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

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 on the formation and evolution of a class of planets 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 P b = 14.3 days. Methods. We use 43 ESPRESSO high-resolution spectra and broad-band photometry information to derive accurate stellar atmospheric and physical parameters of HD 5278. We exploit the TESS light curve and spectroscopic diagnostics to gauge the impact of stellar activity on the ESPRESSO RVs. We perform 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 +0.036 −0.035 M and R = 1.194 +0.017 −0.016 R . We determine HD 5278 b's mass and radius to be M b = 7.8 +1.5 −1.4 M ⊕ and R b = 2.45 ± 0.05 R ⊕ . The derived mean density, b = 2.9 +0.6 −0.5 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 targets orbiting 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 P c = 40.87 +0.18 −0.17 days and a minimum mass M c sin i c = 18.4 +1.8 −1.9 M ⊕ . We study emerging trends in parameters space (mass, radius, stellar insolation, mean density) of the growing population of transiting sub-Neptunes, and provide statistical evidence for a low occurrence of close-in, 10 − 15 M ⊕ companions around G-F primaries with T eff 5500 K.