arXiv subjects

Bun'ei Sato

Publications and source records attributed to Bun'ei Sato.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of an Eccentric Hot Super-Jupiter Leaving the Transiting Geometry of the Early-A-type star TOI-1355

Hot Jupiters orbiting hot stars ($T_\mathrm{eff} > 7000$ K) are suggested to have experienced high-eccentricity migration, often evidenced by the tendency for misaligned orbits, despite their circular orbits. In this paper, we present the discovery of TOI-1355 b: an eccentric ($e\sim0.22$) hot Jupiter with a mass of $m_{\mathrm{p}}\sim5.8M_J$ and a radius of $R_{\mathrm{p}}\sim 1.4R_J$ orbiting an A-type star with a period of about $2.17$ days, identified from the TESS transit survey and subsequent follow-up observations. We measured the stellar parameters using the data from the high-resolution spectrograph Seimei/GAOES-RV and obtained the planetary parameters from the photometric data acquired by TESS and ground-based telescopes. This is one of the rare eccentric hot Jupiters around hot stars. This system could be undergoing high-eccentricity migration. We detected nodal precession by measuring the change in its impact parameter. This implies that its transit will no longer be observable from the middle of 2033. Nevertheless, TOI-1355 b is anticipated to be a compelling target for future atmospheric observations, given the hint of atmospheric variability detected in this study.

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arXiv2026

Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects

We have discovered a compact hierarchical triple main-sequence star system, which is cataloged as Gaia DR3 1010268155897156864 or TIC 21502513. Hereafter, we call it ``G1010''. G1010 consists of a primary (the most massive) star and inner binary that orbit each other. The primary star is a $0.85_{-0.03}^{+0.03}\;{\rm M}_\odot$ main-sequence (MS) star, and the inner binary components are $0.63_{-0.02}^{+0.02}$ and $0.61_{-0.02}^{+0.02}\;{\rm M}_\odot$ MS stars. The outer and inner orbital periods are $277.2_{-1.3}^{+1.6}$ and $\sim 18.26$ days, respectively. G1010 is categorized as a single-lined spectroscopic binary, and its orbital solution indicates that G1010 possibly accompanies a massive compact object, such as a neutron star or massive white dwarf. In order to confirm the presence of a massive compact object, we have performed several-times low signal-to-ratio (SNR) and one-time high SNR spectroscopic observations, and determined the outer orbital parameters. Moreover, we have deeply analyzed the high SNR spectroscopic data, and found that G1010 accompanies not a massive compact object, but an inner binary. We have investigated G1010's light curve in Transiting Exoplanet Survey Satellite (TESS), and concluded that the inner binary is actually an eclipsing binary, not included in TESS Eclipsing Binary Stars. We have obtained the inner orbital parameters from the TESS light curve. G1010 is similar to compact hierarchical triple star systems previously discovered by eclipse timing variation analysis. Our discovery has shown that such triple star systems can be discovered by combination of low- and high-SNR spectroscopic observations with the help of Gaia DR3 and the upcoming Gaia DR4/DR5.

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arXiv2026

Two unseen massive white dwarf candidates in close binaries

We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and 2 .67 d with circular orbits. The visible components have masses of $\sim$0.9--1.0$\,M_\odot$, while the minimum masses of the unseen companions are $1.078^{+0.058}_{-0.060}\, M_\odot$ and $0.943^{+0.048}_{-0.049}\, M_\odot$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.13^{+0.11}_{-0.08} M_\odot$ and $1.05^{+0.15}_{-0.10} M_\odot$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.

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arXiv2026

Unraveling Year-Long Radial Velocity Variations in Red Clump Region -- I: Comprehensive analysis of a K0 Giant star, 2 Draconis

Slow-rotating evolved stars frequently exhibit radial velocity (RV) variations on annual timescales, complicated by instrumental systematics and aliasing in the one-year regime. Here we investigate the origin of the near-yearly periodicity in 2 Dra, a star located in the red-clump region, assessing possible causes between stellar activity, instrumental profile (IP) effects, sampling alias, and planetary companions. We applied two independent approaches: (1) constraining diagnostic signals and performing a correlation analysis ($r$) between period-confined signals, and (2) evaluating phase stability by partitioning Keplerian fits. These methods enabled us to examine the physical connections and phase coherence among stellar activity indicators, RV measurements, and IP diagnostics. Our analysis suggests a stellar rotation period of $\simeq270\text{--}320$\,d for 2~Dra. The 340-d RV signal does not appear to originate from stellar activity in this chromospherically quiet star ($|r| \lesssim 0.33$), nor from instrumental systematics near the annual period ($|r| \lesssim 0.1$). This conclusion is supported by contrasting phase behavior: the RV and stellar activity phases remain stable, whereas the IP phases do not. We therefore propose that the 340-d variation likely arises from either small-amplitude intrinsic variability or a tentative gas giant companion with potential weak activity-induced modulation. The case of 2~Dra provides a framework for distinguishing the origins of $\sim$1-yr RV variations in other evolved stars.

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arXiv2025

Doppler imaging combined with high-cadence photometry. I. Revisiting the surface of a pre-main-sequence flare star

Latitude distribution of stellar magnetic activity is not well constrained by observations, despite its importance for a better understanding of stellar dynamos. We aim to obtain an accurate reconstruction of the surface spot distribution on the young, rapidly rotating K2 star PW And by combining spectroscopic and photometric diagnostics. In particular, we seek to assess how the inclusion of continuous high-precision TESS photometry in parallel with high-resolution spectroscopy improves latitude recovery of starspots, especially at low latitudes and in the southern hemisphere, which are poorly constrained by Doppler imaging (DI) alone. We explore the spatial origins of the observed white-light flares. We performed simultaneous Doppler imaging and light curve inversion (DI+LCI) using contemporaneous high-resolution GAOES-RV spectra from the 3.8 m Seimei telescope (R~65000) and high-precision TESS light curves. Surface reconstructions employ the SpotDIPy code to model both line profiles and continuum brightness variations. We compare DI+LCI maps with DI-only solutions, conduct artificial-spot simulations to evaluate the effects of latitude, phase coverage, and S/N on reconstruction reliability. We also investigate the spatial correlation between the DI+LCI reconstructed map and flares detected in the TESS data. The DI+LCI reconstruction reveals significant spot features at mid-to-low latitudes, equatorial regions, and even in the southern hemisphere. Simulations show that DI+LCI provides more accurate reconstructions than DI-only, especially under conditions of incomplete phase coverage and low S/N, by better recovering both spot latitudes and filling factors. A comparison between the DI+LCI map and the TESS flare timings also suggests potential association between flare occurrence and reconstructed spot longitudes.

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arXiv2024

Three-dimensional atmospheric dynamics of Jupiter from ground-based Doppler imaging spectroscopy in the visible

We present three-dimensional (3D) maps of Jupiter's atmospheric circulation at cloud-top level from Doppler-imaging data obtained in the visible domain with JIVE, the second node of the JOVIAL network, which is mounted on the Dunn Solar Telescope at Sunspot, New Mexico. We report on 12 nights of observations between May 4 and May 30, 2018, representing a total of about 80 hours. Firstly, the average zonal wind profile derived from our data is compatible with that derived from cloud-tracking measurements performed on Hubble Space Telescope images obtained in April 2018 from the Outer Planet Atmospheres Legacy (OPAL) program. Secondly, we present the first ever two-dimensional maps of Jupiter's atmospheric circulation from Doppler measurements. The zonal velocity map highlights well-known atmospheric features, such as the equatorial hot spots and the Great Red Spot (GRS). In addition to zonal winds, we derive meridional and vertical velocity fields from the Doppler data. The motions attributed to vertical flows are mainly located at the boundary between the equatorial belts and tropical zones, which could indicate active motion in theses regions. Qualitatively, these results compare well to recent Juno data that have unveiled the three-dimensional structure of Jupiter's wind field. To the contrary, the motions attributed to meridional circulation are very different from what is obtained by cloud tracking, except at the GRS. Because of limitations with data resolution and processing techniques, we acknowledge that our measurement of vertical or meridional flows of Jupiter are still to be confirmed.

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arXiv2023

Two long-period giant planets around two giant stars: HD 112570 and HD 154391

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 $1.15\pm0.12\,M_{\odot}$, a radius of $9.85\pm0.23\,R_{\odot}$, a metallicity [Fe/H] of $-0.46\pm0.1$ and a ${\rm log}\,g$ of $2.47\pm0.1$. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of $M_{\rm p}={3.42}_{-0.84}^{+1.4}\ M_{\rm Jup}$, a period of $P={2615}_{-77}^{+85}$ days and a moderate eccentricity of $e={0.20}_{-0.14}^{+0.16}$ for the Jovian planet HD 112570 b. For HD 154391, it has a mass of $2.07\pm0.03\,M_{\odot}$, a radius of $8.56\pm0.05\,R_{\odot}$, a metallicity [Fe/H] of $0.07\pm0.1$ and a ${\rm log}\,g$ of $2.86\pm0.1$. The super-Jupiter HD 154391 b has a mass of $M_{\rm p}={9.1}_{-1.9}^{+2.8}\ M_{\rm Jup}$, a period of $P={5163}_{-57}^{+60}$ 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 $4\,M_{\rm Jup}$ 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 ($R_{\star}>21\,R_{\odot}$), remains unclear.

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arXiv2023

Magnetic activity variability of nearby bright Sun-like stars by 4-year intensive H$α$ line monitoring

We report intensive monitoring of the activity variability in the H$α$ line for 10 Sun-like stars using the 1.88-m reflector at Okayama Branch Office, Subaru Telescope, during the last four years 2019-2022. Our aim was to investigate features of the stellar magnetic activity behaviors. We correlated the H$α$ line variability of each star with the stellar activity levels derived from the Ca II H&K line, suggesting its efficiency as a magnetic activity indicator. In analyzing the H$α$ line variation, we observed that some stars exhibited linear or quadratic trends during the observation period. Among several G- and K-type stars expected to have co-existing activity cycles, we confirmed the 2.9-yr short cycle of $ε$ Eri (K2V) from the H$α$ observations. Additionally, we established upper limits on the H$α$ variability of $β$ Com (G0V) and $κ$$^1$ Cet (G5V) concerning their expected shorter cycles. We also detected the possibility of short-term activity cycles in two F-type stars, $β$ Vir (F9V; $\sim$ 530 days) and $α$ CMi (F5IV-V; $\sim$ 130 days). The cycle in $α$ CMi was observed in only one season of our 4-yr observations, suggesting the temporal absence of the cycle period. However, for stars with planets, we did not observe significant magnetic activity variability likely associated with the planetary orbital period. It is speculated that the impact of H$α$ variability on radial velocity (RV) measurements may vary with spectral type.

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arXiv2023

Revisiting Planetary Systems in Okayama Planet Search Program: A new long-period planet, RV astrometry joint analysis, and multiplicity-metallicity trend around evolved stars

In this study, we revisit 32 planetary systems around evolved stars observed within the framework of the Okayama Planet Search Program and its collaborative framework of the 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 ($P_{\rm{c}} = 2051.62_{-40.47}^{+45.98}\ \rm{d}$, $M_{\rm{c}}\sin i=0.912_{-0.090}^{+0.088}\ M_{\rm{J}}$, and $a_{\rm{c}}=3.929_{-0.058}^{+0.052}\ \rm{au}$), 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 $ε$ Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and $ω$ Ser. We further constrain the orbital configuration of the HD 5608, HD 14067, HD 120084, and HD 175679 systems by combining radial velocities 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 ($< 5 M_{\rm{J}}$) 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.

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arXiv2023

Direct Imaging Explorations for Companions around Mid-Late M Stars from the Subaru/IRD Strategic Program

The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late-M~dwarfs via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M~dwarfs. To search for companions around such ``deprioritized" M~dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 observations with pyramid wavefront sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM~J1002+1459~B and LSPM~J2204+1505~B) and two new candidates that are likely companions (LSPM~J0825+6902~B and LSPM~J1645+0444~B) as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between $\sim2-20$~au in total. A comparison of the colors with the spectral library suggests that LSPM~J2204+1505~B and LSPM~J0825+6902~B are located at the boundary between late-M and early-L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions ($\sim10-75\ M_{\rm Jup}$) at 10~au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with JWST to search for smaller-mass companions below the NIRC2 detection limits.

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arXiv2023

Magnetic activity variability from H$α$ line intensive monitoring for two F-type stars having a hot-Jupiter, $τ$ Bootis A and $\upsilon$ Andromedae A

We report the results of intensive monitoring of the variability in the H$α$ line for two F-type stars, $τ$ Boo and $\upsilon$ And, during the last four years 2019-2022, in order to investigate their stellar magnetic activity. The 4-year H$α$ line intensity data taken with the 1.88-m reflector at Okayama Branch Office, Subaru Telescope, shows the existence of a possible $\sim$ 123-day magnetic activity cycle of $τ$ Boo. The result of the H$α$ variability as another tracer of the magnetic activity on the chromosphere is consistent with previous studies of the Ca II H&K line and suggests that the magnetic activity cycle is persisted in $τ$ Boo. For $\upsilon$ And, we suggest a quadratic long-term trend in the H$α$ variability. Meanwhile, the short-term monitoring shows no significant period corresponding to specific variations likely induced by their hot-Jupiter in both cases ($\approx$ 3.31 and 4.62 days, respectively). In this H$α$ observation, we could not find any signature of the Star-Planet Magnetic Interaction. It is speculated that the detected magnetic activity variability of the two F-type stars is related to the stellar intrinsic dynamo.

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arXiv2023

An Earth-sized Planet around an M5 Dwarf Star at 22 pc

We report on the discovery of an Earth-sized transiting planet ($R_p=1.015\pm0.051\,R_\oplus$) in a $P=4.02$ day orbit around K2-415 (EPIC 211414619), an M5V star at 22 pc. The planet candidate was first identified by analyzing the light curve data by the K2 mission, and is here shown to exist in the most recent data from TESS. Combining the light curves with the data secured by our follow-up observations including high-resolution imaging and near infrared spectroscopy with IRD, we rule out false positive scenarios, finding a low false positive probability of $2\times 10^{-4}$. Based on IRD's radial velocities of K2-415, which were sparsely taken over three years, we obtain the planet mass of $3.0\pm 2.7\,M_\oplus$ ($M_p<7.5\,M_\oplus$ at $95\,\%$ confidence) for K2-415b. Being one of the lowest mass stars ($\approx 0.16\,M_\odot$) known to host an Earth-sized transiting planet, K2-415 will be an interesting target for further follow-up observations, including additional radial velocity monitoring and transit spectroscopy.

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arXiv2022

A Close-in Planet Orbiting Giant Star HD 167768

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.

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arXiv2022

A Trio of Giant Planets Orbiting Evolved Star HD 184010

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 ($10^2\ \rm{d} < P < 10^3\ \rm{d}$).

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arXiv2022

Color Dependence of the Transit Detectability for Young Active M-dwarfs

We investigate the planetary transit detectability in the presence of stellar rotational activity from light curves for young M-dwarfs and estimate improvements of the detection at near-infrared (NIR) wavelengths. Making maps of the transit signal detection efficiency over the orbital period and planetary radius with light curves of members of four clusters, Hyades, Praesepe, Pleiades, and Upper Scorpius observed by the K2 mission, we evaluate the detectability for the rotation period and modulation semi-amplitude. We find that the detection efficiency remarkably decreases to about 20% for rapidly rotators with P_{rot} <= 1 d and the lack of planets in Pleiades is likely due to the high fraction of rapidly rotating M-dwarfs. We also evaluate the improvements of the planet detection with NIR photometry via tests using mock light curves assuming that the signal amplitude of stellar rotation decreases at NIR wavelengths. Our results suggest that NIR photometric monitoring would double relative detection efficiency for transiting planetary candidates with P_{rot} <= 1 d and find planets around M-dwarfs with approximately 100 Myr missing in the past transit surveys from the space.

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arXiv2022

Direct Imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS

We present the direct-imaging discovery of a substellar companion in orbit around a Sun-like star member of the Hyades open cluster. So far, no other substellar companions have been unambiguously confirmed via direct imaging around main-sequence stars in Hyades. The star HIP 21152 is an accelerating star as identified by the astrometry from the Gaia and Hipparcos satellites. We have detected the companion, HIP 21152 B, in multi-epoch using the high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. We have also obtained the stellar radial-velocity data from the Okayama 188cm telescope. The CHARIS spectroscopy reveals that HIP 21152 B's spectrum is consistent with the L/T transition, best fit by an early T dwarf. Our orbit modeling determines the semi-major axis and the dynamical mass of HIP 21152 B to be 17.5$^{+7.2}_{-3.8}$ au and 27.8$^{+8.4}_{-5.4}$ $M_{\rm{Jup}}$, respectively. The mass ratio of HIP 21152 B relative to its host is $\approx$2\%, near the planet/brown dwarf boundary suggested from recent surveys. Mass estimates inferred from luminosity evolution models are slightly higher (33--42 $M_{\rm{Jup}}$). With a dynamical mass and a well-constrained age due to the system's Hyades membership, HIP 21152 B will become a critical benchmark in understanding the formation, evolution, and atmosphere of a substellar object as a function of mass and age. Our discovery is yet another key proof-of-concept for using precision astrometry to select direct imaging targets.

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arXiv2022

A Super-Earth Orbiting Near the Inner Edge of the Habitable Zone around the M4.5-dwarf Ross 508

We report the near-infrared radial-velocity (RV) discovery of a super-Earth planet on a 10.77-day orbit around the M4.5 dwarf Ross 508 ($J_\mathrm{mag}=9.1$). Using precision RVs from the Subaru Telescope IRD (InfraRed Doppler) instrument, we derive a semi-amplitude of $3.92^{+0.60}_{-0.58}$ ${\rm m\,s}^{-1}$, corresponding to a planet with a minimum mass $m \sin i = 4.00^{+0.53}_{-0.55}\ M_{\oplus}$. We find no evidence of significant signals at the detected period in spectroscopic stellar activity indicators or MEarth photometry. The planet, Ross 508 b, has a semimajor-axis of $0.05366^{+0.00056}_{-0.00049}$ au. This gives an orbit-averaged insolation of $\approx$1.4 times the Earth's value, placing Ross 508 b near the inner edge of its star's habitable zone. We have explored the possibility that the planet has a high eccentricity and its host is accompanied by an additional unconfirmed companion on a wide orbit. Our discovery demonstrates that the near-infrared RV search can play a crucial role to find a low-mass planet around cool M dwarfs like Ross 508.

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arXiv2022

Nodal Precession of WASP-33b for Eleven Years by Doppler Tomographic and Transit Photometric Observations

WASP-33b, a hot Jupiter around a hot star, is a rare system in which nodal precession has been discovered. We updated the model for the nodal precession of WASP-33b by adding new observational points. Consequently, we found a motion of the nodal precession spanning 11 years. We present homogenous Doppler tomographic analyses of eight datasets, including two new datasets from TS23 and HIDES, obtained between 2008 and 2019, to illustrate the variations in the projected spin-orbit obliquity of WASP-33b and its impact parameter. We also present its impact parameters based on photometric transit observations captured by MuSCAT in 2017 and MuSCAT2 in 2018. We derived its real spin-orbit obliquity $ψ$, stellar spin inclination $i_{s}$, and stellar gravitational quadrupole moment $J_2$ from the time variation models of the two orbital parameters. We obtained $ψ= 108.19^{+0.95}_{-0.97}$ deg, $i_s = 58.3^{+4.6}_{-4.2}$ deg, and $J_2=(1.36^{+0.15}_{-0.12}) \times 10^{-4}$. Our $J_2$ value was slightly smaller than the theoretically predicted value, which may indicate that its actual stellar internal structure is different from the theoretical one. We derived the nodal precession speed $\dotθ=0.507^{+0.025}_{-0.022}$ deg year$^{-1}$, and its period $P_{\mathrm{pre}}=709^{+33}_{-34}$ years, and found that WASP-33b transits in front of WASP-33 for only $\sim$ 20 \% of the entire nodal precession period.

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