HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star

November 2015 • 2015ApJ...813..111B

Authors • Bakos, G. Á. • Penev, K. • Bayliss, D. • Hartman, J. D. • Zhou, G. • Brahm, R. • Mancini, L. • de Val-Borro, M. • Bhatti, W. • Jordán, A. • Rabus, M. • Espinoza, N. • Csubry, Z. • Howard, A. W. • Fulton, B. J. • Buchhave, L. A. • Ciceri, S. • Henning, T. • Schmidt, B. • Isaacson, H. • Noyes, R. W. • Marcy, G. W. • Suc, V. • Howe, A. R. • Burrows, A. S. • Lázár, J. • Papp, I. • Sári, P.

Abstract • We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120 ± 0.012 {M}_{{J}}, a radius of {0.563}_-0.034^+0.046 {R}_{{J}}, and an orbital period of 3.1853 days. The host star is a moderately bright (V=13.340\+/- 0.010 mag, {K}_S=10.976\+/- 0.026 mag) K dwarf star with a mass of 0.849 ± 0.027 {M}_⊙, a radius of {0.815}_-0.035^+0.049 {R}_⊙, and a metallicity of [{Fe}/{{H}}] =+0.250\+/- 0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18 ± 4% (rock-iron core and H₂-He envelope), or 9 ± 4% (ice core and H₂-He envelope), i.e., it has a composition broadly similar to that of Uranus and Neptune, and very different from that of Saturn, which has 75% of its mass in H₂-He. Based on a sample of transiting exoplanets with accurately (<20%) determined parameters, we establish approximate power-law relations for the envelopes of the mass-density distribution of exoplanets. HATS-7b, which, together with the recently discovered HATS-8b, is one of the first two transiting super-Neptunes discovered in the Southern sky, is a prime target for additional follow-up observations with Southern hemisphere facilities to characterize the atmospheres of Super-Neptunes (which we define as objects with mass greater than that of Neptune, and smaller than halfway between that of Neptune and Saturn, i.e., 0.054 {M}_{{J}}\lt {M}_{{p}}\lt 0.18 {M}_{{J}}).

The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the Pontificia Universidad Católica de Chile (PUC). The station at Las Campanas Observatory (LCO) of the Carnegie Institute is operated by PU in conjunction with PUC, the station at the High Energy Spectroscopic Survey (H.E.S.S.) site is operated in conjunction with MPIA, and the station at Siding Spring Observatory (SSO) is operated jointly with ANU. This paper includes data gathered with the 10 m Keck-I telescope at Mauna Kea, the MPG 2.2 m and ESO 3.6 m telescopes at the ESO Observatory in La Silla. This paper uses observations obtained with facilities of the Las Cumbres Observatory Global Telescope.

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Benjamin Fulton

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