Comparative study of small-scale magnetic fields on ξ Boo A using optical and near-infrared spectroscopy

¹ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
² Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, Tautenburg 07778, Germany
⁴ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
⁵ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁶ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁷ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁹ Instituto de Astrofísica de Andalucía – CSIC, c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★ Corresponding author; axel.hahlin@physics.uu.se

Received: 15 November 2024
Accepted: 26 February 2025

Abstract

Context. Magnetic field investigations of Sun-like stars, using Zeeman splitting of non-polarised spectra, in the optical and H-band have found significantly different magnetic field strengths for the same stars, the cause of which is currently unknown.

Aims. We aim to further investigate this issue by systematically analysing the magnetic field of ξ Boo A, a magnetically active G7 dwarf, using spectral lines at different wavelengths.

Methods. We used polarised radiative transfer accounting for the departures from local thermodynamic equilibrium to generate synthetic spectra. To find the magnetic field strengths in the optical, H-band, and K-band, we employed MCMC sampling analysis of high-resolution spectra observed with the spectrographs CRIRES⁺, ESPaDOnS, NARVAL, and UVES. We also determine the formation depth of different lines by calculating the contribution functions for each line employed in the analysis.

Results. We find that the magnetic field strength discrepancy between lines in the optical and H-band persists even when treating the different wavelength regions consistently. In addition, the magnetic measurements derived from the K-band appear to more closely align with the optical. The H-band appears to yield magnetic field strengths ∼0.4 kG with a statistically significant variation while the optical and K-band is stable at ∼0.6 kG for observations spanning about two decades. The contribution functions reveal that the optical lines form at a significantly higher altitude in the photosphere compared to those in the H- and K-band.

Conclusions. While we find that the discrepancy remains, the variation of formation depths could indicate that the disagreement between magnetic field measurements obtained at different wavelengths is linked to the variation of the magnetic field along the line of sight and between different structures, such as star spots and faculae, in the stellar photosphere.