Thursday, May 5, 2016

Hot Jupiter HAT-P-13b's Core is Probably 11 Earth Mass

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


Buhler et al


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. Due to the special orbital configuration of the HAT-P-13 system, the magnitude of HAT-P-13b's eccentricity (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 of eb constrains k2b and allows us to place otherwise elusive constraints on the mass of HAT-P-13b's 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. We fit 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 k2b=0.31+0.08−0.05. Applying structural evolution models, we then find, with 68\% confidence, that Mcore,b is less than 25 Earth masses (M⊕). The most likely value of 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 μm and 4.5 μm bands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day-night circulation.

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