K2-111: an old system with two planets in near-resonance
Bouchy, F.; Rebolo, R.; Zapatero Osorio, M. R.; Allende Prieto, C.; Palle, E.; Vanderburg, A.; Poretti, E.; Cristiani, S.; Sozzetti, A.; Maldonado, J.; Mehner, A.; Piotto, G.; Collier Cameron, A.; Adibekyan, V.; Alibert, Y.; Sousa, S. G.; Wilson, T. G.; Barros, S. C. C.; Demangeon, O. D. S.; Ehrenreich, D.; Lovis, C.; Santos, N. C.; Udry, S.; Lillo-Box, J.; Micela, G.; Oshagh, M.; Cabral, A.; D'Odorico, V.; Di Marcantonio, P.; Figueira, P.; González Hernández, J. I.; Lo Curto, G.; Manescau, A.; Martins, C. J. A. P.; Mégevand, D.; Molaro, P.; Nunes, N. J.; Riva, M.; Suárez Mascareño, A.; Mayor, M.; Bonomo, A. S.; Affer, L.; Benatti, S.; Boschin, W.; Latham, D. W.; Malavolta, L.; Pinamonti, M.; Mortier, A.; Haywood, R. D.; Dumusque, X.; Rice, K.; Cosentino, R.; Ghedina, A.; Harutyunyan, A.; Molinari, E.; Pepe, F. A.; Sasselov, D.; West, R. G.; Lavie, B.; Charbonneau, D.; Watson, C. A.; Phillips, D.; Haldemann, J.; Delgado Mena, E.; Fiorenzano, A.; Lopez-Morales, M.; Buchhave, L.
United Kingdom, Spain, Italy, Switzerland, United States, Denmark, Portugal, Chile, Germany
Abstract
This paper reports on the detailed characterization 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 d, 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.