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. Due 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 ($M_{\mathrm{core},\mathrm{b}}$). 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 $\text{Spitzer Space Telescope}$. We fit the measured secondary eclipse times simultaneously with radial velocity measurements and find that $e_b=0.00700\pm 0.00100$. 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 $M_{\mathrm{core},\mathrm{b}}$ is less than 25 Earth masses ($M_{\oplus }$). The most likely value of $M_{\mathrm{core},\mathrm{b}}=11M_{\oplus }$, 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 $\hat{I}¼$m and 4.5 $\hat{I}¼$m bands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day-night circulation.

12 pages, 5 figures, 1 table, Accepted to ApJ on February 10, 2016