Stable magnetic fields and changing starspots on Vega -- An ultra-deep decadal survey at Pic du Midi and OHP

TL;DR

This study presents an ultra-deep, decadal survey of Vega using high-resolution spectroscopy and spectropolarimetry from SOPHIE and NARVAL/NEO-NARVAL/TBL to map surface magnetism via Zeeman-Doppler Imaging and to reconstruct brightness spot distributions. It finds a long-lived, stable oblique dipole with a polar radial-field patch, suggesting a fossil magnetic component, alongside smaller-scale magnetic features that persist yet may be time-variable. In contrast, brightness spots migrate across years with constant contrast, indicating dynamic surface activity that does not straightforwardly correlate with the large-scale magnetic field. The results point to a two-component magnetism model in Vega, combining a fossil field with a dynamo-generated equatorial component, and have important implications for understanding magnetic fields in radiative envelopes of A-type stars.$P = 0.6705 \,\pm\,0.0019$ d and $P \approx 0.678$ d are used to phase the data and compare across epochs, highlighting the decadal stability of the large-scale field against evolving surface activity.

Abstract

Monitoring magnetic and activity variations in A- and B-type stars with ultra-weak magnetic fields is essential to understand the origin and evolution of these fields in this region of the HR diagram, with Vega standing as the prototype of this category. We collected high-resolution spectroscopic and spectropolarimetric data with SOPHIE (OHP, 2018) and with NARVAL/NEO-NARVAL (TBL, 2018, 2023, and 2024), yielding a total of 13108 individual spectra. Magnetic field maps were reconstructed using the Zeeman-Doppler Imaging method, while brightness maps were derived with a dedicated code developed for this purpose. The average magnetic field confirms a negative radial-field spot at the pole with stable strength, while the maps reveal the long-term stability of an oblique dipole together with smaller magnetic structures consistently detected across observing epochs. In contrast, brightness maps show strong variations in the location of surface spots on timescales of years, possibly shorter, although the spot contrast remains nearly unchanged between 2012, 2018, 2023, and 2024, with a normalized spectral amplitude of 0.0003. No direct correlation between magnetic and brightness features could be established in the simultaneous SOPHIE and NARVAL dataset of 2018. These results suggest that Vega hosts both a persistent fossil magnetic field and a dynamo-generated component, most likely concentrated in equatorial regions.

Figures (12)

• Figure 1: Comparison of the LSD Stokes I intensity profile of Vega obtained with NARVAL/TBL and the new NEXTRA pipeline (red) and a quasi simultaneous observation with SOPHIE/OHP (within one minute, blue), both processed with LSDpy. Stokes I profiles are quasi identical. The difference of the SOPHIE and NARVAL profile is presented in the top panel (orange). Dashed blue line corresponds to the radial velocity of Vega of $-13.9\,\mathrm{km\,s^{-1}}$. The dashed yellow lines show $\pm v\sin i$ ($\hbox{21.6}\,\mathrm{km\,s^{-1}}$), while the dashed red lines correspond to the outer limits of the profile including gaussian broadening (at additional $\pm \hbox{10}\,\mathrm{km\,s^{-1}}$). Data are from August 1$^{\rm st}$ 2018 at 23:11 UT.
• Figure 2: Comparison of the LSD Stokes V profile of $\gamma$ Equ generated with LSDpy and reduced with the new NEXTRA spectropolarimetric data reduction pipeline (blue) and reduced with the historic "Libre-Esprit" (donati1997, red), Both pipelines provide quasi identical results. The exact same data set from August 19$^{\rm th}$ 2018 is compared. The difference of the NEXTRA and LE profile is presented in the top panel (orange).
• Figure 3: Average Stokes V profile of Vega: 2018 (top), 2023 (middle), 2024 (bottom). Black line and grey zone: Averaged Stokes V and $\pm 1\,\sigma$ envelope. Blue line and $1\,\sigma$ error bars: Averaged Null profile. Vertical dashed yellow lines show the position of the negative and positive peak of the 2018 averaged Stokes V profile. The dashed red lines show $\pm v\sin i$, while the dashed black lines correspond to the outer limits of the profile including gaussian broadening (additional $\pm\,10\,\mathrm{km\,s^{-1}}$) as shown in Fig. \ref{['NEXTRA_I']}.
• Figure 4: Comparison of the average LSD Stokes V profile from NARVAL in 2018 (red) with profiles from NEO-NARVAL in 2023 (top panel, blue) and in 2024 (bottom panel, blue). The vertical lines are described in Fig. \ref{['vega_average']}.
• Figure 5: From top to bottom, ZDI reconstructions of magnetic geometries in equatorial projection for 2018, 2023, and 2024. Each column displays a component of the local magnetic vector, in a spherical coordinates frame. The field strength is color-coded (Gauss unit). The phase reference is taken at BJD = 2456892.015, 2456892.185, and 2456892.506 for 2018, 2023, and 2024 (respectively), to ensure that all epochs display an oblique dipole pointing at the same phases.