A sub-Neptune and a non-transiting Neptune-mass companion unveiled by ESPRESSO around the bright late-F dwarf HD 5278 (TOI-130)^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: alessandro.sozzetti@inaf.it
² Physics Institute of University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁵ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Instituto de Astrofisica de Canarias, Via Lactea, 38200 La Laguna, Tenerife, Spain
⁷ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ Centro de Astrobiología (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Lane, Nashville, TN 37235, USA
¹⁰ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹¹ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹² Institute for Fundamental Physics of the Universe, IFPU, Via Beirut 2, 34151 Grignano, Trieste, Italy
¹³ Département d’astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
¹⁵ Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, Montréal, H3T 1J4, Canada
¹⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁷ University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
¹⁸ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, PT1749-016 Lisboa, Portugal
¹⁹ Departamento de Física da Faculdade de Ciências da Univeridade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
²⁰ Caltech/IPAC, 1200 E. California Blvd., Pasadena, CA 91125, USA
²¹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²² ESO, European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago
²³ NASA Ames Research Center, Moffett Field, CA 94035, USA
²⁴ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²⁵ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²⁶ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²⁷ Institute for Computational Science, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
²⁸ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
²⁹ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³¹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³³ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³⁴ SETI Institute, Mountain View, CA 94043, USA

Received: 30 November 2020
Accepted: 27 January 2021

Abstract

Context. Transiting sub-Neptune-type planets, with radii approximately between 2 and 4 R_⊕, are of particular interest as their study allows us to gain insight into the formation and evolution of a class of planets that are not found in our Solar System.

Aims. We exploit the extreme radial velocity (RV) precision of the ultra-stable echelle spectrograph ESPRESSO on the VLT to unveil the physical properties of the transiting sub-Neptune TOI-130 b, uncovered by the TESS mission orbiting the nearby, bright, late F-type star HD 5278 (TOI-130) with a period of P_b = 14.3 days.

Methods. We used 43 ESPRESSO high-resolution spectra and broad-band photometry information to derive accurate stellar atmospheric and physical parameters of HD 5278. We exploited the TESS light curve and spectroscopic diagnostics to gauge the impact of stellar activity on the ESPRESSO RVs. We performed separate as well as joint analyses of the TESS photometry and the ESPRESSO RVs using fully Bayesian frameworks to determine the system parameters.

Results. Based on the ESPRESSO spectra, the updated stellar parameters of HD 5278 are T_eff = 6203 ± 64 K, log g = 4.50 ± 0.11 dex, [Fe/H] = −0.12 ± 0.04 dex, M_⋆ = 1.126_−0.035^+0.036 M_⊙, and R_⋆ = 1.194_−0.016^+0.017 R_⊙. We determine HD 5278 b’s mass and radius to be M_b = 7.8_−1.4^+1.5 M_⊕ and R_b = 2.45 ± 0.05R_⊕. The derived mean density, ϱ_b = 2.9_−0.5^+0.6 g cm⁻³, is consistent with the bulk composition of a sub-Neptune with a substantial (~ 30%) water mass fraction and with a gas envelope comprising ~17% of the measured radius. Given the host brightness and irradiation levels, HD 5278 b is one of the best targetsorbiting G-F primaries for follow-up atmospheric characterization measurements with HST and JWST. We discover a second, non-transiting companion in the system, with a period of P_c = 40.87_−0.17^+0.18 days and a minimum mass of M_c sin i_c = 18.4_−1.9^+1.8 M_⊕. We study emerging trends in parameters space (e.g., mass, radius, stellar insolation, and mean density) of the growing population of transiting sub-Neptunes, and provide statistical evidence for a low occurrence of close-in, 10 − 15M_⊕ companions around G-F primaries with T_eff ≳ 5500 K.