A decade-long magnetic monitoring of Vega

¹ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, 14 avenue Édouard Belin, 31400 Toulouse, France
e-mail: ppetit@irap.omp.eu
² Tartu Observatory, University of Tartu, Observatooriumi 1, Toravere, 61602 Tartumaa, Estonia
³ Department of Astronomy, Beijing Normal University, Beijing 100875, PR China

Received: 24 December 2021
Accepted: 18 August 2022

Abstract

Aims. The very weak magnetic field detected at the surface of Vega hints at a widespread population of weakly magnetic stars of A and B spectral types. We contribute here to gather more clues about the origin of this magnetism by investigating the long-term stability of the field geometry of this prototypical star.

Methods. We use spectropolarimetric data collected as part of a long-term campaign, with more than 2000 observations spread between 2008 and 2018. Using various sub-sets extracted from the whole time series, we reconstruct several maps of the large-scale surface magnetic field.

Results. We confirm that the polarimetric signal is modulated according to a ~0.68 d period, which we interpret as the stellar rotation period. The surface magnetic field is organized in a complex geometry. We confirm the existence of a very localized, polar magnetic spot previously reported for Vega, with a radial field strength of about −5 G. We show that the surface of the star is also covered by a dipole, with a polar strength close to 9 G and a dipole obliquity close to 90°. Both magnetic structures are remarkably stable over one decade. The available data suggest that smaller-scale magnetic spots may not be limited to the polar region, although the poor reliability of their reconstruction does not allow us to firmly conclude about their temporal evolution.