Variable magnetic field geometry of the young sun HN Pegasi (HD 206860)^⋆

¹ Institut für Astrophysik, Universität Göttingen, Friedrich Hund Platz 1, 37077 Göttingen, Germany
e-mail: sudeshna@astro.physik.uni-goettingen.de
² CNRS, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
³ Computational Engineering and Science Research Centre, University of Southern Queensland, 4350 Toowoomba, Australia
⁴ LUPM-UMR 5299, CNRS & Université Montpellier, Place Eugéne Bataillon, 34095 Montpellier Cedex 05, France

Received: 29 April 2014
Accepted: 9 October 2014

Abstract

Context. The large-scale magnetic field of solar-type stars reconstructed from their spectropolarimetric observations provide important insight into their underlying dynamo processes.

Aims. We aim to investigate the temporal variability of the large-scale surface magnetic field and chromospheric activity of a young solar analogue, the G0 dwarf HN Peg.

Methods. The large-scale surface magnetic field topology is reconstructed using Zeeman Doppler imaging at six observational epochs covering seven years. We also investigated the chromospheric activity variations by measuring the flux in the line cores of the three chromospheric activity indicators: Ca II HK, Hα, and the Ca II IRT lines.

Results. The magnetic topology of HN Peg shows a complex and variable geometry. While the radial field exhibits a stable positive polarity magnetic region at the poles at each observational epoch, the azimuthal field is strongly variable in strength, where a strong band of positive polarity magnetic field is present at equatorial latitudes. This field disappears during the middle of our timespan, reappearing again during the last two epochs of observations. The mean magnetic field derived from the magnetic maps also follow a similar trend to the toroidal field, with the field strength at a minimum in epoch 2009.54. Summing the line of sight magnetic field over the visible surface at each observation, HN Peg exhibits a weak longitudinal magnetic field (B_l) ranging from −14 G to 13 G, with no significant long-term trend, although there is significant rotational variability within each epoch. Those chromospheric activity indicators exhibit more long-term variations over the time span of observations, where the minimal is observed in Epoch 2008.71.