The GAPS Programme at TNG

XLII. A characterisation study of the multi-planet system around the 400 Myr-old star HD 63433 (TOI-1726)^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: mario.damasso@inaf.it
² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122, Padova, Italy
⁴ Aix-Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
⁷ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁸ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁹ Department of Physics, University of Rome "Tor Vergata", Via della Ricerca Scientifica 1, 00133, Rome, Italy
¹⁰ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712, Breña Baja, TF, Spain
¹² INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹³ Dipartimento di Fisica – Università di Roma La Sapienza, P.le A. Moro 5, 00185 Roma, Italy
¹⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131, Napoli, Italy
¹⁵ Department of Physics and Astronomy, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
¹⁶ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047, Selargius (CA), Italy
¹⁷ Dipartimento di Fisica e Astronomia "G. Galilei" – Universtà degli Studi di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy

Received: 7 November 2022
Accepted: 31 January 2023

Abstract

Context. The GAPS collaboration is carrying out a spectroscopic and photometric follow-up of a sample of young stars with planets (age ≲600 Myr) to characterise planetary systems at the early stages of their evolution.

Aims. For more than 2 yr, we monitored with the HARPS-N spectrograph the 400 Myr-old star HD 63433, which hosts two close-in (orbital periods P_b ~ 7.1 and P_c ~ 20.5 days) sub-Neptunes detected by the TESS space telescope, and it was announced in 2020. Using radial velocities and additional TESS photometry, we aim to provide the first measurement of their masses, improve the measure of their size and orbital parameters, and study the evolution of the atmospheric mass-loss rate due to photoevaporation.

Methods. We tested state-of-the-art analysis techniques and different models to mitigate the dominant signals due to stellar activity that are detected in the radial velocity time series. We used a hydro-based analytical description of the atmospheric mass-loss rate, coupled with a core-envelope model and stellar evolutionary tracks, to study the past and future evolution of the planetary masses and radii.

Results. We derived new measurements of the planetary orbital periods and radii (P_b = 7.10794 ± 0.000009 days, r_b = 2.02^+0.06_-0.05R_⊕; P_c = 20.54379 ± 0.00002 days, r_c = 2.44 ± 0.07 R_⊕), and determined mass upper limits (m_b ≲11 M_⊕; m_c ≲31 M_⊕; 95% confidence level), with evidence at a 2.1–2.7σ significance level that HD 63433 c might be a dense mini-Neptune with a Neptune-like mass. For a grid of test masses below our derived dynamical upper limits, we found that HD 63433 b has very likely lost any gaseous H-He envelope, supporting HST-based observations that are indicative of there being no ongoing atmospheric will keep evaporating over the next ~5 Gyr if its current mass is m_c ≲15 M_⊕, while it should be hydrodynamically stable for higher masses.