The Sub-Saturn Mass Transiting Planet HAT-P-12b

We present new photometric data of the transiting planet HAT-P-12b observed in 2011. Our three transit curves are modeled using the JKTEBOP code and adopting the quadratic limb-darkening law. Including our measurements, 18 transit times spanning about 4.2 yr were used to determine the improved ephemeris with a transit epoch of 2,454,187.85560 ± 0.00011 BJD and an orbital period of 3.21305961 ± 0.00000035 days. The physical properties of the star-planet system are computed using empirical calibrations from eclipsing binary stars and stellar evolutionary models, combined with both our transit parameters and previously known spectroscopic results. We found that the absolute dimensions of the host star are M _A = 0.73 ± 0.02 M _⊙, R _A = 0.70 ± 0.01 R _⊙, log g _A = 4.61 ± 0.02, ρ_A = 2.10 ± 0.09 ρ_⊙, and L _A = 0.21 ± 0.01 L _⊙. The planetary companion has M _b = 0.21 ± 0.01 M _Jup, R _b = 0.94 ± 0.01 R _Jup, log g _b = 2.77 ± 0.02, ρ_b = 0.24 ± 0.01 ρ_Jup, and T _eq = 960 ± 14 K. Our results agree well with standard models of irradiated gas giants with a core mass of 11.3 M _⊕.