A decade-long magnetic monitoring of Vega

TL;DR

This study presents a decade-long spectropolarimetric analysis of Vega, revealing stable, complex magnetic field structures including a polar spot and dipole, with implications for understanding weak magnetic fields in A and B type stars.

Contribution

It provides the first long-term, detailed magnetic field maps of Vega, demonstrating the stability and complexity of its magnetic geometry over ten years.

Findings

Polar magnetic spot with -5 G strength confirmed

Surface magnetic field includes a stable dipole with 9 G strength

Magnetic structures remain stable over a decade

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 2,000 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 $\sim 0.68$ 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 $90^\circ$. 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.