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 M J and radius of 0.995 ? 0.052 R J, 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 M J and radius of 1.132 ? 0.072 R J, 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.