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Published January 1, 2011 | Published
Journal Article Open

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 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 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 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–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.

Additional Information

© 2011 The American Astronomical Society. Received 2010 July 27; accepted 2010 November 2; published 2010 December 14. Based in part on observations obtained at the W. M. Keck Observatory, which is operated by the University of California and the California Institute of Technology. Keck time has been granted by NOAO (A146Hr, A201Hr, and A264Hr), NASA (N018Hr, N049Hr, N128Hr, and N167Hr), and by the NOAO Keck-Gemini time exchange program (G329Hr). Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based in part on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. HATNet operations have been funded by NASA grants NNG04GN74G, NNX08AF23G and SAO IR&D grants. Work of G.Á.B. and J.A.J. were supported by the Postdoctoral Fellowship of the NSF Astronomy and Astrophysics Program (AST- 0702843 and AST-0702821, respectively). G.T. acknowledges partial support from NASA grant NNX09AF59G. We acknowledge partial support also from the Kepler Mission under NASA Cooperative Agreement NCC2-1390 (D.W.L., PI). G.K. thanks the Hungarian Scientific Research Foundation (OTKA) for support through grant K-81373. This research has made use of Keck telescope time granted through NOAO(programsA146Hr, A201Hr, and A264Hr), NASA (programs N018Hr, N049Hr, N128Hr, and N167Hr), and through the NOAO Keck-Gemini time exchange program (program G329Hr).

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