A Close-in Planet Orbiting Giant Star HD 167768

Huan-Yu Teng¹¹affiliation: Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan ^*^*affiliationmark: Bun’ei Sato¹¹affiliationmark: Masanobu Kunitomo²²affiliation: Department of Physics, Kurume University, 67 Asahi-machi, Kurume, Fukuoka 830-0011, Japan Takuya Takarada³³affiliation: Astrobiology Center, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Masashi Omiya³³affiliationmark: ⁴⁴affiliation: National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Hiroki Harakawa⁵⁵affiliation: Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 650 North A’ohoku Pl., Hilo, HI, 96720, USA Guang-Yao Xiao⁶⁶affiliation: CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China ⁷⁷affiliation: Department of Physics, College of Science, Tibet University, Lhasa 850000, China Yu-Juan Liu⁶⁶affiliationmark: Hideyuki Izumiura⁸⁸affiliation: Okayama Branch Office, Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Kamogata, Asakuchi, Okayama 719-0232, Japan Eiji Kambe⁵⁵affiliationmark: Michitoshi Yoshida⁵⁵affiliationmark: Yoichi Itoh⁹⁹affiliation: Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo, Hyogo 679-5313, Japan Hiroyasu Ando⁴⁴affiliationmark: Eiichiro Kokubo¹⁰¹⁰affiliation: The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan and Shigeru Ida¹¹¹¹affiliation: Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan teng.h.aa@m.titech.ac.jp

Abstract

We report the detection of a giant planet orbiting a G-type giant star HD 167768 from radial velocity measurements using HIgh Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory (OAO). HD 167768 has a mass of $1.08_{-0.12}^{+0.14}M_{\odot}$ , a radius of $9.70_{-0.25}^{+0.25}R_{\odot}$ , a metallicity of $\rm{[Fe/H]}=-0.67_{-0.08}^{+0.09}$ , and a surface gravity of $\log g=2.50_{-0.06}^{+0.06}$ . The planet orbiting the star is a warm Jupiter, having a period of $20.6532_{-0.0032}^{+0.0032}\ \rm{d}$ , a minimum mass of $0.85_{-0.11}^{+0.12}\ M_{\rm{J}}$ , and an orbital semimajor axis of $0.1512_{-0.0063}^{+0.0058}\ \rm{au}$ . The planet has one of the shortest orbital periods among those ever found around deeply evolved stars ( $\log g<3.5$ ) using radial velocity methods. The equilibrium temperature of the planet is $1874\ \rm{K}$ , as high as a hot Jupiter. The radial velocities show two additional regular variations at $41\ \rm{d}$ and $95\ \rm{d}$ , suggesting the possibility of outer companions in the system. Follow-up monitoring will enable validation of the periodicity. We also calculated the orbital evolution of HD 167768 b and found that the planet will be engulfed within 0.15 Gyr.

1 Introduction

Planets around evolved stars have been widely surveyed over the last 20 years, and over 150 planets have been discovered around evolved stars ( $\log g<3.5$ )¹¹1NASA Exoplanet Archive https://exoplanetarchive.ipac.caltech.edu (Akeson2013). However, the lack of close-in planets around giant stars is well-known in the planet population study. Theoretically, radial velocity (RV) measurements should be more capable of discovering these close-in planets rather than wide-orbit ones. However, current surveys reveal that close-in ( $a\lesssim 0.6\ \rm{au}$ ) planets are seldom found around evolved stars (e.g. [Johnson2007, Lillo-Box2016, Medina2018, Teng2022]), while most of the planets survive at distant places to their host stars.

Planet population synthesis suggests the lack of close-in planets could be caused by a scaling of the proto-planetary disk mass with the mass of the central star