HAT-P-18b and HAT-P-19b: Two low-density Saturn-mass planets transiting metal-rich K stars

J. D. Hartman, G. Á Bakos, B. Sato, G. Torres, R. W. Noyes, D. W. Latham, G. Kovács, D. A. Fischer, A. W. Howard, J. A. Johnson, G. W. Marcy, L. A. Buchhave, G. Füresz, G. Perumpilly, B. Béky, R. P. Stefanik, D. D. Sasselov, G. A. Esquerdo, M. Everett, Z. CsubryJ. Lázár, I. Papp, P. Sári

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

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 amass of 0.197 ±0.013 MJ 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 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 amass of 0.292 ±0.018 MJ 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.

Original languageEnglish (US)
JournalAstrophysical Journal
Volume726
Issue number1
DOIs
StatePublished - Jan 1 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • GSC 2283-00589)
  • Planetary systems
  • Stars: individual (HAT-P-18, GSC 2594-00646, HAT-P-19
  • Techniques: photometric
  • Techniques: spectroscopic

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