Secondary Eclipses of HAT-P-13b

We present Spitzer secondary- eclipse observations of the hot Jupiter HAT-P-13 b in the 3.6 and 4.5 mu m bands. HAT-P-13 b inhabits a two-planet system with a configuration that enables constraints on the planet's second Love number, k(2), from precise eccentricity measurements, which in turn constrains models of the planet's interior structure. We exploit the direct measurements of e cos omega from our secondary-eclipse data and combine them with previously published radial velocity data to generate a refined model of the planet's orbit and thus an improved estimate on the possible interval for k(2). We report eclipse phases of 0.49154 +/- 0.00080 and 0.49711 +/- 0.00083 and corresponding e cos omega estimates of -0.0136 +/- 0.0013 and -0.0048 +/- 0.0013. Under the assumptions of previous work, our estimate of k(2) of 0.81 +/- 0.10 is consistent with the lower extremes of possible core masses found by previous models, including models with no solid core. This anomalous result challenges both interior models and the dynamical assumptions that enable them, including the essential assumption of apsidal alignment. We also report eclipse depths of 0.081%+/- 0.008% in the 3.6 mu m channel and 0.088%+/- 0.028% in the 4.5 mu m channel. These photometric results are non-uniquely consistent with solar-abundance composition without any thermal inversion.