Investigating magnetic activity in very stable stellar magnetic fields

Long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Pegasi ^⋆

¹ Konkoly Observatory, MTA CSFK, Konkoly Thege M. út 15-17, 1121 Budapest, Hungary
e-mail: vidakris@konkoly.hu
² University of Graz, Institute of Physics, Department for Geophysics, Astrophysics and Meteorology, NAWI Graz, Universitätsplatz 5, 8010 Graz, Austria
³ Space Research Institute, Austrain Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁴ Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁵ Dark Cosmology Centre, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁶ University of Victoria and Guest Observer, Dominion Astrophysical Observatory, Victoria, BC V9E2E7, Canada
⁷ ELTE Gothard Astrophysical Observatory, Szent Imre herceg út 112, 9704 Szombathely, Hungary

Received: 8 December 2015
Accepted: 2 March 2016

Abstract

The ultrafast-rotating (P_rot ≈ 0.44 d) fully convective single M4 dwarf V374 Peg is a well-known laboratory for studying intense stellar activity in a stable magnetic topology. As an observable proxy for the stellar magnetic field, we study the stability of the light curve, hence the spot configuration. We also measure the occurrence rate of flares and coronal mass ejections (CMEs). We have analysed spectroscopic observations, BV(RI)_C photometry covering 5 yrs, and additional R_C photometry that expands the temporal base over 16 yr. The light curve suggests an almost rigid-body rotation and a spot configuration that is stable over about 16 yrs, confirming the previous indications of a very stable magnetic field. We observed small changes on a nightly timescale and frequent flaring, including a possible sympathetic flare. The strongest flares seem to be more concentrated around the phase where the light curve indicates a smaller active region. Spectral data suggest a complex CME with falling-back and re-ejected material with a maximal projected velocity of ~675 km s^-1. We observed a CME rate that is much lower than expected from extrapolations of the solar flare-CME relation to active stars.