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

• Published in: The Astrophysical Journal
• Main Authors: Bakos, G. A., Penev, K., Bayliss, D., Hartman, J. D., Zhou, G., Brahm, R., Mancini, L., de Val-Borro, M., Bhatti, W., Jordan, 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., Lazar, J., Papp, I., Sari, P.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: IOP Publishing 2015
• Subjects: stars: individual: HATS-7; techniques: photometric; techniques: radial velocities; techniques: spectroscopic; Astrophysics - Earth; Planetary Astrophysics; ice core
• Online Access: https://research.usq.edu.au/item/q6x27/hats-7b-a-hot-super-neptune-transiting-a-quiet-k-dwarf-star; https://research.usq.edu.au/download/4d74ad118ec3e73d07dbcfa2e090ed3431accbc258d93a5373539e9e540b02fc/859729/1507.01024.pdf; https://doi.org/10.1088/0004-637x/813/2/111

We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120 ± 0.012 MJ, a radius of 0.563-0.034 +0.046 RJ, and an orbital period of 3.1853 days. The host star is a moderately bright (V = 13.340 ± 0.010 mag, KS = 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 powerlaw 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 MJ < Mp < 0.18 MJ).