Astronomy and Astrophysics
Aims: We took advantage of the extreme precision of the new-generation echelle spectrograph ESPRESSO to characterize the transiting planetary system orbiting the G2V star K2-38 located at 194 pc from the Sun with V ~ 11.4. This system is particularly interesting because it could contain the densest planet detected to date.
Methods: 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, K2-38b and K2-38c, with Pb = 4.01593 ± 0.00050 d and Pc = 10.56103 ± 0.00090 d, respectively. Using 43 ESPRESSO high-precision RV 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 Markov chain Monte Carlo analysis, significantly improving their mass measurements.
Results: Using ESPRESSO spectra, we derived the stellar parameters, Teff = 5731 ± 66, log g = 4.38 ± 0.11 dex, and [Fe/H] = 0.26 ± 0.05 dex, and thus the mass and radius of K2-38, M⋆ = 1.03-0.02+0.04 M⊕ and R⋆ = 1.06-0.06+0.09 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 = 7.3-1.0+1.1 M⊕, and K2-38c as a sub-Neptune with RP = 2.29 ± 0.26 R⊕ and Mp = 8.3-1.3+1.3 M⊕. Combining the radius and mass measurements, we derived a mean density of ρp = 11.0-2.8+4.1 g cm-3 for K2-38b and ρp = 3.8-1.1+1.8 g cm-3 for K2-38c, confirming K2-38b as one of the densest planets known to date.
Conclusions: 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 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 RV time-series whose origin could be linked to a 0.25-3 MJ planet or stellar activity. The ESPRESSO RVs used in this paper are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/641/A92 Based (in part) on Guaranteed Time Observations collected at the European Southern Observatory under ESO programmes1102.C-0744, 112.C-0958, and 1104.C-0350 by the ESPRESSO Consortium.