Magnetic field structure in single late-type giants: The weak G-band giant 37 Comae from 2008 to 2011^⋆

¹ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko shose, 1784 Sofia, Bulgaria
e-mail: stsvetkova@astro.bas.bg
² CNRS, UMR 5277, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
³ Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, 31400 Toulouse, France
⁴ Department of Physics, Royal Military College of Canada, PO Box 17000, Station “Forces”, Kingston, K7K 4B4 Ontario, Canada
⁵ LUPM, University of Montpellier, CNRS, Place Eugène Bataillon – cc072, 34095 Montpellier, France
⁶ Department of Astronomy, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
⁷ Saint Petersburg State University, Universitetski pr. 28, 198504 Saint Petersburg, Russia
⁸ Observatório Nacional/MCTI, Rua General José Cristino 77, 20921-400 Rio de Janeiro, Brazil

Received: 22 July 2015
Accepted: 22 November 2016

Abstract

Aims. This work studies the magnetic activity of the late-type giant 37 Com. This star belongs to the group of weak G-band stars that present very strong carbon deficiency in their photospheres. The paper is a part of a global investigation into the properties and origin of magnetic fields in cool giants.

Methods. We use spectropolarimetric data, which allows the simultaneous measurement of the longitudinal magnetic field B_l, line activity indicators (Hα, Ca ii IRT, S-index) and radial velocity of the star, and consequently perform a direct comparison of their time variability. Mean Stokes V profiles are extracted using the least squares deconvolution (LSD) method. One map of the surface magnetic field of the star is reconstructed via the Zeeman Doppler imaging (ZDI) inversion technique.

Results. A periodogram analysis is performed on our dataset and it reveals a rotation period of 111 days. We interpret this period to be the rotation period of 37 Com. The reconstructed magnetic map reveals that the structure of the surface magnetic field is complex and features a significant toroidal component. The time variability of the line activity indicators, radial velocity and magnetic field B_l indicates a possible evolution of the surface magnetic structures in the period from 2008 to 2011. For completeness of our study, we use customized stellar evolutionary models suited to a weak G-band star. Synthetic spectra are also calculated to confirm the peculiar abundance of 37 Com.

Conclusions. We deduce that 37 Com is a 6.5 M_⊙ weak G-band star located in the Hertzsprung gap, whose magnetic activity is probably due to dynamo action.