2 | Huanyu Teng

Revisiting Planetary Systems in Okayama Planet Search Program

Fri, 01 Nov 2024 00:00:00 +0000

Abstract

We report the observation of the transiting planet TOI-942c, a Neptunian planet orbiting a young K-type star approximately 50 Myr years old. Using Keck/HIRES, we observed a partial transit of the planet and detected an associated radial velocity anomaly. By modeling the Rossiter-McLaughlin (RM) effect, we derived a sky-projected obliquity of |lambda|=24⁺¹⁴₋₁₄ degrees, indicating TOI-942c is in a prograde and likely aligned orbit. Upon incorporation of the star's inclination and the planet's orbital inclination, we determined a true obliquity for TOI-942c lower than 43 degrees at 84% confidence, while dynamic analysis strongly suggests TOI-942c is aligned with stellar spin and coplanar with the inner planet. Furthermore, TOI-942c is also a suitable target for studying atmospheric loss of young Neptunian planets that are likely still contracting from the heat of formation. We observed a blueshifted excess absorption in the H-alpha line at 6564.7 Ang, potentially indicating atmospheric loss due to photoevaporation. However, due to the lack of pre-ingress data, additional observations are needed to confirm this measurement.

Long-period Giant Planets to evolved stars HD 112570 and HD 154391

Fri, 01 Dec 2023 00:00:00 +0000

Abstract

We present the discoveries of two giant planets orbiting the red giant branch (RGB) star HD 112570 and the red clump (RC) star HD 154391, based on the radial velocity (RV) measurements from Xinglong station and Okayama Astrophysical Observatory (OAO). Spectroscopic and asteroseismic analyses suggest that HD 112570 has a mass of 1.15±0.12 M_⊙, a radius of 9.85±0.23 R_⊙, a metallicity [Fe/H] of −0.46±0.1 and a logg of 2.47±0.1. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of Mp=3.42^+1.4_−0.84 M_Jup, a period of P=2615⁺⁸⁵₋₇₇ days and a moderate eccentricity of e=0.20^+0.16_−0.14 for the Jovian planet HD 112570 b. For HD 154391, it has a mass of 2.07±0.03 M_⊙, a radius of 8.56±0.05 R_⊙, a metallicity [Fe/H] of 0.07±0.1 and a logg of 2.86±0.1. The super-Jupiter HD 154391 b has a mass of Mp=9.1^+2.8_−1.9 M_Jup, a period of P=5163⁺⁶⁰₋₅₇ days and an eccentricity of e=0.20^+0.04_−0.04. We found HD 154391 b has one of the longest orbital period among those ever discovered orbiting evolved stars, which may provide a valuable case in our understanding of planetary formation at wider orbits. Moreover, while a mass gap at 4MJup seems to be present in the population of giant stars, there appears to be no significant differences in the distribution of metallicity among giant planets with masses above or below this threshold. Finally, The origin of the abnormal accumulation near 2 au for planets around large evolved stars (R >21 R_⊙), remains unclear.

Revisiting Planetary Systems in Okayama Planet Search Program

Wed, 01 Mar 2023 00:00:00 +0000

Abstract

In this study, we revisit 32 planetary systems around evolved stars observed within the framework of the Okayama Planet Search Program (OPSP) and its collaborative framework of the East Asian Planet Search Network (EAPS-Net) to search for additional companions and investigate the properties of stars and giant planets in multiple-planet systems. With our latest radial velocities obtained from Okayama Astrophysical Observatory (OAO), we confirm an additional giant planet in the wide orbit of 75 Cet system (P = 2051.62^+45.98_−40.47 d, Msini=0.912^+0.088_−0.090 M_J, a = 3.929^+0.052_−0.058 au), along with five stars exhibiting long-term radial velocity accelerations, which indicates massive companions in the wide orbits. We have also found that the radial velocity variations of several planet-harboring stars may indicate additional planet candidates, stellar activities, or other understudied sources. These stars include eps Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and ome Ser. We further constrain the orbital configuration of the HD 5608, HD 14067, HD 120084, and HD 175679 systems by combining radial veloc- ities with astrometry, as their host central stars exhibit significant astrometric accelerations. For other systems, we simply refine their orbital parameters. Moreover, our study indicates that the OPSP planet-harboring stars are more metal-poor compared to the currently known planet harboring stars, and this is likely due to the B−V color upper limit at 1.0 for star selection in the beginning of the survey. Finally, by investigating the less-massive giant planets (< 5M_J) around currently known planet-harboring evolved stars, we have found that metallicity positively correlates with the multiplicity and total planet mass of the system, which can be evidence for the core-accretion planet formation model.

A Close-in Planet Orbiting Giant Star HD 167768

Thu, 10 Nov 2022 00:00:00 +0000

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^+0.14_−0.12 M_⊙, a radius of 9.70^+0.25_−0.25 R_⊙, a metallicity of [Fe/H] = −0.67^+0.09_−0.08, and a surface gravity of logg=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 d, a minimum mass of 0.85^+0.12_−0.11 MJ, and an orbital semimajor axis of 0.1512^+0.0058_−0.0063 au. The planet has one of the shortest orbital periods among those ever found around deeply evolved stars (logg<3.5cgs) using radial velocity methods. The equilibrium temperature of the planet is 1874 K, as high as a hot Jupiter. The radial velocities show two additional regular variations at 41 d and 95 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.

A Trio of Giant Planets Orbiting Evolved Star HD 184010

Tue, 23 Aug 2022 00:00:00 +0000

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_⊙, a radius of 4.86_-0.49^+0.55R_⊙, and a surface gravity logg 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_bsini=0.31_-0.04^+0.03M_J, M_csini= 0.30_-0.05^+0.04M_J, and M_dsini=0.45_-0.06^+0.04M_J, and orbital periods of P_b=286.6_-0.7^+2.4d, P_c=484.3_-3.5^+5.5d, and P_d=836.4_-8.4^+8.4d, 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_b:P_c:P_d~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 (logg<3.5cgs) and more than two giant planets all with intermediate orbital periods (100d<P<1000d).

Regular radial velocity variations in nine G- and K-type giant stars: Eight planets and one planet candidate

Tue, 21 Dec 2021 00:00:00 +0000

Abstract

We report the detection of radial velocity variations in nine evolved G- and K-type giant stars. The observations were conducted at Okayama Astrophysical Observatory. Planets or planet candidates can best explain these regular variations. However, a coincidence of near 280-day variability among five of them prevents us from fully ruling out stellar origins for some of the variations, s ince all nine stars behave similarly in stellar properties. In the planet hypotheses to the RV variations, the planets (including one candidate) may survive close to the boundary of the so-called "planet desert" around evolved stars, having orbital periods between 255 and 555 days. Besides, they are the least-massive giant planets detected around G- and K-type giant stars, with minimum masses between 0.45 M_J and 1.34 M_J. We further investigated other hypotheses for our detection, yet none of them can better explain regular RV variation. With our detection, it is convinced that year-long regular variation with amplitude down to 15 m/s for G- and K-type giant stars is detectable. Moreover, we performed simulations to further confirm the detectability of planets around these stars. Finally, we explored giant planets around intermediate-mass stars, and likewise found a 4 Jupiter mass gap (e.g. Santos et al. 2017), which is probably a boundary of the giant planet population.