Characterization of the K2-38 planetary system. Unraveling one of the densest planets known to date
Creators: name: Toledo-Padrón, B.
name: Lovis, C.
name: Mascareño, A. Suárez
name: Barros, S. C. C.
name: Hernández, J. I. González
name: Sozzetti, A.
name: Bouchy, F.
name: Osorio, M. R. Zapatero
name: Rebolo, R.
name: Cristiani, S.
name: Pepe, F. A.
name: Santos, N. C.
name: Sousa, S. G.
name: Tabernero, H. M.
name: Lillo-Box, J.
name: Bossini, D.
name: Adibekyan, V.
name: Allart, R.
name: Damasso, M.
name: D'Odorico, V.
name: Figueira, P.
name: Lavie, B.
name: Curto, G. Lo
name: Mehner, A.
name: Micela, G.
name: Modigliani, A.
name: Nunes, N. J.
name: Pallé, E.
name: Abreu, M.
name: Affolter, M.
name: Alibert, Y.
name: Aliverti, M.
name: Prieto, C. Allende
name: Alves, D.
name: Amate, M.
name: Avila, G.
name: Baldini, V.
name: Bandy, T.
name: Benatti, S.
name: Benz, W.
name: Bianco, A.
name: Broeg, C.
name: Cabral, A.
name: Calderone, G.
name: Cirami, R.
name: Coelho, J.
name: Conconi, P.
name: Coretti, I.
name: Cumani, C.
name: Cupani, G.
name: Deiries, S.
name: Dekker, H.
name: Delabre, B.
name: Demangeon, O.
name: Di Marcantonio, P.
name: Ehrenreich, D.
name: Fragoso, A.
name: Genolet, L.
name: Genoni, M.
name: Santos, R. Génova
name: Hughes, I.
name: Iwert, O.
name: Knudstrup, J.
name: Landoni, M.
name: Lizon, J. L.
name: Maire, C.
name: Manescau, A.
name: Martins, C. J. A. P.
name: Mégevand, D.
name: Molaro, P.
name: Monteiro, M. J. P. F. G.
name: Monteiro, M. A.
name: Moschetti, M.
name: Mueller, E.
name: Oggioni, L.
name: Oliveira, A.
name: Oshagh, M.
name: Pariani, G.
name: Pasquini, L.
name: Poretti, E.
name: Rasilla, J. L.
name: Redaelli, E.
name: Riva, M.
name: Tschudi, S. Santana
name: Santin, P.
name: Santos, P.
name: Segovia, A.
name: Sosnowska, D.
name: Spanò, P.
name: Tenegi, F.
name: Udry, S.
name: Zanutta, A.
name: Zerbi, F.
Publisher: name: arXiv
Year: 2020
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $ρ_{\rm p}$=11.0$^{+4.1}_{-2.8}$~g cm$^{-3}$ for K2-38b and $ρ_{\rm p}$=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.
description: 16 pages, 14 figures