A Trio of Giant Planets Orbiting Evolved Star HD 184010

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: Takuya Takarada²²affiliation: Astrobiology Center, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Masashi Omiya²²affiliationmark: ³³affiliation: National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Hiroki Harakawa⁴⁴affiliation: Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 650 North A’ohoku Pl., Hilo, HI, 96720, USA Makiko Nagasawa⁵⁵affiliation: Department of Physics, Kurume University School of Medicine, 67 Asahi-machi, Kurume, Fukuoka 830-0011, Japan Ryo Hasegawa¹¹affiliationmark: Hideyuki Izumiura⁶⁶affiliation: Okayama Branch Office, Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Kamogata, Asakuchi, Okayama 719-0232, Japan Eiji Kambe⁴⁴affiliationmark: Michitoshi Yoshida⁴⁴affiliationmark: Yoichi Itoh⁷⁷affiliation: Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo, Hyogo 679-5313, Japan Hiroyasu Ando³³affiliationmark: Eiichiro Kokubo⁸⁸affiliation: The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan and Shigeru Ida⁹⁹affiliation: Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan [email protected]

Abstract

We report the discovery of a triple-giant-planet system around an evolved star HD 184010 (HR 7421, HIP 96016). This discovery is based on observations from Okayama Planet Search Program, a precise radial velocity survey, undertaken at Okayama Astrophysical Observatory between 2004 April and 2021 June. The star is K0 type and located at beginning of the red-giant branch. It has a mass of $1.35_{-0.21}^{+0.19}M_{\odot}$ , a radius of $4.86_{-0.49}^{+0.55}R_{\odot}$ , and a surface gravity $\log g$ of $3.18_{-0.07}^{+0.08}$ . The planetary system is composed of three giant planets in a compact configuration: The planets have minimum masses of $M_{\rm{b}}\sin i=0.31_{-0.04}^{+0.03}M_{\rm{J}}$ , $M_{\rm{c}}\sin i=0.30_{-0.05}^{+0.04}M_{\rm{J}}$ , and $M_{\rm{d}}\sin i=0.45_{-0.06}^{+0.04}M_{\rm{J}}$ , and orbital periods of $P_{\rm{b}}=286.6_{-0.7}^{+2.4}\ \rm{d}$ , $P_{\rm{c}}=484.3_{-3.5}^{+5.5}\ \rm{d}$ , and $P_{\rm{d}}=836.4_{-8.4}^{+8.4}\ \rm{d}$ , respectively, which are derived from a triple Keplerian orbital fit to three sets of radial velocity data. The ratio of orbital periods are close to $P_{\rm{d}}:P_{\rm{c}}:P_{\rm{b}}\sim 21:12:7$ , which means the period ratios between neighboring planets are both lower than $2:1$ . The dynamical stability analysis reveals that the planets should have near-circular orbits. The system could remain stable over 1 Gyr, initialized from co-planar orbits, low eccentricities ( $e=0.05$ ), and planet masses equal to the minimum mass derived from the best-fit circular orbit fitting. Besides, the planets are not likely in mean motion resonance. HD 184010 system is unique: it is the first system discovered to have a highly evolved star ( $\log g<3.5$ cgs) and more than two giant planets all with intermediate orbital periods ( $10^{2}\ \rm{d}<P<10^{3}\ \rm{d}$ ).

1 Introduction

Nowadays, planets around evolved stars (giants and subgiants) have been extensively surveyed for over 20 years, so as to explore the world of planets around more massive stars, and to investigate how the planetary system evolves after the host star moving out of their main-sequence phase. So far, over 150 planet-harboring evolved stars have been confirmed with both radial velocity (RV) method and transit method¹¹1Evolved stars are simply defined as $\log g\lesssim 3.5$ , and data acquisition is from NASA Exoplanet Archive (Akeson2013). Among these systems around evolved stars, only less than 15% of the systems are known to have more than one planet (shown in Figure LABEL:fig:logg_prot), whereas they could be the key to enigma of evolution of the planetary systems. Thanks to long-baseline RV surveys, these multi-planet systems have been discovered.

As shown in Figure LABEL:fig:logg_prot, the majority of these systems are in the pattern of massive planet pairs and have intermediate orbital periods, i.e. $\sim 10^{2}$ to $\sim 10^{3}$ days, while the minority includes some hot Jupiters orbiting the host star with short orbital periods. Anyhow, these planets are likely to have passed Type I, II, or III migration (e.g. [Goldreich1980, Lin1996, Masset2003, Ida2004]), where planets have interactions with the disk, or they may have experienced gravitational interactions to reach the current position, i.e. Kozai mechanism