Characterization of the K2-38 planetary system

Toledo-Padrón, B.; Lovis, C.; Suárez Mascareño, A.; Barros, S. C. C.; González Hernández, J. I.; Sozzetti, A.; Bouchy, F.; Zapatero Osorio, M. R.; Rebolo, R.; Cristiani, S.; Pepe, F. A.; Santos, N. C.; Sousa, S. G.; Tabernero, H. M.; Lillo-Box, J.; Bossini, D.; Adibekyan, V.; Allart, R.; Damasso, M.; D’Odorico, V.; ... (2020). Characterization of the K2-38 planetary system. Astronomy and astrophysics, 641, A92. EDP Sciences 10.1051/0004-6361/202038187

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We characterized the transiting planetary system orbiting the G2V star K2-38 using the new-generation echelle spectrograph ESPRESSO. We carried out a photometric analysis of the available K2 photometric light curve of this star to measure the radius of its two known planets. Using 43 ESPRESSO high-precision radial velocity measurements taken over the course of 8 months along with the 14 previously published HIRES RV measurements, we modeled the orbits of the two planets through a MCMC analysis, significantly improving their mass measurements. Using ESPRESSO spectra, we derived the stellar parameters, Teff=5731±66, logg=4.38±0.11~dex, and [Fe/H]=0.26±0.05~dex, and thus the mass and radius of K2-38, M⋆=1.03 +0.04−0.02~M⊕ and R⋆=1.06 +0.09−0.06~R⊕. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with RP=1.54±0.14~R⊕ and Mp=7.3+1.1−1.0~M⊕, and K2-38c as a sub-Neptune with RP=2.29±0.26~R⊕ and Mp=8.3+1.3−1.3~M⊕. We derived a mean density of ρp=11.0+4.1−2.8~g cm−3 for K2-38b and ρp=3.8+1.8−1.1~g~cm−3 for K2-38c, confirming K2-38b as one of the densest planets known to date. The best description for the composition of K2-38b comes from an iron-rich Mercury-like model, while K2-38c is better described by a rocky model with a H2 envelope. The maximum collision stripping boundary shows how giant impacts could be the cause for the high density of K2-38b. The irradiation received by each planet places them on opposite sides of the radius valley. We find evidence of a long-period signal in the radial velocity time-series whose origin could be linked to a 0.25-3~MJ planet or stellar activity.

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