Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities

September 2018 • 2018AJ....156...89P

Authors • Petigura, Erik A. • Benneke, Björn • Batygin, Konstantin • Fulton, Benjamin J. • Werner, Michael • Krick, Jessica E. • Gorjian, Varoujan • Sinukoff, Evan • Deck, Katherine M. • Mills, Sean M. • Deming, Drake

Abstract • While planets between the size of Uranus and Saturn are absent within the solar system, the star K2-24 hosts two such planets, K2-24b and c, with radii equal to 5.4 {R}_\oplus and 7.5 {R}_\oplus , respectively. The two planets have orbital periods of 20.9 days and 42.4 days, residing only 1% outside the nominal 2:1 mean-motion resonance. In this work, we present results from a coordinated observing campaign to measure planet masses and eccentricities that combines radial velocity measurements from Keck/HIRES and transit-timing measurements from K2 and Spitzer. K2-24b and c have low, but nonzero, eccentricities of {e}₁∼ {e}₂∼ 0.08. The low observed eccentricities provide clues to the formation and dynamical evolution of K2-24b and K2-24c, suggesting that they could be the result of stochastic gravitational interactions with a turbulent protoplanetary disk, among other mechanisms. K2-24b and c are {19.0}_-2.1^+2.2 {M}_\oplus and {15.4}_-1.8^+1.9 {M}_\oplus , respectively; K2-24c is 20% less massive than K2-24b, despite being 40% larger. Their large sizes and low masses imply large envelope fractions, which we estimate at {26}_-3⁺³% and {52}_-3⁺⁵%. In particular, K2-24c’s large envelope presents an intriguing challenge to the standard model of core-nucleated accretion that predicts the onset of runaway accretion when {f}_env} ≈ 50%.

Links

• PREPRINT http://arxiv.org/abs/1806.08959
• ELECTR https://doi.org/10.3847%2F1538-3881%2Faaceac
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2018AJ....156...89P
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• DATA http://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/156/89
• DATA http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?type=REFERENCE&bibcode=2018AJ....156...89P
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2018AJ....156...89P
• DATA https://irsa.ipac.caltech.edu/bibdata/2018/P/2018AJ....156...89P.html