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

Abstract: We present new photometric data of the transiting planet HAT-P-12b observed in 2011. Our three transit curves are modelled 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$\pm$0.00011 BJD and an orbital period of 3.21305961$\pm$0.00000035 d. 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 $M_{\rm A}$=0.73$\pm$0.02 M$\odot$, $R{\rm A}$=0.70$\pm$0.01 R$\odot$, log $g{\rm A}$=4.61$\pm$0.02, $\rho_{\rm A}$=2.10$\pm$0.09 $\rho_{\odot}$, and $L_{\rm A}$=0.21$\pm$0.01 L$\odot$. The planetary companion has $M{\rm b}$=0.21$\pm$0.01 M${\rm Jup}$, $R{\rm b}$=0.94$\pm$0.01 R${\rm Jup}$, log $g{\rm b}$=2.77$\pm$0.02, $\rho_{\rm b}$=0.24$\pm$0.01 $\rho_{\rm Jup}$, and $T_{\rm eq}$=960$\pm$14 K. Our results agree well with standard models of irradiated gas giants with a core mass of 11.3 M$_{\oplus}$.