Detailed stellar activity analysis and modelling of GJ 832

Reassessment of the putative habitable zone planet GJ 832c^★

¹ Universidad de Conceptión, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
e-mail: paula.gorrini@uni-goettingen.de
² Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera, 2850 Concepción, Chile
⁴ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ International Center for Advanced Studies (ICAS) and ICIFI (CON-ICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, (1650) Buenos Aires, Argentina
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Max Planck Institute for Solar System Research, Justus-von-Liebig-weg 3, 37077 Göttingen, Germany
⁸ School of Physical Sciences, The Open University, Walton Hall, MK7 6AA, Milton Keynes, UK
⁹ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra, Spain
¹⁰ INAF–Osservatorio Astrofisico di Catania, via Santa Sofia, 78 Catania, Italy
¹¹ Institut de Recherche sur les Exoplanètes, Université de Montréal, Département de Physique, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7, Canada
¹² Observatoire du Mont-Mégantic, Université de Montréal, Montréal, QC H3C 3J7, Canada
¹³ Observatoire de Genève, Université de Genève, chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ European Southern Observatory, Vitacura, Santiago, Chile
¹⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁷ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008, Granada, Spain
¹⁸ Portuguese Space Agency, Estrada das Laranjeiras, n.° 205, RC, 1649-018 Lisboa, Portugal
¹⁹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
²⁰ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²¹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²² Department of Physics, Ariel University, Ariel 40700 Israel
²³ Astrophysics Geophysics And Space Science Research Center, Ariel University, Ariel 40700, Israel
²⁴ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany

Received: 7 January 2022
Accepted: 25 May 2022

Abstract

Context. Gliese-832 (GJ 832) is an M2V star hosting a massive planet on a decade-long orbit, GJ 832b, discovered by radial velocity (RV). Later, a super Earth or mini-Neptune orbiting within the stellar habitable zone was reported (GJ 832c). The recently determined stellar rotation period (45.7 ± 9.3 days) is close to the orbital period of putative planet c (35.68 ± 0.03 days).

Aims. We aim to confirm or dismiss the planetary nature of the RV signature attributed to GJ 832c, by adding 119 new RV data points, new photometric data, and an analysis of the spectroscopic stellar activity indicators. Additionally, we update the orbital parameters of the planetary system and search for additional signals.

Methods. We performed a frequency content analysis of the RVs to search for periodic and stable signals. Radial velocity time series were modelled with Keplerians and Gaussian process (GP) regressions alongside activity indicators to subsequently compare them within a Bayesian framework.

Results. We updated the stellar rotational period of GJ 832 from activity indicators, obtaining 37.5^+1.4_-1.5 days, improving the precision by a factor of 6. The new photometric data are in agreement with this value. We detected an RV signal near 18 days (FAP < 4.6%), which is half of the stellar rotation period. Two Keplerians alone fail at modelling GJ 832b and a second planet with a 35-day orbital period. Moreover, the Bayesian evidence from the GP analysis of the RV data with simultaneous activity indices prefers a model without a second Keplerian, therefore negating the existence of planet c.