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 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 Pb = 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 Teff = 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 Mb = 7.8−1.4+1.5 M⊕ and Rb = 2.45 ± 0.05R⊕. The derived mean density, ϱb = 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 Pc = 40.87−0.17+0.18 days and a minimum mass of Mc sin ic = 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 Teff ≳ 5500 K.

The TESS-Keck Survey. XI. Mass Measurements for Four Transiting Sub-Neptunes Orbiting K Dwarf TOI–1246

“…TOI-1246 c (8.8 ± 1.2 M ⊕ , 2.47 ± 0.08 R ⊕ ) is consistent in mass and radius with HD 5278 b, K2-138 d (Christiansen et al 2018), HD 15337c (Gandolfi et al 2019), andK2-38 c (Sinukoff et al 2016). HD 5278 b (Sozzetti et al 2021), which has a insolation flux similar to that of TOI-1246c (S c = 129 S ⊕ , S HD 5278 b = 132 S ⊕ ), highlights how little can be inferred from a planet's bulk density. All of these planets lie near the 100% H 2 O composition line, and yet their masses and radii could be explained by a diversity of compositions.…”

“…All of these planets lie near the 100% H 2 O composition line, and yet their masses and radii could be explained by a diversity of compositions. Sozzetti et al (2021) suggest two options: a "wet" differentiated planet with a substantial water layer and a H/He gas envelope, and a "dry" planet with an iron core and silicate mantle. While Sozzetti et al (2021) find that sub-Neptunes are expected to retain their volatile contents, the bulk density of TOI-1246c is consistent with a variety of compositions.…”

“…Sozzetti et al (2021) suggest two options: a "wet" differentiated planet with a substantial water layer and a H/He gas envelope, and a "dry" planet with an iron core and silicate mantle. While Sozzetti et al (2021) find that sub-Neptunes are expected to retain their volatile contents, the bulk density of TOI-1246c is consistent with a variety of compositions. Atmospheric observations, such as transmission spectroscopy (see Section 5.3), are critical to distinguish between degenerate planet composition models (Rogers & Seager 2010).…”

Revisiting WASP-47 with ESPRESSO and TESS

“…In this work we use the next generation of high precision spectrographs, ESPRESSO, in order to obtain the most precise and accurate measurements of the planet masses and densities, particularly focusing on the super-Earth WASP-47 e. (Sozzetti et al 2021), and the planets in the TOI-178 system (Leleu et al 2021). ESPRESSO radial velocity measurements have also been used to improve upon the precision of the masses of LHS-1140 b andc (Lillo-Box et al 2020).…”

New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State

“…Examples of such recent discoveries from TESS include the TOI-561 system (Lacedelli et al 2021;Weiss et al 2020), TOI-130 system (Sozzetti et al 2021), and TOI-178 system (Leleu et al 2021).…”