B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD −57° 3509^⋆

¹ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
e-mail: norbert.przybilla@uibk.ac.at
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
³ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
⁷ Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège, Belgium
¹⁰ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 26 October 2015
Accepted: 5 December 2015

Abstract

Aims. We report the detection of a magnetic field in the helium-strong star CPD −57° 3509 (B2 IV), a member of the Galactic open cluster NGC 3293, and characterise the star’s atmospheric and fundamental parameters.

Methods. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star.

Results. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD −57° 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in Hα is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23 750 ± 250 K and 4.05 ± 0.10, respectively, and a surface helium fraction of 0.28 ± 0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD −57° 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its cluster membership.