EDP Sciences
Free access
Volume 406, Number 3, August II 2003
Page(s) 1019 - 1031
Section Formation and evolution of planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20030742

A&A 406, 1019-1031 (2003)
DOI: 10.1051/0004-6361:20030742

Discovery of a magnetic field in the Slowly Pulsating B star $\zeta$ Cassiopeiae

C. Neiner1, 2, 3, V. C. Geers1, H. F. Henrichs1, M. Floquet2, Y. Frémat2, A.-M. Hubert2, O. Preuss4 and K. Wiersema1

1  Sterrenkundig Instituut "Anton Pannekoek", Universiteit van Amsterdam, The Netherlands
2  GEPI/ UMR 8111 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
3  Current affiliation: RSSD, Estec/ESA, Keplerlaan 1, 2201 AZ, Noordwijk ZH, The Netherlands
4  Max Plank Institut für Aeronomie, Lindau, Germany

(Received 13 August 2002 / Accepted 16 May 2003)

$\zeta$ Cas is a B2 IV star with $v\sin i = 17$ km s -1. Time-resolved circular spectropolarimetric observations of  $\zeta$ Cas obtained in 2001 and 2002 with the Musicos échelle spectropolarimeter at the 2 m Télescope Bernard Lyot (TBL) show a sinusoidally varying longitudinal magnetic field with a strength between 10 G and -46 G for the averaged line-of-sight component, corresponding to  $B_{\rm pol}=335^{+120}_{-65}$ G. The period corresponds very accurately with the 5.37045 day period as derived from stellar wind variations observed in the ultraviolet. The epoch of the positive maximum field corresponds in phase with the maximum emission in the UV wind lines. This gives compelling evidence for a magnetic rotator model for this star, with an unambiguous rotation period of 5.37 days. We searched for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves in the ~400 optical spectra. We found a non-radial pulsation mode with  $l=2 \pm 1$ at the frequency  f=0.64 c d -1. From this periodicity and from stellar parameters derived from model fits, we propose to classify  $\zeta$ Cas as a Slowly Pulsating B (SPB) star. This is the third detection of a magnetic field in an early B-type pulsating star and the first one in a SPB star.

Key words: stars: magnetic fields -- stars: oscillations -- stars: winds, outflows

Offprint request: C. Neiner, cneiner@rssd.esa.int

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