B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars β CMa and ϵ CMa

Possible lack of a “magnetic desert” in massive stars^⋆

¹ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: lfossati@astro.uni-bonn.de
² Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège, Belgium
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁶ Institute for Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁸ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
¹⁰ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306 Santiago 22, Chile
¹³ European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 15 September 2014
Accepted: 3 November 2014

Abstract

Only a small fraction of massive stars seem to host a measurable structured magnetic field, whose origin is still unknown and whose implications for stellar evolution still need to be assessed. Within the context of the “B fields in OB stars (BOB)” collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars β CMa (HD 44743; B1 II/III) and ϵ CMa (HD 52089; B1.5II) in December 2013 and April 2014. For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field, approximately constant with time. We determined the physical parameters of both stars and characterise their X-ray spectrum. For the β Cep star β CMa, our mode identification analysis led to determining a rotation period of 13.6 ± 1.2 days and of an inclination angle of the rotation axis of 57.6 ± 1.7°, with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of about 22° and a dipolar magnetic field strength (B_d) of about 100 G (60 <B_d< 230 G within the 1σ level), below what is typically found for other magnetic massive stars. This conclusion is strengthened further by considerations of the star’s X-ray spectrum. For ϵ CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Our results imply that both stars are expected to have a dynamical magnetosphere, so the magnetic field is not able to support a circumstellar disk. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. Ahistogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field “desert” observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational effort is probably still needed to properly access the magnetic field incidence.