The strong magnetic field of the large-amplitude $beta$,Cephei pulsator V1449,Aql

S.~Hubrig$^1$, I.~Ilyin$^1$, M.~Briquet$^{2,3}$, M.~Sch"oller$^4$, J.~F.~Gonz'alez$^5$, N.~Nu~nez$^5$, P. De Cat$^6$, T. Morel$^7$

$^1$ Leibniz-Institut f"ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
$^2$ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001~Leuven, Belgium
$^3$ LESIA, Observatoire de Paris, CNRS, UPMC, Universit'e Paris-Diderot, 92195 Meudon, France
$^4$ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei M"unchen, Germany
$^5$ Instituto de Ciencias Astronomicas, de la Tierra, y del Espacio (ICATE), 5400 San Juan, Argentina
$^6$ Koninklijke Sterrenwacht van Belgi"e, Ringlaan 3, 1180 Brussel, Belgium
$^7$ Institut d'Astrophysique et de G'eophysique, Universit'e de Li`ege, All'ee du 6 Ao^ut, B^at. B5c, 4000~Li`ege, Belgium

Only for very few $beta$~Cephei stars has the behaviour of the magnetic field
been studied over the rotation cycle. During the past two years we have obtained
multi-epoch polarimetric spectra of the $beta$~Cephei star V1449,Aql with
SOFIN at the Nordic Optical Telescope to search for a rotation period and to
constrain the geometry of the magnetic field.

The mean longitudinal magnetic field is measured at 13 different epochs. The new
measurements, together with the previous FORS,1 measurements, have been used
for the frequency analysis and the characterization of the magnetic field.

V1449,Aql so far possesses the strongest longitudinal magnetic field of up to
700,G among the $beta$,Cephei stars. The resulting periodogram displays three
dominant peaks with the highest peak at $f=0.0720$,d$^{-1}$ corresponding to a
period $P=13fd893$. The magnetic field geometry can likely be described by a
centred dipole with a polar magnetic field strength $B_{rm d}$ around 3,kG and
an inclination angle $beta$ of the magnetic axis to the rotation axis of
76$pm$4$^{circ}$. As of today, the strongest longitudinal magnetic fields are
detected in the $beta$,Cephei stars V1449,Aql and $xi^1$,CMa with large
radial velocity amplitudes. Their peak-to-peak amplitudes reach
$sim$90,km,s$^{-1}$ and $sim$33,km,s$^{-1}$, respectively. Concluding, we
briefly discuss the position of the currently known eight magnetic
$beta$,Cephei and candidate $beta$,Cephei stars in the Hertzsprung-Russell
(H-R) diagram.

Reference: A&A 531, L20 (2011)
Status: Manuscript has been accepted