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

2010; IOP Publishing; Volume: 726; Issue: 1 Linguagem: Inglês

10.1088/0004-637x/726/1/52

1538-4357

J. D. Hartman, G. Á. Bakos, B. Sato, Guillermo Torres, R. W. Noyes, David W. Latham, G. Kovács, Debra A. Fischer, Andrew W. Howard, John Asher Johnson, Geoffrey W. Marcy, Lars A. Buchhave, Gábor Fűrész, G. Perumpilly, B. Béky, R. P. Stefanik, Dimitar Sasselov, Gilbert A. Esquerdo, Mark E. Everett, Z. Csubry, J. Lázár, I. Papp, P. Sári,

Astrophysics and Star Formation Studies

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.