* Astronomy

Title: The K2-ESPRINT Project VI: K2-105 b, a Hot-Neptune around a Metal-rich G-dwarf
Author: Norio Narita, Teruyuki Hirano, Akihiko Fukui, Yasunori Hori, Fei Dai, Liang Yu, John Livingston, Tsuguru Ryu, Grzegorz Nowak, Masayuki Kuzuhara, Yoichi Takeda, Simon Albrecht, Tomoyuki Kudo, Nobuhiko Kusakabe, Enric Palle, Ignasi Ribas, Motohide Tamura, Vincent Van Eylen, Joshua N. Winn

We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26±0.05) G-dwarf (T_eff = 5430±70 K and log g = 4.48±0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2m telescope. High-spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be <10^-6 using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multi-colour transit photometry with MuSCAT on the Okayama 1.88m telescope gives the orbital period of P = 8.266902±0.000070 days and consistent transit depths of R_p/R_*~0.035 or (R_p/R_*)^2~0.0012. The transit depth corresponds to a planetary radius of R_p = 3.59_{-0.39}^{+0.44} earth radii, indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3\sigma\ upper limit of 90 earth masses(0.00027 solar masses) on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, spin-orbit (mis)alignment, as well as investigate the possibility of additional planets in the system.

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