The CARMENES search for exoplanets around M dwarfs

Rotational variation in activity indicators of Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac

¹ Instituto de Astrofísica de Andalucia (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: pschoefer.astro@freenet.de
² Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ de Can Magrans s/n, E-08193 Bellaterra, Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capità 2-4, 08034 Barcelona, Spain
⁷ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL UK
⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁹ Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
¹⁰ School of Physics and Astronomy, Queen Mary, University of London, 327 Mile End Road, London, E1 4NS, UK
¹¹ Instituto de Astrofísica de Canarias, Via Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹³ 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
¹⁴ Departamento de Explotación y Prospección de Minas, Escuela de Minas, Energía y Materiales, Universidad de Oviedo, 33003 Oviedo, Asturias, Spain
¹⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁶ Space Research Institute, Austrian Academy of Sciences, Schmiedl-strasse 6, 8042 Graz, Austria
¹⁷ Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁸ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁹ School of Earth & Environmental Sciences, University of St Andrews, Irvine Building, St Andrews, KY16 9AL, UK
²⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²¹ Centro Astronómico Hispano-Alemán (CSIC-MPG), Observatorio Astronómico de Calar Alto, Sierra de los Filabres, E-04550 Gérgal, Almería, Spain
²² Department of Physics, Ariel University, Ariel 40700 Israel

Received: 14 June 2019
Accepted: 7 April 2022

Abstract

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Échelle Spectrographs (CARMENES) instrument is searching for periodic radial-velocity (RV) variations of M dwarfs, which are induced by orbiting planets. However, there are other potential sources of such variations, including rotational modulation caused by stellar activity.

Aims. We aim to investigate four M dwarfs (Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac) with different activity levels and spectral sub-types. Our goal is to compare the periodicities seen in 22 activity indicators and the stellar RVs, and to examine their stability over time.

Methods. For each star, we calculated generalised Lomb-Scargle periodograms of pseudo-equivalent widths of chromospheric lines, indices of photospheric bands, the differential line width as a measure of the width of the average photospheric absorption line, the RV, the chromatic index that describes the wavelength dependence of the RV, and parameters of the cross-correlation function. We also calculated periodograms for subsets of the data and compared our results to TESS photometry.

Results. We find the rotation periods of all four stars to manifest themselves in the RV and photospheric indicators, particularly the TiO 7050 index, whereas the chromospheric lines show clear signals only at lower activity levels. For EV Lac and TYC 3529-1437-1, we find episodes during which indicators vary with the rotation period, and episodes during which they vary with half the rotation period, similarly to photometric light curves.

Conclusions. The changing periodicities reflect the evolution of stellar activity features on the stellar surface. We therefore conclude that our results not only emphasise the importance of carefully analysing indicators complementary to the RV in RV surveys, but they also suggest that it is also useful to search for signals in activity indicators in subsets of the dataset, because an activity signal that is present in the RV may not be visible in the activity indicators all the time, in particular for the most active stars.