Dynamics and Formation of the Near-resonant K2-24 System: Insights from Transit-timing Variations and Radial Velocities
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%.
- Publication:
-
The Astronomical Journal
- Pub Date:
- September 2018
- DOI:
- 10.3847/1538-3881/aaceac
- arXiv:
- arXiv:1806.08959
- Bibcode:
- 2018AJ....156...89P
- Keywords:
-
- planets and satellites: dynamical evolution and stability;
- planets and satellites: formation;
- planets and satellites: individual: K2-24b;
- K2-24c;
- techniques: radial velocities;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 14 pages, 9 figures, 2 tables, accepted to AJ