Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey^{⋆,⋆⋆,⋆⋆⋆}

¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: evelyne.alecian@obs.ujf-grenoble.fr
² LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
⁴ Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA
⁵ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁶ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
⁷ Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
⁸ Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic
⁹ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston Ontario, K7K 7B4, Canada
¹⁰ Dominion Astrophysical Observatory, Herzberg Astronomy and Astrophysics Program, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

Received: 19 December 2013
Accepted: 8 April 2014

Abstract

Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment.

Aims. One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution.

Methods. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e.g., surface abundance peculiarities, Hα emission), which are also of interest for future studies.

Results. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Hα emission with respect to the theory of centrifugally-supported magnetospheres.