Mitigating stellar activity jitter with different line lists for least-squares deconvolution

Analysis of a parametric and a randomised line selection

¹ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
e-mail: stefano.bellotti@irap.omp.eu
² Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, 34095 Montpellier, France
³ Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁴ Department of Physics & Space Science, Royal Military College of Canada, PO Box 17000 Station Forces, K7K 0C6 Kingston, ON, Canada
⁵ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere, 61602 Tartumaa, Estonia
⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 16 July 2021
Accepted: 19 October 2021

Abstract

Context. Stellar activity limits the radial velocity (RV) search and characterisation of exoplanets, as it introduces spurious noise (called jitter) in the data sets and prevents the correct retrieval of a planetary signal. This is key for M dwarfs, considering that they manifest high activity levels and are primary targets for present and future searches of habitable Earth-like planets. To perform precise RV measurements, multi-line numerical techniques like cross-correlation and least-squares deconvolution (LSD) are typically employed.

Aims. Effective filtering of activity is crucial to achieving the sensitivity required for small planet detections. Here we analyse the impact of selecting different line lists for LSD on the dispersion in our RV data sets, to identify the line list that most effectively reduces the jitter.

Methods. We employ optical spectropolarimetric observations of the active M dwarf EV Lac collected with ESPaDOnS and NARVAL, and study two line down-selection approaches: a parametric method based on line properties (depth, wavelength, magnetic sensitivity) and a randomised algorithm that samples the line combination space. We test the latter further to find the line list that singles out the activity signal from other sources of noise, and on AD Leo and DS Leo to examine its consistency at mitigating jitter for different activity levels. The analysis is complemented with planetary injection tests.

Results. The parametric selection yields a RV RMS reduction of less than 10%, while the randomised selection yields a systematic improvement (>50%) regardless of the activity level of the star examined. Furthermore, if activity is the dominant source of noise, this approach allows the construction of lists containing mainly activity-sensitive lines, which could be used to enhance the rotational modulation of the resulting data sets and determine the stellar rotation period more robustly. Finally, the output line lists allow the recovery of a synthetic planet (0.3–0.6 M_Jup on a 10 d orbit) in the presence of both moderate (20 m s⁻¹ semi-amplitude) and high (200 m s⁻¹) activity levels, without substantially affecting the planet signal (between 60 and 120 m s⁻¹).