The CARMENES search for exoplanets around M dwarfs

Stable radial-velocity variations at the rotation period of AD Leonis: A test case study of current limitations to treating stellar activity^★

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: diana.kossakowski@gmail.com
² Department of Astronomy, Sofia University “St Kliment Ohridski”, 5 James Bourchier Blvd, 1164 Sofia, Bulgaria
³ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Canada, Madrid, Spain
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/ de Can Magrans s/n, 08193 Cerdanyola del Vallès, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capità 2–4, 08034 Barcelona, Spain
⁷ Instituto de Astrofísica de Andalucia (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig Weg 3, 37077 Göttingen, Germany
¹⁰ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹² Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁴ Departamento de Fisica 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
¹⁵ Department of Physics, Ariel University, Ariel 40700, Israel
¹⁶ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 13 April 2022
Accepted: 31 August 2022

Abstract

Context. A challenge with radial-velocity (RV) data is disentangling the origin of signals either due to a planetary companion or to stellar activity. In fact, the existence of a planetary companion has been proposed, as well as contested, around the relatively bright, nearby M3.0 V star AD Leo at the same period as the stellar rotation of 2.23 days.

Aims. We further investigate the nature of this signal. We introduce new CARMENES optical and near-IR RV data and an analysis in combination with archival data taken by HIRES and HARPS, along with more recent data from HARPS-N, GIANO-B, and HPF. Additionally, we address the confusion concerning the binarity of AD Leo.

Methods. We consider possible correlations between the RVs and various stellar activity indicators accessible with CARMENES. We additionally applied models within a Bayesian framework to determine whether a Keplerian model, a red-noise quasi-periodic model using a Gaussian process, or a mixed model would explain the observed data best. We also exclusively focus on spectral lines potentially associated with stellar activity.

Results. The CARMENES RV data agree with the previously reported periodicity of 2.23 days, correlate with some activity indicators, and exhibit chromaticity. However, when considering the entire RV data set, we find that a mixed model composed of a stable and a variable component performs best. Moreover, when recomputing the RVs using only spectral lines insensitive to activity, there appears to be some residual power at the period of interest. We therefore conclude that it is not possible to determinedly prove that there is no planet orbiting in synchronization with the stellar rotation given our data, current tools, machinery, and knowledge of how stellar activity affects RVs. We do rule out planets more massive than 27 M_⊕ (=0.084 M_Jup). Likewise, we exclude any binary companion around AD Leo with M sin i greater than 3–6 M_Jup on orbital periods <14 yr.