The CARMENES search for exoplanets around M dwarfs

Line-by-line sensitivity to activity in M dwarfs^★

¹ Institut de Ciències de l'Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Spain
e-mail: lafarga@ice.cat
² Institut d'Estudis Espacials de Catalunya (IEEC), c/ Gran Capità 2-4, 08034 Barcelona, Spain
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ Centre for Exoplanets and Habitability, University of Warwick, Coventry CV4 7AL, UK
⁵ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁶ 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
⁷ Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Landessternwarte, Zentrum für Astronomie der Universtät Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁹ Instituto de Astrofísica de Andalucia (IAA-CSIC), Glorieta de la Astronomia s/n, 18008 Granada, Spain
¹⁰ Centro de Astrobiologia, CSIC-INTA, ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Canada, Madrid, Spain
¹¹ Centro Astronónomico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
¹² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna, 38026 La Laguna, Tenerife, Spain
¹⁴ Thüringer Landesstenwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁶ Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany
¹⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 1 December 2022
Accepted: 9 February 2023

Abstract

Context. Radial velocities (RVs) measured from high-resolution stellar spectra are routinely used to detect and characterise orbiting exoplanet companions. The different lines present in stellar spectra are created by several species, which are non-uniformly affected by stellar variability features such as spots or faculae. Stellar variability distorts the shape of the spectral absorption lines from which precise RVs are measured, posing one of the main problems in the study of exoplanets.

Aims. In this work we aim to study how the spectral lines present in M dwarfs are independently impacted by stellar activity.

Methods. We used CARMENES optical spectra of six active early- and mid-type M dwarfs to compute line-by-line RVs and study their correlation with several well-studied proxies of stellar activity.

Results. We are able to classify spectral lines based on their sensitivity to activity in five M dwarfs displaying high levels of stellar activity. We further used this line classification to compute RVs with activity-sensitive lines and less sensitive lines, enhancing or mitigating stellar activity effects in the RV time series. For specific sets of the least activity-sensitive lines, the RV scatter decreases by ~2 to 5 times the initial one, depending on the star. Finally, we compare these lines in the different stars analysed, finding the sensitivity to activity to vary from star to star.

Conclusions. Despite the high density of lines and blends present in M dwarf stellar spectra, we find that a line-by-line approach is able to deliver precise RVs. Line-by-line RVs are also sensitive to stellar activity effects, and they allow for an accurate selection of activity-insensitive lines to mitigate activity effects in RV. However, we find stellar activity effects to vary in the same insensitive lines from star to star.