2011ApJ...726...52H

We report the discovery of two new transiting extrasolar planets. HAT-P-18b orbits the V = 12.759 K2 dwarf star GSC 2594-00646, with a period P = 5.508023 ±0.000006 days, transit epoch T_c= 2454715.02174±0.00020 (BJD), and transit duration 0.1131±0.0009 days. The host star has a mass of 0.77 ± 0.03 M_☉, radius of 0.75±0.04 R_☉, effective temperature 4803 ±80 K, and metallicity [Fe/H] = +0.10±0.08. The planetary companion has a mass of 0.197±0.013 M_J and radius of 0.995±0.052 R_J, yielding a mean density of 0.25±0.04 g/cm3. HAT-P-19b orbits the V = 12.901 K1 dwarf star GSC 2283-00589, with a period P = 4.008778±0.000006 days, transit epoch T_c= 2455091.53417±0.00034 (BJD), and transit duration 0.1182 ± 0.0014 days. The host star has a mass of 0.84±0.04 M_☉, radius of 0.82±0.05 R_☉, effective temperature 4990±130 K, and metallicity [Fe/H] = +0.23±0.08. The planetary companion has a mass of 0.292±0.018 M _J and radius of 1.132±0.072 R_J, yielding a mean density of 0.25 ± 0.04 g/cm3. The radial velocity residuals for HAT-P-19 exhibit a linear trend in time, which indicates the presence of a third body in the system. Comparing these observations with theoretical models, we find that HAT-P-18b and HAT-P-19b are each consistent with a hydrogen-helium-dominated gas giant planet with negligible core mass. HAT-P-18b and HAT-P-19b join HAT-P-12b and WASP-21b in an emerging group of low-density Saturn-mass planets, with negligible inferred core masses. However, unlike HAT-P-12b and WASP-21b, both HAT-P-18b and HAT-P-19b orbit stars with super-solar metallicity. This calls into question the heretofore suggestive correlation between the inferred core mass and host star metallicity for Saturn-mass planets.