Volume 586, February 2016
|Number of page(s)||6|
|Section||Stellar structure and evolution|
|Published online||28 January 2016|
1 Joint ALMA Observatory & European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
2 Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
3 Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
4 Department of Physics & Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
5 Physics/Astronomy Department, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4002, USA
6 Department of Physics, The Citadel, 171 Moultrie Street, Charleston, SC 29409, USA
Received: 29 September 2015
Accepted: 18 November 2015
Context. The existence of a significant population of Ap stars with very long rotation periods (up to several hundred years) has progressively emerged over the past two decades. However, only lower limits of the periods are known for most of them because their variations have not yet been observed over a sufficient timebase.
Aims. We determine the rotation period of the slowly rotating Ap star HD 18078 and we derive constraints on the geometrical structure of its magnetic field.
Methods. We combine measurements of the mean magnetic field modulus obtained from 1990 to 1997 with determinations of the mean longitudinal magnetic field spanning the 1999–2007 time interval to derive an unambiguous value of the rotation period. We show that this value is consistent with photometric variations recorded in the Strömgren uvby photometric system between 1995 and 2004. We fit the variations of the two above-mentioned field moments with a simple model to constrain the magnetic structure.
Results. The rotation period of HD 18078 is (1358 ± 12) d. The geometrical structure of its magnetic field is consistent to first order with a colinear multipole model whose axis is offset from the centre of the star.
Conclusions. HD 18078 is only the fifth Ap star with a rotation period longer than 1000 d for which the exact value of that period (as opposed to a lower limit) could be determined. The strong anharmonicity of the variations of its mean longitudinal magnetic field and the shift between their extrema and those of the mean magnetic field modulus are exceptional and indicative of a very unusual magnetic structure.
Key words: stars: individual: HD 18078 / stars: chemically peculiar / stars: rotation / stars: magnetic field
Based in part on observations made at Observatoire de Haute Provence (CNRS), France; at Kitt Peak National Observatory, National Optical Astronomy Observatory (NOAO Prop. ID: KP2442; PI: T. Lanz), which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation; at the Canada-France-Hawaii Telescope (CFHT) which is operated from the summit of Maunakea by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii; and at the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences.
Table 3 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A85
© ESO, 2016
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