Star-spot activity, orbital obliquity, transmission spectrum, physical properties, and transit time variations of the HATS-2 planetary system

¹ Department of Physics, University of Rome “La Sapienza”, Piazzale Aldo Moro 2, 00185 Rome, Italy
e-mail: francesco.biagiotti@uniroma1.it
² INAF – Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), Via Fosso del Cavaliere 100, 00133 Rome, Italy
³ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ INAF – Turin Astrophysical Observatory, via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Astrophysics Group, Keele University, Keele ST5 5BG, UK
⁷ Instituto de Astronomia y Ciencias Planetarias de Atacama, Universidad de Atacama, Copayapu 485, Copiapo, Chile
⁸ Department of Physics and Astronomy, University of Florence, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁹ Centre for ExoLife Sciences, Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5, 1350 Copenhagen, Denmark
¹⁰ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
¹¹ Department of Physics “E.R. Caianiello”, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano, Italy
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy
¹³ Universität Hamburg, Department of Earth Sciences, Meteorological Institute, Bundesstrasse 55, 20146 Hamburg, Germany
¹⁴ Centre for Exoplanet Science, SUPA, School of Phys. & Astron., University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁵ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
¹⁶ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁷ University of Southern Denmark, Department of Physics, Chemistry and Pharmacy, Campusvej 55, 5230 Odense M, Denmark
¹⁸ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg (ZAH), 69120 Heidelberg, Germany
¹⁹ Astronomy Research Center, Research Institute of Basic Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea
²⁰ Centro de Astronomía, Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
²¹ Departamento de Matemática y Física Aplicadas, Facultad de Ingeniería, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
²² Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran
²³ Centre for Electronic Imaging, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
²⁴ Observatoire de Vaison-La-Romaine, Départementale 51, près du Centre Equestre au Palis, 84110 Vaison-La-Romaine, France
²⁵ KNC Deep Sky Chile Observatory, Chile

Received: 4 September 2023
Accepted: 12 January 2024

Abstract

Aims. Our aim in this paper is to refine the orbital and physical parameters of the HATS-2 planetary system and study transit timing variations and atmospheric composition thanks to transit observations that span more than 10 yr and that were collected using different instruments and pass-band filters. We also investigate the orbital alignment of the system by studying the anomalies in the transit light curves induced by starspots on the photosphere of the parent star.

Methods. We analysed new transit events from both ground-based telescopes and NASA’s TESS mission. Anomalies were detected in most of the light curves and modelled as starspots occulted by the planet during transit events. We fitted the clean and symmetric light curves with the JKTEBOP code and those affected by anomalies with the PRISM+GEMC codes to simultaneously model the photometric parameters of the transits and the position, size, and contrast of each starspot.

Results. We found consistency between the values we found for the physical and orbital parameters and those from the discovery paper and ATLAS9 stellar atmospherical models. We identified different sets of consecutive starspot-crossing events that temporally occurred in less than five days. Under the hypothesis that we are dealing with the same starspots, occulted twice by the planet during two consecutive transits, we estimated the rotational period of the parent star and, in turn the projected and the true orbital obliquity of the planet. We find that the system is well aligned. We identified the possible presence of transit timing variations in the system, which can be caused by tidal orbital decay, and we derived a low-resolution transmission spectrum.