Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the Formation of Warm Jupiters

Abstract

I report the discovery of non-transiting close companions to two transiting warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period $P=17.9\,\mathrm{days}$, radius $R_{\rm p}=1.23^{+0.06}_{-0.05}\,R_{\rm Jup}$) and Kepler-693/KOI-824b ($P=15.4\,\mathrm{days}$, $R_{\rm p}=0.91\pm0.05\,R_{\rm Jup}$), via dynamical modeling of their transit timing and duration variations (TTVs and TDVs). The companions have masses of $22^{+7}_{-5}\,M_{\rm Jup}$ (Kepler-448c) and $150^{+60}_{-40}\,M_{\rm Jup}$ (Kepler-693c), and both are on eccentric orbits ($e=0.65^{+0.13}_{-0.09}$ for Kepler-448c and $e=0.47^{+0.11}_{-0.06}$ for Kepler-693c) with periastron distances of $1.5\,\mathrm{au}$. Moderate eccentricities are detected for the inner orbits as well ($e=0.34^{+0.08}_{-0.07}$ for Kepler-448b and $e=0.2^{+0.2}_{-0.1}$ for Kepler-693b). In the Kepler-693 system, a large mutual inclination between the inner and outer orbits ($53^{+7}_{-9}\,\mathrm{deg}$ or $134^{+11}_{-10}\,\mathrm{deg}$) is also revealed by the TDVs. This is likely to induce a secular oscillation of the inner WJ's eccentricity that brings its periastron close enough to the host star for tidal star-planet interactions to be significant. In the Kepler-448 system, the mutual inclination is weakly constrained and such an eccentricity oscillation is possible for a fraction of the solutions. Thus these WJs may be undergoing tidal migration to become hot Jupiters (HJs), although the migration via this process from beyond the snow line is disfavored by the close-in and massive nature of the companions. This may indicate that WJs can be formed in situ and could even evolve into HJs via high-eccentricity migration inside the snow line.