B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD −62° 2124^⋆

¹ Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107, USA
e-mail: ncastror@umich.edu
² Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁷ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁸ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
¹⁰ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Quartier Agora, Allée du 6 Août 19c, Bât. B5C, 4000 Liège, Belgium
¹¹ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany

Received: 20 September 2016
Accepted: 30 November 2016

Abstract

The origin and evolution of magnetism in OB stars is far from being well understood. With approximately 70 magnetic OB stars known, any new object with unusual characteristics may turn out to be a key piece of the puzzle. We report the detection of an exceptionally strong magnetic field in the He-strong B2IV star CPD −62° 2124. Spectropolarimetric FORS2 and HARPSpol observations were analysed by two independent teams and procedures, concluding on a strong longitudinal magnetic field of approximately 5.2 kG. The quantitative characterisation of the stellar atmosphere yields an effective temperature of 23 650 ± 250 K, a surface gravity of 3.95 ± 0.10 dex and a surface helium fraction of 0.35 ± 0.02 by number. The metal composition is in agreement with the cosmic abundance standard, except for Mg, Si and S, which are slightly non-solar. The strong and broad (~300 km s^-1) disc-like emission displayed by the Hα line suggests a centrifugal magnetosphere supported by the strong magnetic field. Our results imply that CPD −62° 2124 is an early B-type star hosting one of the strongest magnetic fields discovered to date, and one of the most evolved He-strong stars known, with a fractional main-sequence lifetime of approximately 0.6.