HAT-P-18b AND HAT-P-19b: TWO LOW-DENSITY SATURN-MASS PLANETS TRANSITING METAL-RICH K STARS

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 Tc = 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 MJ and radius of 0.995 ± 0.052 RJ, yielding a mean density of 0.25 ± 0.04 g cm−3. 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 Tc = 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 MJ and radius of 1.132 ± 0.072 RJ, yielding a mean density of 0.25 ± 0.04 g cm−3. 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.