Dynamical Constraints on the Core Mass of Hot Jupiter HAT-P-13b

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-13bʼseccentricity(eb)is in part dictated by its Love number(k2b), which is in turn a proxy for the degree of central mass concentration in its interior. Thus, the measurement ofebconstrainsk2band allows us to place otherwise elusive constraints on the mass of HAT-P-13bʼs core(Mcore,b). In this study we derive new constraints on the value of eb by observing two secondary eclipses of HAT-P-13b with the Infrared Array Camera on board the Spitzer Space Telescope.Wefit the measured secondary eclipse times simultaneously with radial velocity measurements and find that eb=0.00700±0.00100. We then use octupole-order secular perturbation theory to find the corresponding=-+k0.3120.050.08b. Applying structural evolution models, we then find, with 68% confidence, that Mcore,b is less than 25 Earth masses(M⊕). The most likely value isMcore,b=11M⊕, 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 4.5μmbands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day–night circulation.