Stellar-activity analysis of the nearby M dwarf GJ 526

Multi-dimensional Gaussian-process modelling of RV, FWHM, and S-index

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
³ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
⁴ Light Bridges S. L., 35004 Las Palmas de Gran Canaria, Spain
⁵ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁶ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁸ Centro de Astrobiología (CAB), CSIC-INTA, ESAC campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
⁹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
¹⁰ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹² INAF – Osservatorio Astrofisico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹³ IFPU – Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
¹⁴ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

^★ Corresponding author: atanas.stefanov@iac.es

Received: 10 July 2025
Accepted: 2 September 2025

Abstract

M dwarfs are the most abundant stars in the Galaxy, and their low masses make them natural favourites for exoplanet radial-velocity (RV) searches. Nevertheless, these stars often demonstrate strong stellar activity that is yet to be fully understood. We used high-precision ESPRESSO, HIRES, and HARPS spectroscopy to perform a stellar-activity analysis on the nearby early M dwarf GJ 526 (d = 5.4 pc). We carried out a joint modelling of (i) the stellar rotation in RV, full width at half maximum (FWHM), and Mount Wilson S-index through Gaussian processes; (ii) long-term trends in these three physical quantities; and (iii) RV planetary signals. We constrained the stellar-rotation period to P_rot = 48.7 ± 0.3 d, and discovered a weak sinusoidal signature in RV, FWHM, and S-index of period P_cyc = 1680₋₄₀⁺⁵⁰d. We propose phase-space trajectories between RV and activity indicators as a novel means to visualise and interpret stellar activity. Current evidence does not support planetary companions of GJ 526.