The CARMENES search for exoplanets around M dwarfs

Characterization of the nearby ultra-compact multiplanetary system YZ Ceti^★

¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: sstock@lsw.uni-heidelberg.de
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ School of Physics and Astronomy, Queen Mary University of London, 327 Mile End Road, London, E1 4NS, UK
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/Can Magrans s/n, 08193 Bellaterra, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁹ School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
¹⁰ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Casilla 36-D, Santiago, Chile
¹¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹³ Physikalisches Institut, Universitaet Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁴ 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
¹⁵ Department of Exploitation and Exploration of Mines, University of Oviedo, Oviedo, Spain
¹⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁷ Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁸ Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, Santiago, Chile
¹⁹ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
²⁰ Department of Earth Sciences and Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
²¹ Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain

Received: 18 September 2019
Accepted: 20 January 2020

Abstract

Context. The nearby ultra-compact multiplanetary system YZ Ceti consists of at least three planets, and a fourth tentative signal. The orbital period of each planet is the subject of discussion in the literature due to strong aliasing in the radial velocity data. The stellar activity of this M dwarf also hampers significantly the derivation of the planetary parameters.

Aims. With an additional 229 radial velocity measurements obtained since the discovery publication, we reanalyze the YZ Ceti system and resolve the alias issues.

Methods. We use model comparison in the framework of Bayesian statistics and periodogram simulations based on a method by Dawson and Fabrycky to resolve the aliases. We discuss additional signals in the RV data, and derive the planetary parameters by simultaneously modeling the stellar activity with a Gaussian process regression model. To constrain the planetary parameters further we apply a stability analysis on our ensemble of Keplerian fits.

Results. We find no evidence for a fourth possible companion. We resolve the aliases: the three planets orbit the star with periods of 2.02 d, 3.06 d, and 4.66 d. We also investigate an effect of the stellar rotational signal on the derivation of the planetary parameters, in particular the eccentricity of the innermost planet. Using photometry we determine the stellar rotational period to be close to 68 d and we also detect this signal in the residuals of a three-planet fit to the RV data and the spectral activity indicators. From our stability analysis we derive a lower limit on the inclination of the system with the assumption of coplanar orbits which is i_min = 0.9 deg. From the absence of a transit event with TESS, we derive an upper limit of the inclination of i_max = 87.43 deg.

Conclusions. YZ Ceti is a prime example of a system where strong aliasing hindered the determination of the orbital periods of exoplanets. Additionally, stellar activity influences the derivation of planetary parameters and modeling them correctly is important for the reliable estimation of the orbital parameters in this specific compact system. Stability considerations then allow additional constraints to be placed on the planetary parameters.