K2-111: an old system with two planets in near-resonance†

Abstract

This paper reports on the detailed characterisation of the K2-111 planetary system with K2, WASP, and ASAS-SN photometry as well as high-resolution spectroscopic data from HARPS-N and ESPRESSO. The host, K2-111, is confirmed to be a mildly evolved (log g = 4.17), iron-poor ([Fe/H]=−0.46), but alpha-enhanced ([α/Fe]=0.27), chromospherically quiet, very old thick disc G2 star. A global fit, performed by using PyORBIT shows that the transiting planet, K2-111 b, orbits with a period Pb = 5.3518 ± 0.0004 d, and has a planet radius of $1.82^{+0.11}_{-0.09}$ R⊕ and a mass of $5.29^{+0.76}_{-0.77}$ M⊕, resulting in a bulk density slightly lower than that of the Earth. The stellar chemical composition and the planet properties are consistent with K2-111 b being a terrestrial planet with an iron core mass fraction lower than the Earth. We announce the existence of a second signal in the radial velocity data that we attribute to a non-transiting planet, K2-111 c, with an orbital period of 15.6785 ± 0.0064 days, orbiting in near-3:1 mean-motion resonance with the transiting planet, and a minimum planet mass of 11.3 ± 1.1 M⊕. Both planet signals are independently detected in the HARPS-N and ESPRESSO data when fitted separately. There are potentially more planets in this resonant system, but more well-sampled data are required to confirm their presence and physical parameters.

Transit detection of the long-period volatile-rich super-Earth ν2 Lupi d with CHEOPS

“…We assumed the planet to be made of four different layers: a central core made of iron and sulfur, a silicate mantle (containing Si, Mg, and Fe), a water layer, and a gas layer made of pure H and He. Compared to previous similar models 23,24 , the physical models used here have been improved 112 , namely with a new equation of state (EOS) for the water layer 113 , and the EOS for the iron core 114 (which can also contain some sulfur). The EOS for the silicate mantle 115 depends on the mole fractions of Si, Mg, and Fe, and the gas envelope model 116 gives the thickness of the gas envelope as a function of age, planetary mass, etc.…”

Composition of super-Earths, super-Mercuries, and their host stars

“…For the derivation of chemical abundances we closely followed the methods described in Adibekyan et al (2012Adibekyan et al ( , 2015b. The stellar parameters and abundances of Mg, Si, and Fe of HD 137496 are taken from Silva et al ( 2021) and for K2-111 are taken from Mortier et al (2020) 2 . Based on the abundances of Mg, Si, and Fe, and using the stoichiometric models of Santos et al (2015) we estimated the iron-to-silicate mass fraction (f star iron ) of planetary building blocks under assumption that the stellar atmospheric composition reflects the composition of the proto-stellar (protoplanetary) disk where the star and the planets are formed.…”

“…Several attempts have been made in the last years trying to link the composition of low-mass planets and their host stars. However, these attempts were either based on single planetary systems (Lillo-Box et al, 2020, Mortier et al, 2020, on a small sample of planets (Plotnykov & Valencia, 2020, Santos et al, 2015, Schulze et al, 2021, or on a comparison of the overall properties of planets and overall properties of planet host stars in a population sense (Plotnykov & Valencia, 2020). As a result, it was not possible to reach a firm conclusion either because of low-number statistics or because the results were not as informative (especially if the composition of the stars are not derived in a homogeneous way) as they would be if a direct star-planet comparison was performed.…”

A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals

“…Our final adopted stellar atmospheric parameters, which appear in Table 3, are inverse variance-weighted averages of the results from these three methods, following a methodology first used by Malavolta et al (2018) that has since been well tested and widely adopted (e.g. Rice et al 2019;Mortier et al 2020). We obtained stellar mass, radius, age, and distance estimates from isochrones and evolutionary tracks.…”

“…We used the isochrones package (Morton 2015) and two separate stellar evolution models, viz. Dartmouth (Dotter et al 2008) and MIST (MESA Isochrones and Stellar Tracks;Dotter 2016) to estimate these parameters, following the methodology described in Mortier et al (2020). As inputs, we used our spectroscopically determined effective temperature and iron abundance, the Gaia EDR3 parallax, and magnitudes from the visible to mid-infrared (as listed in Table 1).…”