A decade-long magnetic monitoring of Vega

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 ∼ 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.

Asteroseismology and Spectropolarimetry of the Exoplanet Host Star λ Serpentis

“…The LSD profiles are shown in Figure 6. Running ZDI (Semel 1989) on polarization signatures close to the detection threshold is not ideal, but previous studies have shown that it is possible to reconstruct magnetic maps even when the signatures are dominated by noise, provided that a sufficient number of observations are combined together in the inversion process (Petit et al 2010(Petit et al , 2022. Here we use the ZDI implementation of Folsom et al (2018aFolsom et al ( , 2018b in a procedure closely following the one presented by Petit et al (2021).…”

Magnetism in High-Mass Stars

“…The strongest magnetic field reported (≈20 kG inferred dipolar strength) for a seemingly chemically normal O-type star is of NGC 1624-2 [61,67]. The detection of "ultra-weak" magnetic fields in Vega [68,69], an intermediate-mass A-type star, has been surprising. A few other A-type stars have been discovered to host ultra-weak fields, although they possess chemical peculiarities typically associated with the Am class [70,71].…”