Stable magnetic fields and changing starspots on Vega

An ultra-deep decadal survey at Pic du Midi and OHP

¹ Université de Toulouse; UPS-OMP; IRAP ; Toulouse, France
² CNRS; IRAP ; 14, avenue Edouard Belin, 31400 Toulouse, France
³ Observatoire Midi-Pyrénées, 14 ave Edouard Belin, 31400 Toulouse, France
⁴ Institut für Mathematik, Universität Potsdam, 14476 Potsdam, Germany
⁵ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
⁶ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

^★ Corresponding author: torsten.boehm@irap.omp.eu

Received: 8 July 2025
Accepted: 13 August 2025

Abstract

Aims. Monitoring magnetic and activity variations in A and B stars with ultra-weak magnetic fields is essential to our understanding of the origin and evolution of these fields in this domain of the Hertzsprung-Russell (HR) diagram. Vega is a prototype star of its type and its long-term monitoring offers the most promising opportunity to study these properties.

Methods. High-resolution spectrocopic and spectropolarimetric data were gathered with SOPHIE/OHP in 2018 and NARVAL/NEO-NARVAL/TBL in 2018, 2023, and 2024. A total of 13 108 individual spectra of Vega were obtained, which are the basis for the analysis carried out in this work. Magnetic field maps were reconstructed with the Zeeman-Doppler imaging (ZDI) method, while activity maps were reconstructed with an innovative code built for this purpose. These maps were compared to previous results.

Results. The rotation period of Vega was confirmed to be very close to the former reference period of 0.678 d. The average magnetic field confirms a negative spot of radial field on the pole with stable strength, while the magnetic maps confirm the long-term stability of an oblique dipole, as well as smaller magnetic features showing up consistently throughout our three observing epochs. However, brightness maps show strong variations of the spot location over a timescale of years (and possibly even shorter). The spot contrast is shown to be very similar in the observations from 2012, 2018, 2023, and 2024, with a normalised spectral amplitude of 0.0003. A direct correlation between the magnetic field and brightness patches could not be revealed in the simultaneous SOPHIE and NARVAL 2018 dataset.

Conclusions. Vega’s so-called magnetic mystery has gained complexity since indications point towards the presence of a fossil magnetic field, but also to the presence of a dynamo-generated field, most likely concentrated on equatorial regions.