Chromaticity of stellar activity in radial velocities

Context . In the search for exoplanets using radial velocities (RV), stellar activity has become one of the main limiting factors for detectability. Fortunately, activity-induced RV signals are wavelength-dependent or chromatic, unlike planetary signals. This study exploits the broad spectral coverage provided by the combined use of SOPHIE and SPIRou velocimeters to investigate the chromatic nature of the activity signal of the highly active M dwarf EV Lac. Aims . We aim to understand the origin of the strong wavelength dependence (chromaticity) observed in the RV signal of EV Lac by selecting spectral lines based on physical properties. In particular, we explore the impact of starspots by defining the contrast effect at the level of individual lines. The Zeeman effect is also considered in this study. Methods . SPIRou and SOPHIE spectra were reduced using the line-by-line (LBL) method. We performed custom RV calculations, using groups of spectral lines selected for their sensitivity to either the spot-to-photosphere contrast or the Zeeman effect. The sensitivity of each line to the spot is defined using a two-temperature model based on PHOENIX spectra, while Landé factors were used to quantify Zeeman sensitivity. Results . We find that the spectral lines are distributed in two distinct families of contrasts, producing anti-correlated RV signals. This leads to a partial cancellation of the total RV signal, especially at longer wavelengths and provides a natural explanation for the strong chromaticity observed in EV Lac. This sign-reversal effect is demonstrated here, for the first time, on empirical data. Building on this discovery, we propose a new approach to constraining spot temperatures and to mitigating stellar activity. This will open up promising avenues for improving activity corrections and enhancing the detection of exoplanets around active M dwarfs.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-08-14
DOI
https://doi.org/10.1051/0004-6361/202554963
Citations
2
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
2.31

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article

Chromaticity of stellar activity in radial velocities

G. Hébrard, N. Heidari, R. F. Díaz, L. Mignon et al.
2 citations
Astronomy and Astrophysics
Stellar, planetary, and galactic studies
2.31
article

Chromaticity of stellar activity in radial velocities

G. Hébrard, N. Heidari, R. F. Díaz, L. Mignon, Aurélien Maurel-Pantel, H. G. Vivien, N. Hara, A. Santerne, Charles Cadieux, D. Ségransan, I. Boisse, S. Grouffal, A. Chomez, C. Moutou, X. Bonfıls, P. Larue, J. Serrano Bell, L. Arnold, T. Forveille, J. F. Donati, Paul Charpentier, A. Carmona, J. Morin, Antoine C. Petit, P. Cortés-Zuleta, X. Delfosse, S. Bellotti, P. I. Cristofari, N. C. Santos, F. Kiefer, Neil J. Cook, N. Meunier, Étienne Artigau, V. Bourrier, P. Petit, R. Doyon
article en
2 citations

Abstract

Context . In the search for exoplanets using radial velocities (RV), stellar activity has become one of the main limiting factors for detectability. Fortunately, activity-induced RV signals are wavelength-dependent or chromatic, unlike planetary signals. This study exploits the broad spectral coverage provided by the combined use of SOPHIE and SPIRou velocimeters to investigate the chromatic nature of the activity signal of the highly active M dwarf EV Lac. Aims . We aim to understand the origin of the strong wavelength dependence (chromaticity) observed in the RV signal of EV Lac by selecting spectral lines based on physical properties. In particular, we explore the impact of starspots by defining the contrast effect at the level of individual lines. The Zeeman effect is also considered in this study. Methods . SPIRou and SOPHIE spectra were reduced using the line-by-line (LBL) method. We performed custom RV calculations, using groups of spectral lines selected for their sensitivity to either the spot-to-photosphere contrast or the Zeeman effect. The sensitivity of each line to the spot is defined using a two-temperature model based on PHOENIX spectra, while Landé factors were used to quantify Zeeman sensitivity. Results . We find that the spectral lines are distributed in two distinct families of contrasts, producing anti-correlated RV signals. This leads to a partial cancellation of the total RV signal, especially at longer wavelengths and provides a natural explanation for the strong chromaticity observed in EV Lac. This sign-reversal effect is demonstrated here, for the first time, on empirical data. Building on this discovery, we propose a new approach to constraining spot temperatures and to mitigating stellar activity. This will open up promising avenues for improving activity corrections and enhancing the detection of exoplanets around active M dwarfs.

Astronomy and AstrophysicsVol. 701
University of Geneva (CH), Leiden University (NL), Centre National de la Recherche Scientifique (FR), Université du Québec à Montréal (CA), University of St Andrews (GB), Université Fédérale de Toulouse Midi-Pyrénées (FR), Universidade do Porto (PT), Université de Montpellier (FR), Université Paris Cité (FR), Aix-Marseille Université (FR), Institut d'Astrophysique de Paris (FR), Sorbonne Université (FR), Institute of Astrophysics and Space Sciences (PT), Sorbonne Paris Cité (FR), Laboratoire Univers et Particules de Montpellier (FR), Center for Astrophysics Harvard & Smithsonian (US), Observatoire de la Côte d’Azur (FR), International Center for Advanced Studies (AR), Laboratório Nacional de Astrofísica (BR), Canada–France–Hawaii Telescope (US), Université de Montréal (CA), Université Grenoble Alpes (FR)
European Commission, Agence Nationale de la Recherche, Centre National d’Etudes Spatiales, Universität Wien, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Centre National de la Recherche Scientifique, Uppsala Universitet, Canadian Space Agency, Fundação para a Ciência e a Tecnologia, Fonds de recherche du Québec – Nature et technologies, Institut national des sciences de l'Univers
Openalex Percentile: Top 10%
Stellar, planetary, and galactic studies
2.31
DOIOpen Access PDFOpenAlex
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