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 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 amass 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 gcm^-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 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 amass 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 gcm^-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-heliumdominated 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.