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, $T_{ \rm eff}$=5731$\pm$66, $\log g$=4.38$\pm$0.11~dex, and $[Fe/H]$=0.26$\pm$0.05~dex, and thus the mass and radius of K2-38, $M_{ \star}$=1.03 $^{ +0.04}_{ -0.02}$~M$_{ \oplus}$ and $R_{ \star}$=1.06 $^{ +0.09}_{ -0.06}$~R$_{ \oplus}$. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with $R_{ \rm P}$=1.54$\pm$0.14~R$_{ \rm \oplus}$ and $M_{ \rm p}$=7.3$^{ +1.1}_{ -1.0}$~M$_{ \oplus}$, and K2-38c as a sub-Neptune with $R_{ \rm P}$=2.29$\pm$0.26~R$_{ \rm \oplus}$ and $M_{ \rm p}$=8.3$^{ +1.3}_{ -1.3}$~M$_{ \oplus}$. We derived a mean density of $\rho_{ \rm p}$=11.0$^{ +4.1}_{ -2.8}$~g cm$^{ -3}$ for K2-38b and $\rho_{ \rm p}$=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~M$_{ \rm J}$ planet or stellar activity.
Categories: astro-ph.EP astro-ph.SR