Characterization of the K2-38 planetary system. Unraveling one of the densest planets known to date

Abstract

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, $T_{\mathrm{eff}}$=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 ${}_{-0.02}^{+0.04}$~M${}_{\oplus }$ and $R_{\star }$=1.06 ${}_{-0.06}^{+0.09}$~R${}_{\oplus }$. We determine new values for the planetary properties of both planets. We characterize K2-38b as a super-Earth with $R_{\mathrm{P}}$=1.54$\pm$0.14~R${}_{\oplus }$ and $M_{\mathrm{p}}$=7.3${}_{-1.0}^{+1.1}$~M${}_{\oplus }$, and K2-38c as a sub-Neptune with $R_{\mathrm{P}}$=2.29$\pm$0.26~R${}_{\oplus }$ and $M_{\mathrm{p}}$=8.3${}_{-1.3}^{+1.3}$~M${}_{\oplus }$. We derived a mean density of ${\rho }_{\mathrm{p}}$=11.0${}_{-2.8}^{+4.1}$~g cm${}^{-3}$ for K2-38b and ${\rho }_{\mathrm{p}}$=3.8${}_{-1.1}^{+1.8}$~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${}_{\mathrm{J}}$ planet or stellar activity.

16 pages, 14 figures

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