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

A. Sozzetti, M. Damasso, A. S. Bonomo, Y. Alibert, S. G. Sousa, V. Zh. Adibekyan, M. R. Zapatero Osorio, J. I. González Hernández, S. C. C. Barros, J. Lillo-Box, K. G. Stassun, J. N. Winn, S. Cristiani, F. Pepe, R. Rebolo López, N. C. Santos, R. Allart, T. Barclay, F. Bouchy, A. Cabral, D. Ciardi, P. Di Marcantonio, V. D'Odorico, D. Ehrenreich, M. Fasnaugh, P. Figueira, J. Haldemann, J. M. Jenkins, D. W. Latham, B. Lavie, G. Lo Curto, C. Lovis, C. J. A. P. Martins, D. Mégevand, A. Mehner, G. Micela, P. Molaro, N. J. Nunes, M. Oshagh, J. F. Otegi, E. Pallé, E. Poretti, G. R. Ricker, D. R. Rodriguez, S. Seager, A. Suárez Mascareño, J. D. Twicken, S. Udry

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 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 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.

planetary systems; planets and satellites: composition; stars: individual: HD 5278 (TOI-130); techniques: radial velocities; techniques: photometric; methods: miscellaneous; Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Solar and Stellar

Based on Guaranteed Time Observations collected at the European Southern Observatory under ESO programme 1102.C-0744 by the ESPRESSO Consortium.

Astronomy & Astrophysics
Volume 648, Article Number A75, Number of pages 25
2021 April

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia