individual: β Cep – stars: magnetic fields – stars: early-type – stars: activity –binaries: close1. IntroductionThe well-known pulsating star β Cephei (HD 205021) has beenclassified as B1IVe. Its Be status was assigned after the starshowed prominent emission in Hα. The presence of this emis-sion has been reported from time to time since 1933 (Karpov1933), but often the emission disappeared or was not noticed.A new Hα emission episode was discovered in 1990 (Mathiaset al. 1991; Kaper & Mathias 1995), which decayed in about10 years. Neiner et al. (2001) found that the emission was backagain within several years. A summary of the emission phasesuntil 1995 is given by Pan’ko & Tarasov (1997).This behaviour is typical of Be stars. The enigma is thatnearly all Be stars are rapid rotators with equatorial rotationrates of typically ∼70–80% of the critical rotation velocity (e.g.Porter & Rivinius 2003), or perhaps even higher (Townsend et al.2004). However, β Cep is a very slow rotator with v sin i ≈25 km s−1 and has a very well-determined rotation period of12.00 days (Henrichs et al. 1993), much longer than the inferredrotation periods of other Be stars. Interestingly, the star was dis-covered to be an oblique magnetic rotator (Henrichs et al. 2000)with a polar field of ∼360 G (see also Donati et al. 2001), whichstrongly modulates the outflowing stellar wind with the rotationperiod. This has been very clearly observed in the UV resonancelines of C iv, Si iv, and Nv with the IUE satellite over morethan 15 years. This spectral line modulation could be modelledreasonably well as being due to the interaction of the magneticfield with the stellar wind (Schnerr et al. 2006), similar to therotationally modulated winds of the magnetic Bp stars (e.g.Townsend et al. 2005), which also show Hα emission.The serious problem, however, is that every model so far pre-dicts that this 12-day rotation period of β Cep should also beclearly visible in the Hα emission (probing the outflow near thestellar surface), whereas no sign of any 12-day modulation couldbe found in more than 300 high-resolution Hα profiles taken over6 years (Henrichs et al. 2006). This discrepancy seriously ham-pers our understanding of the Be phenomenon: if β Cep reallybelongs to the (phenomenologically defined) class of Be stars,rapid rotation would not be required for the explanation of theBe phenomenon, opposed to all existing models. In addition, theorigin of the unmodulated Hα emission would remain a mystery.Current modelling efforts would clearly benefit from resolvingthis critical issue.The aim of this study is to investigate the hypothesis thatthe source of the Hα emission is not β Cep itself, but its nearbycompanion, which has been resolved by speckle techniques.This suggestion has already been put forward by Tarasov (seeHenrichs et al. 2003), which was at that time, ironically, rejectedby one of the current authors. If this close companion were toturn out to be a Be star, this would clearly mean a major stepforwards in understanding the β Cep system, and also removethe unfulfillable constraint on Be star models it poses now.1.1. The binary componentsThe star β Cep (V = 3.2) has a visual companion (V = 7.9)at a distance of 13.4′′. A second companion was detected usingArticle published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20066392L22 R. S. Schnerr et al.: On the Hα emission from the β Cephei systemspeckle interferometry by Gezari et al. (1972) at a distance of∼0.25′′, which was later found to have a visual magnitude ofV = 6.6. The parameters of the close binary orbit have beendetermined from the variations in the pulsation period due tothe light time effect and speckle interferometry by Pigulski &Boratyn (1992, see also Hadrava & Harmanec 1996). When re-cent, additional speckle measurements (Hartkopf et al. 2001) aretaken into account, the current position of the companion is ata distance of about 0.1′′ from the primary, at a position angleof 42◦ (in the NE) on the sky. From the mass ratio determinedfrom the binary orbit, the companion has an estimated spectraltype around B6-8.As the target is very bright and the approximate orbit isknown, the technique of spectro-astrometry is particularly well-suited to resolving the question of the origin of the Hα emis-sion. Spectro-astrometry measures the relative spatial positionof spectral features from a long-slit spectrum (see Bailey 1998a;Porter et al. 2004, and references therein). If one star in an other-wise unresolved binary has, for example, Hα emission, the pho-tocentre across the line perpendicular to the dispersion direc-tion will shift towards that star. So far the technique has mainlybeen used to detect close binary companions (e.g. Bailey 1998a;Baines et al. 2006), but also the individual spectra of binarieswith a separation down to tens of milliarcseconds (mas) can beobtained.2. Observations and data reductionLongslit spectra of β Cep were obtained with the ALFOSC spec-trograph at the Nordic Optical Telescope (NOT) on La Palma.We used grism #17 (2400 l/mm VPH), which gives a disper-sion of 0.25 Å/pixel for the ∼6350–6850 Å range. The 1.9′′ off-centre slit was used to avoid a ghost near Hα. We observed witha typical seeing of ∼1.1′′, resulting in an effective resolution ofR ≈ 4500. The CCD with 2048 × 2048 pixels gives a spatialresolution of 0.19′′/pixel, thereby giving a good sampling of thespatial profile of the spectrum.A total of 11 spectra were obtained on 28 August 2006, be-tween 5:40 and 5:53 UT (HJD 2453975.74), with exposure timesbetween 2 and 5 s. The star was positioned at three different lo-cations on the slit, to check for possible instrumental effects. Theangle of the slit on the sky was set to 42◦ (NE), which was con-firmed by images obtained without the slit, leaving the orienta-tion of the sky unchanged with this instrument.Data were reduced using the IRAF software package. TheCCD-frames were corrected for the bias level and divided by anormalised flatfield. Scattered light was subtracted. Wavelengthcalibration spectra were obtained using an Ne lamp. The result-ing two-dimensional spectra were fitted by a Gaussian profilein the spatial direction at each wavelength step with the fitprofsroutine, using a 5-point running average in the dispersion direc-tion (comparable to the spectral resolution). We have checkedthat similar results were obtained when no correction for scat-tered light was applied, or when Voigt instead of Gaussian pro-files were used. Further consistency checks were carried out bycomparing the results for all individual spectra taken at differ-ent slit positions. All traces were similar to each other, stronglysuggesting that instrumental artefacts are not present.3. ResultsThe average Hα profile is plotted in Fig. 1, together with spectrataken in 2000 and 2001 (Henrichs et al. 2003). Although it is notFig. 1. Average Hα profile (full line), the profile of 7 July 2000 and19 June 2001 (dashed and dotted lines respectively, see Henrichs et al.2003). During our observations more emission was present than in2000.directly clear from the new spectra that emission is present, com-parison with the spectrum of July 2000 shows that the emissionis currently stronger than it was in 2000.3.1. The source of the Hα emissionThe spectro-astrometric results for Hα (6563 Å) and the He i lineat 6678 Å are shown in Fig. 2. It is clear that near Hα the pho-tocentre of the spatial profile of the spectrum shifts towards thecompanion (in the NE direction). This is due to an increased rel-ative contribution to the flux of the companion, indicating thatthe companion is the source of the Hα emission. The width ofthe signature in Hα is from about −400 to +400 km s−1, which ismuch broader than the width of the absorption line and is typicalfor a Be star emission line.3.2. The spectra of the individual starsFor close binaries with smaller separations than the slit, it ispossible to determine the two spectra of the individual, un-resolved, binary components with the technique described inBailey (1998b) and Takami et al. (2003, see also Porter et al.2004). Using average photocentre shifts, we determined the in-dividual spectra, adopting the measured magnitude difference of3.4 mag (Hartkopf et al. 2001) and separations of 0.07′′, 0.1′′,and 0.15′′, bracketing the estimated separation.The resulting spectra are shown in Fig. 3. The results forthree possible separations are shown, and apart from the strengthof Hα the results are qualitatively similar. The conclusion thatthe NE component is the source of the Hα emission is con-firmed when the spectra are split. We find that the secondaryhas a double-peaked emission profile, characteristic of a classi-cal Be star.In the He i line the signal is also in the direction of the com-panion, but it has the same width as the absorption line in thetotal intensity spectrum. In the separated spectra it can be seenthat this line is present only in the primary and not in the sec-ondary, as expected for its later spectral type.R. S. Schnerr et al.: On the Hα emission from the β Cephei system L23Fig. 2. Spectro-astrometric observations of Hα (left) and the He i line at 6678 Å (right) of the β Cep system. Shown are the normalised intensityline profile (top) and the position of the photocentre of the spatial profile relative to that of the continuum (bottom). In the plot of the offset of thephotocentre the results of all individual spectra are shown (dotted lines) as well as the average (full line). In both the Hα and He i plots a shift ofthe photocentre towards the companion is visible. However, in Hα the photocentre is offset from about −400 to +400 km s−1, while in He i thewidth of the offset is similar to the width of the line.Fig. 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 of0.07′′ (dashed lines), 0.1′′ (full line, corresponding to the best estimate of the separation), and 0.15′′ (dotted line). A double-peaked Hα emissionline with a width of ∼400 km s−1, typical of a classical Be star, is found in the secondary star. The He i line at 6678 Å is only present in the primarystar.4. Conclusions and discussionWe have shown that the Hα emission observed from the β Cepsystem is not related to the slowly rotating primary star, but to thesecondary, which is most likely a classical Be star. This explainswhy the Hα emission is not modulated by the rotation of theprimary. This removes the exceptional status of β Cep amongthe fast rotating Be stars, which therefore no longer contradictsthe current models that require rapid rotation for explaining theBe phenomenon.L24 R. S. Schnerr et al.: On the Hα emission from the β Cephei systemWe find that the Hα 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 indepen-dently confirmed by the extent of the variability shown in Fig. 1.The large width of the Hα emission suggests a relatively highvalue for v sin i, which points to a high inclination angle. Withthe orbital inclination angle of 87◦ (Pigulski & Boratyn 1992)and the high inclination angle of β Cep itself (>60◦, Telting et al.1997; Donati et al. 2001) this means that the spin and orbitalangular momentum vectors could well be aligned. An interest-ing question is how such a binary system with one, presumablyspun-down, magnetic B star and a Be star may have evolved.Our result implies that the observed Hα emission is not re-lated to the magnetic field of the primary star. This agrees withmodels explaining the variability observed in the UV wind-linesas due to the rotation of the magnetic field.New spectro-astrometric observations to obtain a wider spec-tral coverage are being planned and will allow us to further con-strain the v sin i and spectral type of the secondary star.Acknowledgements. Based on observations obtained with the Nordic OpticalTelescope, which is operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, at the Spanish Observatorio del Roquede los Muchachos of the Instituto de Astrofisica de Canarias. The data pre-sented here were taken using ALFOSC, which is owned by the Instituto deAstrofisica de Andalucia (IAA) and operated at the Nordic Optical Telescope un-der agreement between IAA and the NBIfAFG of the Astronomical Observatoryof Copenhagen. RS and HFH thank F. Leone for useful discussions.ReferencesBailey, J. 1998a, MNRAS, 301, 161Bailey, J. A. 1998b, in Optical Astronomical Instrumentation, ed. S. D’Odorico,Proc. SPIE, 3355, 932Baines, D., Oudmaijer, R. D., Porter, J. M., & Pozzo, M. 2006, MNRAS, 367,737Donati, J.-F., Wade, G. A., Babel, J., et al. 2001, MNRAS, 326, 1265Gezari, D. Y., Labeyrie, A., & Stachnik, R. V. 1972, ApJ, 173, L1Hadrava, P., & Harmanec, P. 1996, A&A, 315, L401Hartkopf, W., Mason, B., Wycoff, G., & McAlister, H. 2001,Fourth Catalog of Interferometric Measurements of Binary Stars,http://www.ad.usno.navy.mil/wds/int4.htmlHenrichs, H. F., Bauer, F., Hill, G. M., et al. 1993, in New Perspectives on StellarPulsation and Pulsating Variable Stars, ed. J. M. Nemec, & J. M. Matthews,IAU Coll., 139, 186Henrichs, H. F., de Jong, J. A., Donati, J.-F., et al. 2000, in IAU Coll. 175, TheBe Phenomenon in Early-Type Stars, ed. M. A. Smith, H. F. Henrichs, &J. Fabregat, ASP Conf. Ser., 214, 324Henrichs, H. F., de Jong, J. A., Verdugo, E., et al. 2006, in prep.Henrichs, H. F., Neiner, C., & Geers, V. C. 2003, in Magnetic Fields in O, Band A Stars: Origin and Connection to Pulsation, Rotation and Mass Loss,ed. L. A. Balona, H. F. Henrichs, & R. Medupe, ASP Conf. Ser., 305, 301Kaper, L., & Mathias, P. 1995, in IAU Coll. 155, Astrophysical Applications ofStellar Pulsation, ed. R. S. Stobie, & P. A. Whitelock, ASP Conf. Ser., 83,295Karpov, B. G. 1933, Lick Observatory Bull., 16, No. 457, 167Mathias, P., Gillet, D., & Kaper, L. 1991, in Rapid Variability of OB-stars:Nature and Diagnostic Value, ed. D. Baade, 193Neiner, C., Henrichs, H., Geers, V., & Donati, J.-F. 2001, IAU Circ., 7651, 3Pan’ko, E. A., & Tarasov, A. E. 1997, Astron. Lett., 23, 545Pigulski, A., & Boratyn, D. A. 1992, A&A, 253, 178Porter, J. M., & Rivinius, T. 2003, PASP, 115, 1153Porter, J. M., Oudmaijer, R. D., & Baines, D. 2004, A&A, 428, 327Schnerr, R. S., Owocki, S. P., ud-Doula, A., Henrichs, H. F., & Townsend,R. H. D. 2006, A&A, in prep.Takami, M., Bailey, J., & Chrysostomou, A. 2003, A&A, 397, 675Telting, J. H., Aerts, C., & Mathias, P. 1997, A&A, 322, 493Townsend, R. H. D., Owocki, S. P., & Howarth, I. D. 2004, MNRAS, 350, 189Townsend, R. H. D., Owocki, S. P., & Groote, D. 2005, ApJ, 630, L81

On the Hα emission from the β Cephei system

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 β Cephe

EDP Sciences OAI-PMH repository (1.2.0)

On the H

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)On the Hα emission from the β Cephei systemSchnerr, R.S.; Henrichs, H.F.; Oudmaijer, R.D.; Telting, J.H.DOI10.1051/0004-6361:20066392Publication date2006Document VersionFinal published versionPublished inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Schnerr, R. S., Henrichs, H. F., Oudmaijer, R. D., & Telting, J. H. (2006). On the Hα emissionfrom the β Cephei system. Astronomy & Astrophysics, 459(1), L21-L24.https://doi.org/10.1051/0004-6361:20066392General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:31 Aug 2023A&A 459, L21–L24 (2006)DOI: 10.1051/0004-6361:20066392c© ESO 2006Astronomy&AstrophysicsLetter to the EditorOn the Hα emission from the β Cephei systemR. S. Schnerr1, H. F. Henrichs1, R. D. Oudmaijer2, and J. H. Telting31 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlandse-mail: rschnerr@science.uva.nl2 School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK3 Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, SpainReceived 13 September 2006 / Accepted 20 September 2006ABSTRACTContext. Be stars, which are characterised by intermittent emission in their hydrogen lines, are known to be fast rotators. This fastrotation is a requirement for the formation of a Keplerian disk, which in turn gives rise to the emission. However, the pulsating,magnetic B1IV star β Cephei is a very slow rotator that still shows Hα emission episodes like in other Be stars, contradicting currenttheories.Aims. We investigate the hypothesis that the Hα emission stems from the spectroscopically unresolved companion of β Cep.Methods. Spectra of the two unresolved components have been separated in the 6350–6850 Å range with spectro-astrometric tech-niques, using 11 longslit spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma.Results. We find that the Hα emission is not related to the primary in β Cep, but is due to its 3.4 mag fainter companion. This com-panion has been resolved by speckle techniques, but it remains unresolved by traditional spectroscopy. The emission extends fromabout −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 Hα emission behaviour, the enigma behind the Be status ofthe slow rotator β Cep has been resolved.Key words. stars: emission-line, Be – stars: individual: β Cep – stars: magnetic fields – stars: early-type – stars: activity –binaries: close1. IntroductionThe well-known pulsating star β Cephei (HD 205021) has beenclassified as B1IVe. Its Be status was assigned after the starshowed prominent emission in Hα. The presence of this emis-sion has been reported from time to time since 1933 (Karpov1933), but often the emission disappeared or was not noticed.A new Hα emission episode was discovered in 1990 (Mathiaset al. 1991; Kaper & Mathias 1995), which decayed in about10 years. Neiner et al. (2001) found that the emission was backagain within several years. A summary of the emission phasesuntil 1995 is given by Pan’ko & Tarasov (1997).This behaviour is typical of Be stars. The enigma is thatnearly all Be stars are rapid rotators with equatorial rotationrates of typically ∼70–80% of the critical rotation velocity (e.g.Porter & Rivinius 2003), or perhaps even higher (Townsend et al.2004). However, β Cep is a very slow rotator with v sin i ≈25 km s−1 and has a very well-determined rotation period of12.00 days (Henrichs et al. 1993), much longer than the inferredrotation periods of other Be stars. Interestingly, the star was dis-covered to be an oblique magnetic rotator (Henrichs et al. 2000)with a polar field of ∼360 G (see also Donati et al. 2001), whichstrongly modulates the outflowing stellar wind with the rotationperiod. This has been very clearly observed in the UV resonancelines of C iv, Si iv, and Nv with the IUE satellite over morethan 15 years. This spectral line modulation could be modelledreasonably well as being due to the interaction of the magneticfield with the stellar wind (Schnerr et al. 2006), similar to therotationally modulated winds of the magnetic Bp stars (e.g.Townsend et al. 2005), which also show Hα emission.The serious problem, however, is that every model so far pre-dicts that this 12-day rotation period of β Cep should also beclearly visible in the Hα emission (probing the outflow near thestellar surface), whereas no sign of any 12-day modulation couldbe found in more than 300 high-resolution Hα profiles taken over6 years (Henrichs et al. 2006). This discrepancy seriously ham-pers our understanding of the Be phenomenon: if β Cep reallybelongs to the (phenomenologically defined) class of Be stars,rapid rotation would not be required for the explanation of theBe phenomenon, opposed to all existing models. In addition, theorigin of the unmodulated Hα emission would remain a mystery.Current modelling efforts would clearly benefit from resolvingthis critical issue.The aim of this study is to investigate the hypothesis thatthe source of the Hα emission is not β Cep itself, but its nearbycompanion, which has been resolved by speckle techniques.This suggestion has already been put forward by Tarasov (seeHenrichs et al. 2003), which was at that time, ironically, rejectedby one of the current authors. If this close companion were toturn out to be a Be star, this would clearly mean a major stepforwards in understanding the β Cep system, and also removethe unfulfillable constraint on Be star models it poses now.1.1. The binary componentsThe star β Cep (V = 3.2) has a visual companion (V = 7.9)at a distance of 13.4′′. A second companion was detected usingArticle published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20066392L22 R. S. Schnerr et al.: On the Hα emission from the β Cephei systemspeckle interferometry by Gezari et al. (1972) at a distance of∼0.25′′, which was later found to have a visual magnitude ofV = 6.6. The parameters of the close binary orbit have beendetermined from the variations in the pulsation period due tothe light time effect and speckle interferometry by Pigulski &Boratyn (1992, see also Hadrava & Harmanec 1996). When re-cent, additional speckle measurements (Hartkopf et al. 2001) aretaken into account, the current position of the companion is ata distance of about 0.1′′ from the primary, at a position angleof 42◦ (in the NE) on the sky. From the mass ratio determinedfrom the binary orbit, the companion has an estimated spectraltype around B6-8.As the target is very bright and the approximate orbit isknown, the technique of spectro-astrometry is particularly well-suited to resolving the question of the origin of the Hα emis-sion. Spectro-astrometry measures the relative spatial positionof spectral features from a long-slit spectrum (see Bailey 1998a;Porter et al. 2004, and references therein). If one star in an other-wise unresolved binary has, for example, Hα emission, the pho-tocentre across the line perpendicular to the dispersion direc-tion will shift towards that star. So far the technique has mainlybeen used to detect close binary companions (e.g. Bailey 1998a;Baines et al. 2006), but also the individual spectra of binarieswith a separation down to tens of milliarcseconds (mas) can beobtained.2. Observations and data reductionLongslit spectra of β Cep were obtained with the ALFOSC spec-trograph at the Nordic Optical Telescope (NOT) on La Palma.We used grism #17 (2400 l/mm VPH), which gives a disper-sion of 0.25 Å/pixel for the ∼6350–6850 Å range. The 1.9′′ off-centre slit was used to avoid a ghost near Hα. We observed witha typical seeing of ∼1.1′′, resulting in an effective resolution ofR ≈ 4500. The CCD with 2048 × 2048 pixels gives a spatialresolution of 0.19′′/pixel, thereby giving a good sampling of thespatial profile of the spectrum.A total of 11 spectra were obtained on 28 August 2006, be-tween 5:40 and 5:53 UT (HJD 2453975.74), with exposure timesbetween 2 and 5 s. The star was positioned at three different lo-cations on the slit, to check for possible instrumental effects. Theangle of the slit on the sky was set to 42◦ (NE), which was con-firmed by images obtained without the slit, leaving the orienta-tion of the sky unchanged with this instrument.Data were reduced using the IRAF software package. TheCCD-frames were corrected for the bias level and divided by anormalised flatfield. Scattered light was subtracted. Wavelengthcalibration spectra were obtained using an Ne lamp. The result-ing two-dimensional spectra were fitted by a Gaussian profilein the spatial direction at each wavelength step with the fitprofsroutine, using a 5-point running average in the dispersion direc-tion (comparable to the spectral resolution). We have checkedthat similar results were obtained when no correction for scat-tered light was applied, or when Voigt instead of Gaussian pro-files were used. Further consistency checks were carried out bycomparing the results for all individual spectra taken at differ-ent slit positions. All traces were similar to each other, stronglysuggesting that instrumental artefacts are not present.3. ResultsThe average Hα profile is plotted in Fig. 1, together with spectrataken in 2000 and 2001 (Henrichs et al. 2003). Although it is notFig. 1. Average Hα profile (full line), the profile of 7 July 2000 and19 June 2001 (dashed and dotted lines respectively, see Henrichs et al.2003). During our observations more emission was present than in2000.directly clear from the new spectra that emission is present, com-parison with the spectrum of July 2000 shows that the emissionis currently stronger than it was in 2000.3.1. The source of the Hα emissionThe spectro-astrometric results for Hα (6563 Å) and the He i lineat 6678 Å are shown in Fig. 2. It is clear that near Hα the pho-tocentre of the spatial profile of the spectrum shifts towards thecompanion (in the NE direction). This is due to an increased rel-ative contribution to the flux of the companion, indicating thatthe companion is the source of the Hα emission. The width ofthe signature in Hα is from about −400 to +400 km s−1, which ismuch broader than the width of the absorption line and is typicalfor a Be star emission line.3.2. The spectra of the individual starsFor close binaries with smaller separations than the slit, it ispossible to determine the two spectra of the individual, un-resolved, binary components with the technique described inBailey (1998b) and Takami et al. (2003, see also Porter et al.2004). Using average photocentre shifts, we determined the in-dividual spectra, adopting the measured magnitude difference of3.4 mag (Hartkopf et al. 2001) and separations of 0.07′′, 0.1′′,and 0.15′′, bracketing the estimated separation.The resulting spectra are shown in Fig. 3. The results forthree possible separations are shown, and apart from the strengthof Hα the results are qualitatively similar. The conclusion thatthe NE component is the source of the Hα emission is con-firmed when the spectra are split. We find that the secondaryhas a double-peaked emission profile, characteristic of a classi-cal Be star.In the He i line the signal is also in the direction of the com-panion, but it has the same width as the absorption line in thetotal intensity spectrum. In the separated spectra it can be seenthat this line is present only in the primary and not in the sec-ondary, as expected for its later spectral type.R. S. Schnerr et al.: On the Hα emission from the β Cephei system L23Fig. 2. Spectro-astrometric observations of Hα (left) and the He i line at 6678 Å (right) of the β Cep system. Shown are the normalised intensityline profile (top) and the position of the photocentre of the spatial profile relative to that of the continuum (bottom). In the plot of the offset of thephotocentre the results of all individual spectra are shown (dotted lines) as well as the average (full line). In both the Hα and He i plots a shift ofthe photocentre towards the companion is visible. However, in Hα the photocentre is offset from about −400 to +400 km s−1, while in He i thewidth of the offset is similar to the width of the line.Fig. 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 of0.07′′ (dashed lines), 0.1′′ (full line, corresponding to the best estimate of the separation), and 0.15′′ (dotted line). A double-peaked Hα emissionline with a width of ∼400 km s−1, typical of a classical Be star, is found in the secondary star. The He i line at 6678 Å is only present in the primarystar.4. Conclusions and discussionWe have shown that the Hα emission observed from the β Cepsystem is not related to the slowly rotating primary star, but to thesecondary, which is most likely a classical Be star. This explainswhy the Hα emission is not modulated by the rotation of theprimary. This removes the exceptional status of β Cep amongthe fast rotating Be stars, which therefore no longer contradictsthe current models that require rapid rotation for explaining theBe phenomenon.L24 R. S. Schnerr et al.: On the Hα emission from the β Cephei systemWe find that the Hα 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 indepen-dently confirmed by the extent of the variability shown in Fig. 1.The large width of the Hα emission suggests a relatively highvalue for v sin i, which points to a high inclination angle. Withthe orbital inclination angle of 87◦ (Pigulski & Boratyn 1992)and the high inclination angle of β Cep itself (>60◦, Telting et al.1997; Donati et al. 2001) this means that the spin and orbitalangular momentum vectors could well be aligned. An interest-ing question is how such a binary system with one, presumablyspun-down, magnetic B star and a Be star may have evolved.Our result implies that the observed Hα emission is not re-lated to the magnetic field of the primary star. This agrees withmodels explaining the variability observed in the UV wind-linesas due to the rotation of the magnetic field.New spectro-astrometric observations to obtain a wider spec-tral coverage are being planned and will allow us to further con-strain the v sin i and spectral type of the secondary star.Acknowledgements. Based on observations obtained with the Nordic OpticalTelescope, which is operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, at the Spanish Observatorio del Roquede los Muchachos of the Instituto de Astrofisica de Canarias. The data pre-sented here were taken using ALFOSC, which is owned by the Instituto deAstrofisica de Andalucia (IAA) and operated at the Nordic Optical Telescope un-der agreement between IAA and the NBIfAFG of the Astronomical Observatoryof Copenhagen. RS and HFH thank F. Leone for useful discussions.ReferencesBailey, J. 1998a, MNRAS, 301, 161Bailey, J. A. 1998b, in Optical Astronomical Instrumentation, ed. S. D’Odorico,Proc. SPIE, 3355, 932Baines, D., Oudmaijer, R. D., Porter, J. M., & Pozzo, M. 2006, MNRAS, 367,737Donati, J.-F., Wade, G. A., Babel, J., et al. 2001, MNRAS, 326, 1265Gezari, D. Y., Labeyrie, A., & Stachnik, R. V. 1972, ApJ, 173, L1Hadrava, P., & Harmanec, P. 1996, A&A, 315, L401Hartkopf, W., Mason, B., Wycoff, G., & McAlister, H. 2001,Fourth Catalog of Interferometric Measurements of Binary Stars,http://www.ad.usno.navy.mil/wds/int4.htmlHenrichs, H. F., Bauer, F., Hill, G. M., et al. 1993, in New Perspectives on StellarPulsation and Pulsating Variable Stars, ed. J. M. Nemec, & J. M. Matthews,IAU Coll., 139, 186Henrichs, H. F., de Jong, J. A., Donati, J.-F., et al. 2000, in IAU Coll. 175, TheBe Phenomenon in Early-Type Stars, ed. M. A. Smith, H. F. Henrichs, &J. Fabregat, ASP Conf. Ser., 214, 324Henrichs, H. F., de Jong, J. A., Verdugo, E., et al. 2006, in prep.Henrichs, H. F., Neiner, C., & Geers, V. C. 2003, in Magnetic Fields in O, Band A Stars: Origin and Connection to Pulsation, Rotation and Mass Loss,ed. L. A. Balona, H. F. Henrichs, & R. Medupe, ASP Conf. Ser., 305, 301Kaper, L., & Mathias, P. 1995, in IAU Coll. 155, Astrophysical Applications ofStellar Pulsation, ed. R. S. Stobie, & P. A. Whitelock, ASP Conf. Ser., 83,295Karpov, B. G. 1933, Lick Observatory Bull., 16, No. 457, 167Mathias, P., Gillet, D., & Kaper, L. 1991, in Rapid Variability of OB-stars:Nature and Diagnostic Value, ed. D. Baade, 193Neiner, C., Henrichs, H., Geers, V., & Donati, J.-F. 2001, IAU Circ., 7651, 3Pan’ko, E. A., & Tarasov, A. E. 1997, Astron. Lett., 23, 545Pigulski, A., & Boratyn, D. A. 1992, A&A, 253, 178Porter, J. M., & Rivinius, T. 2003, PASP, 115, 1153Porter, J. M., Oudmaijer, R. D., & Baines, D. 2004, A&A, 428, 327Schnerr, R. S., Owocki, S. P., ud-Doula, A., Henrichs, H. F., & Townsend,R. H. D. 2006, A&A, in prep.Takami, M., Bailey, J., & Chrysostomou, A. 2003, A&A, 397, 675Telting, J. H., Aerts, C., & Mathias, P. 1997, A&A, 322, 493Townsend, R. H. D., Owocki, S. P., & Howarth, I. D. 2004, MNRAS, 350, 189Townsend, R. H. D., Owocki, S. P., & Groote, D. 2005, ApJ, 630, L81

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

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