DYNAMICAL CONSTRAINTS ON THE CORE MASS OF HOT JUPITER HAT-P-13B

Abstract

ABSTRACT HAT-P-13b is a Jupiter-mass transiting exoplanet that has settled onto a stable, short-period, and mildly eccentric orbit as a consequence of the action of tidal dissipation and perturbations from a second, highly eccentric, outer companion. Owing to the special orbital configuration of the HAT-P-13 system, the magnitude of HAT-P-13b's eccentricity (e b ) is in part dictated by its Love number ( k 2 b ), which is in turn a proxy for the degree of central mass concentration in its interior. Thus, the measurement of e b constrains k 2 b and allows us to place otherwise elusive constraints on the mass of HAT-P-13b's core (M core,b). In this study we derive new constraints on the value of e b by observing two secondary eclipses of HAT-P-13b with the Infrared Array Camera on board the Spitzer Space Telescope. We fit the measured secondary eclipse times simultaneously with radial velocity measurements and find that e b = 0.00700 ± 0.00100. We then use octupole-order secular perturbation theory to find the corresponding k 2 b = 0.31 − 0.05 + 0.08 . Applying structural evolution models, we then find, with 68% confidence, that M core,b is less than 25 Earth masses (M ⊕). The most likely value is M core,b = 11 M ⊕, which is similar to the core mass theoretically required for runaway gas accretion. This is the tightest constraint to date on the core mass of a hot Jupiter. Additionally, we find that the measured secondary eclipse depths, which are in the 3.6 and 4.5 μm bands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day–night circulation.