A low-mass sub-Neptune planet transiting the bright active star HD 73344^★

¹ Université Aix Marseille, CNRS, CNES, LAM, Marseille, France
e-mail: sophia.sulis@lam.fr
² Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
³ IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université de Lille, 75014 Paris, France
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁶ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA
⁷ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁰ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹¹ Institut d’astrophysique de Paris, UMR 7095 CNRS Université Pierre et Marie curie, 98 bis, Bd Arago, 75014 Paris, France
¹² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁴ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁵ Astrophysics Group, University of Exeter, Exeter EX4 2QL, UK
¹⁶ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁷ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁸ Department of Physics & Astronomy, Tufts University, 574 Boston Avenue, Medford, MA 02155, USA
¹⁹ Institut Universitaire de France (IUF), France
²⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
²¹ Geneva Observatory, University of Geneva, chemin des Maillettes 51, 1290 Versoix, Switzerland
²² Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France
²³ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received: 9 February 2024
Accepted: 6 May 2024

Abstract

Context. Planets with radii of between 2 and 4 R_⊕ closely orbiting solar-type stars are of significant importance for studying the transition from rocky to giant planets, and are prime targets for atmospheric characterization by missions such as JWST and ARIEL. Unfortunately, only a handful of examples with precise mass measurements are known to orbit bright stars.

Aims. Our goal is to determine the mass of a transiting planet around the very bright F6 star HD 73344 (Vmag = 6.9). This star exhibits high activity and has a rotation period that is close to the orbital period of the planet (P_b = 15.6 days).

Methods. The transiting planet, initially a K2 candidate, is confirmed through TESS observations (TOI 5140.01). We refined its parameters using TESS data and rule out a false positive with Spitzer observations. We analyzed high-precision radial velocity (RV) data from the SOPHIE and HIRES spectrographs. We conducted separate and joint analyses of K2, TESS, SOPHIE, and HIRES data using the PASTIS software. Given the star’s early type and high activity, we used a novel observing strategy, targeting the star at high cadence for two consecutive nights with SOPHIE to understand the short-term stellar variability. We modeled stellar noise with two Gaussian processes: one for rotationally modulated stellar processes, and one for short-term stellar variability.

Results. High-cadence RV observations provide better constraints on stellar variability and precise orbital parameters for the transiting planet: a radius of R_b = 2.88_−0.07^+0.08 R_⊕ and a mass of M_b = 2.98_−1.90^+2.50 M_⊕ (upper-limit at 3σ is <10.48 M_⊕). The derived mean density suggests a sub-Neptune-type composition, but uncertainties in the planet’s mass prevent a detailed characterization. In addition, we find a periodic signal in the RV data that we attribute to the signature of a nontransiting exoplanet, without totally excluding the possibility of a nonplanetary origin. This planetary candidate would have a minimum mass of about M_c sin i_c = 116.3 ± _−13.0^+12.8 M_⊕ and a period of P_c = 66.45_−0.25^+0.10 days. Dynamical analyses confirm the stability of the two-planet system and provide constraints on the inclination of the candidate planet; these findings favor a near-coplanar system.

Conclusions. While the transiting planet orbits the bright star at a short period, stellar activity prevented us from precise mass measurements despite intensive RV follow-up. Long-term RV tracking of this planet could improve this measurement, as well as our understanding of the activity of the host star. The latter will be essential if we are to characterize the atmosphere of planets around F-type stars using transmission spectroscopy.