Search

Kepler showed a paucity of planets with radii of 1.5–2 R⊕ around solar mass stars but this radius-gap has not been well studied for low-mass star planets. Energy-driven escape models like photoevaporation and core-powered mass-loss predict opposing transition regimes between rocky and non-rocky planets when compared to models depicting planets forming in gas-poor environments. Here, we present transit observations of three super-Earth sized planets in the radius-gap around low-mass stars using high-dispersion InfraRed Doppler spectrograph on the Subaru 8.2 m telescope. The planets GJ 9827 b and d orbit around a K6V star and TOI-1235 b orbits a M0.5 star. We limit any planet-related absorption in the 1083.3 nm lines of triplet He i by placing an upper-limit on the equivalent width of 14.71, 18.39, and 1.44 mÅ for GJ 9827 b (99 per cent confidence), GJ 9827 d (99 per cent confidence), and TOI-1235 b (95 per cent confidence), respectively. Using a Parker wind model, we cap the mass-loss at >0.25 M⊕ Gyr−1 and >0.2 M⊕ Gyr−1 for GJ 9827 b and d, respectively (99 per cent confidence), and >0.05 M⊕ Gyr−1 for TOI-1235 b (95 per cent confidence) for a representative wind temperature of 5000 K. Our observed results for the three planets are more consistent with the predictions from photoevaporation and/or core-powered mass-loss models than the gas-poor formation models. However, more planets in the radius-gap regime around the low-mass stars are needed to robustly predict the atmospheric evolution in planets around low-mass stars.

We report a search for excess absorption in the 1083.2 nm line of ortho (triplet) helium during transits of TOI-1807b and TOI-2076b, 1.25 and 2.5R$_{\rm Earth}$ planets on 0.55- and 10.4-day orbits around nearby $\sim$200~Myr-old K dwarf stars. We limit the equivalent width of any transit-associated absorption to $, Accepted to MNRAS

Silicon and Strontium are key elements to explore the nucleosynthesis and chemical evolution of the Galaxy by measurements of very metal-poor stars. There are, however, only a few useful spectral lines of these elements in the optical range that are measurable for such low-metallicity stars. Here we report on abundances of these two elements determined from near-infrared high-resolution spectra obtained with the Subaru Telescope Infrared Doppler instrument (IRD). Si abundances are determined for as many as 26 Si lines for six very and extremely metal-poor stars (-4.0, Comment: 3 figures, 6 tables, PASJ, in press

Remote sensing of the Earth has demonstrated that photosynthesis is traceable as the vegetation red edge (VRE), which is a steep rise in the reflection spectrum of vegetation, and as solar-induced fluorescence. This study examines the detectability of biological fluorescence from two types of photosynthetic pigments, chlorophylls (Chls) and bacteriochlorophylls (BChls), on Earthlike planets with oxygen-rich/poor and anoxic atmospheres around the Sun and M dwarfs. Atmospheric absorption, such as H2O, CH4, O2, and O3, and the VRE obscure the fluorescence emissions from Chls and BChls. We find that the BChl-based fluorescence for wavelengths of 1000–1100 nm, assuming the spectrum of BChl b–bearing purple bacteria, could provide a suitable biosignature, but only in the absence of water cloud coverage or other strong absorbers near 1000 nm. The Chl fluorescence is weaker for several reasons, e.g., spectral blending with the VRE. The apparent reflectance excess is greatly increased in both the Chl and BChl cases around TRAPPIST-1, due to the fluorescence and stellar absorption lines. This could be a promising feature for detecting the fluorescence around ultracool red dwarfs using follow-up ground-based observations at high spectral resolution; however, this would require a long time around Sunlike stars, even for a LUVOIR-like space mission. Moreover, the simultaneous detection of fluorescence and the VRE is the key to identifying traces of photosynthesis, because absorption, reflectance, and fluorescence are physically connected. For further validation of the fluorescence detection, the nonlinear response of biological fluorescence as a function of light intensity could be considered.

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., Accepted by Publications of the Astronomical Society of Japan, 9 pages, 7 figures, 2 tables

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 ($10^2\ \rm{d} < P < 10^3\ \rm{d}$)., 20 pages, 5 figures, Published in PASJ

MWC 560 (V694 Mon) is classified as a symbiotic binary consisting of a white dwarf and an M5III red giant. In this object, a very- high-speed component, thought to be a jet, has been reported in the hydrogen Balmer line. We have been conducting long-term spectroscopic monitoring since the brightening in 2016 February. An irregular rebrightening was reported by Goranskij et al. (2018, Astronomer’s Telegram, 12227), who also showed that the high-velocity absorption component was not seen on 2018 November 16. Our low-resolution spectroscopic observations showed no high-velocity absorption component on 2018 November 14, and wide emission lines in Balmer line region (Hα, Hβ). However, high-resolution spectroscopy revealed the presence of slow, weak absorbing components and an emission wing on 2018 December 25. MWC 560 began to brighten in 2018 November, and it is gradually brightening as of 2020 April 14. For the absorption component, we propose a shell expansion of a classical symbiotic nova that occurred in 2016. During the 2018 brightening a new emission wing with vFWHM ≈ 700 km s−1 was confirmed, which could be triggered by disk instability like a dwarf nova. This paper summarizes the results of long-term monitoring observations and the recent changes in MWC 560.

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of $3.09 \pm 0.11 \,R_\oplus$ and an orbital period of $2.5 \,\rm{days}$, using a combination of TESS and follow-up observations using ground-based telescopes. Joint analysis of multi-band photometry from TESS, MuSCAT, MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/'Alopeke and high-resolution spectroscopy with the Subaru/IRD confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and IRTF/SpeX were used to determine the stellar parameters, and found the host star is an M4 dwarf with an effective temperature of $T_{eff} = 3185 \pm 76\,\rm{K}$ and a metallicity of [Fe/H] $=0.336 \pm 0.060 \,\rm{dex}$. The radial velocities measured from IRD set a $2$-$\sigma$ upper limit on the planetary mass to be $48.8 \,M_\oplus$. The large radius ratio ($R_p/R_\star \sim 0.1$) and the relatively bright NIR magnitude ($J=12.2 \,\rm{mag}$) make this planet an attractive target for further followup observations. TOI-1696b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST., Comment: 25 pages, 18 figures, 3 tables, Submitted to AJ

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.--Full lis of authors: Fukui, Akihiko; Kimura, Tadahiro; Hirano, Teruyuki; Narita, Norio; Kodama, Takanori; Hori, Yasunori; Ikoma, Masahiro; Palle, Enric; Murgas, Felipe; Parviainen, Hannu; Kawauchi, Kiyoe; Mori, Mayuko; Esparza-Borges, Emma; Bieryla, Allyson; Irwin, Jonathan; Safonov, Boris S.; Stassun, Keivan G.; Alvarez-Hernandez, Leticia; Bejar, Victor J. S.; Casasayas-Barris, Nuria; Chen, Guo; Crouzet, Nicolas; de Leon, Jerome P.; Isogai, Keisuke; Kagetani, Taiki; Klagyivik, Peter; Korth, Judith; Kurita, Seiya; Kusakabe, Nobuhiko; Livingston, John; Luque, Rafael; Madrigal-Aguado, Alberto; Morello, Giuseppe; Nishiumi, Taku; Orell-Miquel, Jaume; Oshagh, Mahmoudreza; Sanchez-Benavente, Manuel; Stangret, Monika; Terada, Yuka; Watanabe, Noriharu; Zou, Yujie; Tamura, Motohide; Kurokawa, Takashi; Kuzuhara, Masayuki; Nishikawa, Jun; Omiya, Masashi; Vievard, Sebastien; Ueda, Akitoshi; Latham, David W.; Quinn, Samuel N.; Strakhov, Ivan S.; Belinski, Alexandr A.; Jenkins, Jon M.; Ricker, George R.; Seager, Sara; Vanderspek, Roland; Winn, Joshua N.; Charbonneau, David; Ciardi, David R.; Collins, Karen A.; Doty, John P.; Bachelet, Etienne; Harbeck, Daniel., We report the discovery of TOI-2285b, a sub-Neptune-sized planet transiting a nearby (42 pc) M dwarf with a period of 27.3 d. We identified the transit signal from the Transiting Exoplanet Survey Satellite photometric data, which we confirmed with ground-based photometric observations using the multiband imagers MuSCAT2 and MuSCAT3. Combining these data with other follow-up observations including high-resolution spectroscopy with the Tillinghast Reflector Echelle Spectrograph, high-resolution imaging with the SPeckle Polarimeter, and radial velocity (RV) measurements with the InfraRed Doppler instrument, we find that the planet has a radius of 1.74±0.08R⊕⁠, a mass of, Funding for the TESS mission is provided by NASA’s Science Mission Directorate. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS SPOC. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This paper includes data that are publicly available from the MAST. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the NASA under the Exoplanet Exploration Program. This work makes use of observations from the LCO global telescope network. This paper is based on observations made with the MuSCAT3 instrument, developed by the Astrobiology Center (ABC) and under financial supports by JSPS KAKENHI (JP18H05439) and JST PRESTO (JPMJPR1775), at FTN on Maui, HI, operated by the LCO, and observations made with the MuSCAT2 instrument, developed by ABC, at TCS operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide. This paper is partially based on observations made at the CMO SAI MSU with the support by M.V. Lomonosov Moscow State University Program of Development. This work is partly supported by JSPS KAKENHI Grant Numbers 22000005, JP15H02063, JP17H04574, JP18H05439, JP18H05442, JP20K14518, Grant-in-Aid for JSPS Fellows Grant Number JP20J21872, JST PRESTO Grant Number JPMJPR1775, and the ABC of National Institutes of Natural Sciences (Grant Number AB031010). This work is partly financed by the Spanish Ministry of Economics and Competitiveness through grants PGC2018-098153-B-C31 and PID2019-109522GB-C53. A.A.B., B.S.S., and I.A.S. acknowledge the support of Ministry of Science and Higher Education of the Russian Federation under the grant 075-15-2020-780 (N13.1902.21.0039). N.C.B. and G.M. acknowledge the funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 694513 and under the Marie Skłodowska-Curie grant agreement No. 895525, respectively. J.K. acknowledges the support of the Swedish National Space Agency (SNSA; DNR 2020-00104). R.L. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación, through project PID2019-109522GB-C52/AEI/10.13039/501100011033, and the Centre of Excellence “Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

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, since 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$M_{\rm{J}}$ and 1.34$M_{\rm{J}}$. 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 $\rm{m\ s^{-1}}$ 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. Santo et al. 2017), which is probably a boundary of the giant planet population., 59 pages, 32 figures, 12 tables, accepted for publication in PASJ, all RVs will be available online as supplementary after the publication

We obtained high-resolution spectra of the ultra-cool M-dwarf TRAPPIST-1 during the transit of its planet `b' using two high dispersion near-infrared spectrographs, IRD instrument on the Subaru 8.2m telescope and HPF instrument on the 10m Hobby-Eberly Telescope. These spectroscopic observations are complemented by a photometric transit observation for planet `b' using the APO/ARCTIC, which assisted us to capture the correct transit times for our transit spectroscopy. Using the data obtained by the new IRD and HPF observations, as well as the prior transit observations of planets `b', `e' and `f' from IRD, we attempt to constrain the atmospheric escape of the planet using the He I triplet 10830 �� absorption line. We do not detect evidence for any primordial extended H-He atmospheres in all three planets. To limit any planet related absorption, we place an upper limit on the equivalent widths of, 11 pages; 4 figures; Accepted for publication in AJ

Detailed chemical analyses of M dwarfs are scarce but necessary to constrain the formation environment and internal structure of planets being found around them. We present elemental abundances of 13 M dwarfs (2900 < Teff < 3500 K) observed in the Subaru/IRD planet search project. They are mid-to-late M dwarfs whose abundance of individual elements has not been well studied. We use the high-resolution (~70,000) near-infrared (970-1750 nm) spectra to measure the abundances of Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Sr by the line-by-line analysis based on model atmospheres, with typical errors ranging from 0.2 dex for [Fe/H] to 0.3-0.4 dex for other [X/H]. We measure radial velocities from the spectra and combine them with Gaia astrometry to calculate the Galactocentric space velocities UVW. The resulting [Fe/H] values agree with previous estimates based on medium-resolution K-band spectroscopy, showing a wide distribution of metallicity (-0.6 < [Fe/H] < +0.4). The abundance ratios of individual elements [X/Fe] are generally aligned with the solar values in all targets. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin disk population, the most metal-poor object, GJ 699, could be a thick disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and Galactic stellar population of nearby M dwarfs., 31 pages, 16 figures, accepted for publication in AJ

Full list of authors: Soto, M. G.; Anglada-Escudé, G.; Dreizler, S.; Molaverdikhani, K.; Kemmer, J.; Rodríguez-López, C.; Lillo-Box, J.; Pallé, E.; Espinoza, N.; Caballero, J. A.; Quirrenbach, A.; Ribas, I.; Reiners, A.; Narita, N.; Hirano, T.; Amado, P. J.; Béjar, V. J. S.; Bluhm, P.; Burke, C. J.; Caldwell, D. A.; Charbonneau, D.; Cloutier, R.; Collins, K. A.; Cortés-Contreras, M.; Girardin, E.; Guerra, P.; Harakawa, H.; Hatzes, A. P.; Irwin, J.; Jenkins, J. M.; Jensen, E.; Kawauchi, K.; Kotani, T.; Kudo, T.; Kunimoto, M.; Kuzuhara, M.; Latham, D. W.; Montes, D.; Morales, J. C.; Mori, M.; Nelson, R. P.; Omiya, M.; Pedraz, S.; Passegger, V. M.; Rackham, B. V.; Rudat, A.; Schlieder, J. E.; Schöfer, P.; Schweitzer, A.; Selezneva, A.; Stockdale, C.; Tamura, M.; Trifonov, T.; Vanderspek, R.; Watanabe, D., One of the main objectives of the Transiting Exoplanet Survey Satellite (TESS) mission is the discovery of small rocky planets around relatively bright nearby stars. Here, we report the discovery and characterization of the transiting super-Earth planet orbiting LHS 1478 (TOI-1640). The star is an inactive red dwarf (J ∼ 9.6 mag and spectral type m3 V) with mass and radius estimates of 0.20 ± 0.01M· and 0.25 ± 0.01R· , respectively, and an effective temperature of 3381 ± 54 K. It was observed by TESS in four sectors. These data revealed a transit-like feature with a period of 1.949 days. We combined the TESS data with three ground-based transit measurements, 57 radial velocity (RV) measurements from CARMENES, and 13 RV measurements from IRD, determining that the signal is produced by a planet with a mass of 2.33-0.20+0.20 Mâ• and a radius of 1.24-0.05+0.05 Ra• . The resulting bulk density of this planet is 6.67 g cm-3, which is consistent with a rocky planet with an Fe-and MgSiO3-dominated composition. Although the planet would be too hot to sustain liquid water on its surface (its equilibrium temperature is about ∼595 K, suggesting aVenus-like atmosphere), spectroscopic metrics based on the capabilities of the forthcoming James Webb Space Telescope and the fact that the host star is rather inactive indicate that this is one of the most favorable known rocky exoplanets for atmospheric characterization. © 2021 ESO., CARMENES is an instrument at the Centro Astronómico Hispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Junta de Andalucía and the Instituto de Astrofísica de Andalucía (CSIC). CARMENES was funded by the Max-Planck-Gesellschaft (MPG), the Consejo Superior de Investigaciones Científicas (CSIC), the Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (ERDF) through projects FICTS-2011-02, ICTS-2017-07-CAHA-4, and CAHA16-CE-3978, and the members of the CARMENES Consortium (Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Landessternwarte Königstuhl, Institut de Ciències de l’Espai, Institut für Astrophysik Göttingen, Universidad Complutense de Madrid, Thüringer Landessternwarte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte, Centro de Astrobiología and Centro Astronómico Hispano-Alemán), with additional contributions by the MINECO, the Deutsche Forschungsgemeinschaft through the Major Research Instrumentation Programme and Research Unit FOR2544 “Blue Planets around Red Stars”, the Klaus Tschira Stiftung, the states of Baden-Württemberg and iedersachsen, and by the Junta de Andalucía. This paper included data collected by the TESS mission. Funding for the TESS mission is provided by the NASA’s Science Mission Directorate. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This work made use of observations from the LCOGT network. LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP), which is funded by the National Science Foundation. We acknowledge financial support from STFC grants ST/P000592/1 and ST/T000341/1, NASA grant NNX17AG24G, the Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades and the ERDF through projects PID2019-109522GB-C5[1:4]/AEI/10.13039/501100011033, PGC2018-098153-B-C3[1,3] and the Centre of Excellence “Severo Ochoa” and “María de Maeztu” awards to the Instituto de Astrofísica de Canarias (SEV-2015-0548), Instituto de Astrofísica de Andalucía (SEV-2017-0709), and Centro de Astrobiología (MDM-2017-0737), the Generalitat de Catalunya/CERCA programme, Grant-in-Aid for JSPS Fellows Grant (JP20J21872), JSPS KAKENHI Grant (22000005, JP15H02063,JP18H01265, JP18H05439, JP18H05442, JP19K14783), JST PRESTO Grant (JPMJPR1775), and University Research Support Grant from the National Astronomical Observatory of Japan.

We are developing a high-resolution near-infrared spectrograph for exoplanet searches with telescopes in Suther- land, South Africa; the South Africa Near-infrared Doppler (SAND) instrument. It covers the z- and Y -bands (0.83–1.10 µm) simultaneously with a maximum spectral resolution of 55, 000. This specification enables the precise radial velocity (RV) measurements to search for both giant planets around young stars and habitable planets around nearby M-dwarfs. The SAND is a fiber-fed instrument and, therefore, can be installed into several telescopes at the same site by changing the fiber connection. It will be operated mainly on the PRIME telescope, which is newly developed by Osaka University in Japan. The InfraRed Survey Facility (IRSF) and the Southern African Large Telescope (SALT) are also candidate telescopes for the installation. Inside the SAND spectrograph, a star image ejected from the fiber is sliced half and each sliced image is spectrally dispersed by an echelle grating. As a reference source for the wavelength calibration, light transmitted from a Fabry-Perot Etalon is simultaneously injected into the spectrograph and share the almost identical optical pass. The SAND is expected to realize RV-measurements for targets in large stellar associations at the southern sky, with exclusive and flexible use of the telescope time.

We present the optical spectra of 338 nearby M dwarfs, and compute their spectral types, effective temperatures ($T_{\mathrm{eff}}$), and radii. Our spectra have been obtained using several optical spectrometers with spectral resolutions that range from 1200 to 10000. As many as 97% of the observed M-type dwarfs have a spectral type of M3-M6, with a typical error of 0.4 sub-type, among which the spectral types M4-M5 are the most common. We infer the $T_{\mathrm{eff}}$ of our sample by fitting our spectra with theoretical spectra from the PHOENIX model. Our inferred $T_{\mathrm{eff}}$ is calibrated with the optical spectra of M dwarfs whose $T_{\mathrm{eff}}$ have been well determined with the calibrations that are supported by previous interferometric observations. Our fitting procedures utilize the VO absorption band (7320-7570 {\AA}) and the optical region (5000-8000 {\AA}), yielding typical errors of 128 K (VO band) and 85 K (optical region). We also determine the radii of our sample from their spectral energy distributions (SEDs). We find most of our sample stars have radii of $, Comment: 24 pages, 17 figures, 11 tables. Accepted to be published in Publications of the Astronomical Society of Japan. Please contact us if you need complete version of long tables

We report radial-velocity search for short-period planets in the Pleiades open cluster. We observed 30 Pleiades member stars at the Okayama Astrophysical Observatory (OAO) with High Dispersion Echelle Spectrograph (HIDES). To evaluate and mitigate the effects of stellar activity on radial-velocity measurements, we computed four activity indicators (FWHM, $V_{\rm span}$, $W_{\rm span}$ and $S_{\rm H{\alpha}}$). Among our sample, no short-period planet candidates were detected. Stellar intrinsic RV jitter was estimated to be ${\rm 52\ m\,s^{-1}}$, ${\rm 128\ m\,s^{-1}}$ and ${\rm 173\ m\,s^{-1}}$ for stars with $v\sin i$ of ${\rm 10\ km\,s^{-1}}$, ${\rm 15\ km\,s^{-1}}$ and ${\rm 20\ km\,s^{-1}}$, respectively. We determined the planet occurrence rate from our survey and set the upper limit to 11.4\% for the planets with masses 1--13 $M_{\rm JUP}$ and period 1--10 days. To set a more stringent constraint on the planet occurrence rate, we combined the result of our survey with those of other surveys targeting open clusters with ages between 30--300 Myr. As a result, the planet occurrence rate in young open clusters was found to be less than 7.4\%, 2.9\% and 1.9\% for the planets with an orbital period of three days and masses between 1--5, 5--13, and 13--80 $M_{\rm JUP}$, respectively., Comment: 54 pages, 39 figures, accepted for publication in PASJ

Precision radial velocity (RV) measurements in the near-infrared are a powerful tool to detect and characterize exoplanets around low-mass stars or young stars with higher magnetic activity. However, the presence of strong telluric absorption lines and emission lines in the near infrared that significantly vary in time can prevent extraction of RV information from these spectra by classical techniques, which ignore or mask the telluric lines. We present a methodology and pipeline to derive precision RVs from near-infrared spectra using a forward-modeling technique. We applied this to spectra with a wide wavelength coverage (Y, J, and H bands, simultaneously), taken by the InfraRed Doppler (IRD) spectrograph on the Subaru 8.2-m telescope. Our pipeline extracts the instantaneous instrumental profile of the spectrograph for each spectral segment, based on a reference spectrum of the laser-frequency comb that is injected into the spectrograph simultaneously with the stellar light. These profiles are used to derive the intrinsic stellar template spectrum, which is free from instrumental broadening and telluric features, as well as model and fit individual observed spectra in the RV analysis. Implementing a series of numerical simulations using theoretical spectra that mimic IRD data, we test the pipeline and show that IRD can achieve $ 100$ per pixel at 1000 nm. Dependences of RV precision on various stellar parameters (e.g., $T_{\rm eff}$, $v\sin i$, [Fe/H]) and the impact of telluric-line blendings on the RV accuracy are discussed through the mock spectra analyses. We also apply the RV-analysis pipeline to the observed spectra of GJ 699 and TRAPPIST-1, demonstrating that the spectrograph and the pipeline are capable of an RV accuracy of $, 21 pages, 20 figures; accepted for publication in PASJ; typos corrected

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: $J=9.5-9.6$) hosting ultra-short period (USP) planets, identified by the TESS mission. The two stars are similar in temperature, mass, and radius ($T_\mathrm{eff}\,\approx\,3500$ K, $M_\star\,\approx\,0.45-0.46\,M_\odot$, and $R_\star\approx 0.45-0.46\,R_\odot$), and the planets are both super-Earth-sized ($1.25\,R_\oplus, Comment: 29 pages, 21 figures, 4 tables; AJ in press

In an effort to measure the Rossiter-McLaughlin effect for the TRAPPIST-1 system, we performed high-resolution spectroscopy during transits of planets e, f, and b. The spectra were obtained with the InfraRed Doppler spectrograph on the Subaru 8.2-m telescope, and were supplemented with simultaneous photometry obtained with a 1-m telescope of the Las Cumbres Observatory Global Telescope. By analyzing the anomalous radial velocities, we found the projected stellar obliquity to be $\lambda=1\pm 28$ degrees under the assumption that the three planets have coplanar orbits, although we caution that the radial-velocity data show correlated noise of unknown origin. We also sought evidence for the expected deformations of the stellar absorption lines, and thereby detected the "Doppler shadow" of planet b with a false alarm probability of $1.7\,\%$. The joint analysis of the observed residual cross-correlation map including the three transits gave $\lambda=19_{-15}^{+13}$ degrees. These results indicate that the the TRAPPIST-1 star is not strongly misaligned with the common orbital plane of the planets, although further observations are encouraged to verify this conclusion., Comment: 9 pages, 3 figures, accepted for publication in ApJ Letters

M dwarfs are prominent targets of planet search projects, and their chemical composition is crucial to understanding the formation process or interior of orbiting exoplanets. However, measurements of elemental abundances of M dwarfs have been limited due to difficulties in the analysis of their optical spectra. We conducted a detailed chemical analysis of five M dwarfs (T_eff ~3200--3800 K), which form binary systems with G/K-type stars, by performing a line-by-line analysis based on high-resolution (R ~80,000) near-infrared (960--1710 nm) spectra obtained with CARMENES. We determined the chemical abundances of eight elements (Na, Mg, K, Ca, Ti, Cr, Mn, and Fe), which are in agreement with those of the primary stars within measurement errors (~0.2 dex). Through the analysis process, we investigated the unique behavior of atomic lines in a cool atmosphere. Most atomic lines are sensitive to changes in abundance not only of the corresponding elements but also of other elements, especially dominant electron donors such as Na and Ca. The Ti I lines show a negative correlation with the overall metallicity at T_eff < 3400 K due to the consumption of neutral titanium by the formation of TiO molecules. These findings indicate that to correctly estimate the overall metallicity or the abundance of any element, we need to determine the abundances of other individual elements consistently., 19 pages, 11 figures, accepted for publication in PASJ

We obtained spectra of the pre-main sequence star AU Microscopii during a transit of its Neptune-sized planet to investigate its orbit and atmosphere. We used the high-dispersion near-infrared spectrograph IRD on the Subaru telescope to detect the Doppler "shadow" from the planet and constrain the projected stellar obliquity. Modeling of the observed planetary Doppler shadow suggests a spin-orbit alignment of the system ($\lambda=-4.7_{-6.4}^{+6.8}$ degrees), but additional observations are needed to confirm this finding. We use both the IRD data and spectra obtained with NIRSPEC on Keck-II to search for absorption in the 1083 nm line of metastable triplet He I by the planet's atmosphere and place an upper limit for the equivalent width of 3.7 m\AA at 99 $\%$ confidence. With this limit and a Parker wind model we constrain the escape rate from the atmosphere to $, Comment: 10 pages, 5 figures, accepted for publication in ApJ Letters

Nitrogen abundances and carbon isotope ratios (12C/13C) in the atmospheres of red giants are known to be influenced by dredge-up of H-burning products and serve as useful probes to study the nature of evolution-induced envelope mixing. We determined the [N/Fe] and 12C/13C ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the 12CN and 13CN lines in the ~8002-8005A region, with an aim to investigate how these quantities are related to other similar mixing-affected indicators which were already reported in our previous work. It was confirmed that [N/Fe] values are generally supersolar (typically by several tenths dex though widely differ from star to star), anti-correlated with [C/Fe], and correlated with [Na/Fe], as expected from theory. As seen from their dependence upon stellar parameters, it appears that mixing tends to be enhanced with an increase of stellar luminosity (or mass) and rotational velocity, which is also reasonable from the theoretical viewpoint. In contrast, the resulting 12C/13C ratios turned out to be considerably diversified in the range of ~5-50 (with a peak around ~20), without showing any systematic dependence upon C or N abundance anomalies caused by the mixing of CN-cycled material. It thus appears that our understanding on the photospheric 12C/13C ratios in red giants is still incomplete, for which more observational studies would be required., Comment: 21 pages, 13 figures, 2 tables, and ancillary data (online material); accepted for publication in Publ. Astron. Soc. Japan

The star S0-2, orbiting the Galactic central massive black hole candidate Sgr A*, passed its pericenter in May 2018. This event is the first chance to detect the general relativistic (GR) effect of a massive black hole, free from non-gravitational physics. The observable GR evidence in the event is the difference between the GR redshift and the Newtonian redshift of photons coming from S0-2. Within the present observational precision, the 1st post-Newtonian (1PN) GR evidence is detectable. In this paper, we give a theoretical analysis of the time evolution of the 1PN GR evidence, under a presupposition that is different from used in previous papers. Our presupposition is that the GR/Newtonian redshift is always calculated with the parameter values (the mass of Sgr A*, the initial conditions of S0-2, and so on) determined by fitting the GR/Newtonian motion of S0-2 with the observational data. It is then revealed that the difference of the GR redshift and the Newtonian one shows two peaks before and after the pericenter passage. This double-peak-appearance is due to our presupposition, and reduces to a single peak if the same parameter values are used in both GR and Newtonian redshifts as considered in previous papers. In addition to this theoretical discussion, we report our observational data obtained with the Subaru telescope by 2018. The quality and the number of Subaru data in 2018 are not sufficient to confirm the detection of the double-peak-appearance., Comment: Accepted for publication in PASJ (Publications of the Astronomical Society of Japan), 23 pages, 11 figures, 6 tables

The Infrared Doppler (IRD) instrument is a fiber-fed high-resolution NIR spectrometer for the Subaru telescope covering the Y,J,H-bands simultaneously with a maximum spectral resolution of 70,000. The main purpose of IRD is a search for Earth-mass planets around nearby M-dwarfs by precise radial velocity measurements, as well as a spectroscopic characterization of exoplanet atmospheres. We report the current status of the instrument, which is undergoing commissioning at the Subaru Telescope, and the first light observation successfully done in August 2017. The general description of the instrument will be given including spectrometer optics, fiber injection system, cryogenic system, scrambler, and laser frequency comb. A large strategic survey mainly focused on late-type M-dwarfs is planned to start from 2019.

The InfraRed Doppler (IRD) instrument is a high-dispersion spectrograph that is available on Subaru Telescope to explore extrasolar planets via infrared radial velocity (RV) observations. The Subaru/IRD is especially useful in the search of a low-mass planet around cool M-type dwarfs for which infrared RV observations are essential. We report our early performance tests for IRD. IRD’s two H2RG detectors have been evaluated with our detector readout technique, ensuring that their readout noise is made sufficiently smaller than the stellar photon noise expected in our planned survey. We have also tested the instrumental stability of RV measurements from the laboratory data obtained with the IRD’s calibration systems including a laser frequency comb (LFC). Among our tested three types of velocity stability, the stability of comb spectra obtained with a multi-mode fiber (MMF) relative to that with another MMF is measured to be ∼1 m s−1. We also infer from these tests that stellar RV measurements with an MMF can be calibrated with a short-term stability of 2 m s−1 or better by the simultaneously-observed reference spectra of LFC. Furthermore, we report preliminary on-sky RV measurements calibrated with a Thorium-Argon hollow-cathode lamp for RV-stable stars (τ Ceti and Barnard's star) and a planet-host (51 Pegasi). These preliminary RV measurements help the further performance test of IRD that will be performed by the on-sky observations with LFC.

We report the detection of planets around two evolved giant stars from radial velocity measurements at Okayama Astrophysical observatory. 24 Boo (G3IV) has a mass of $0.99\,M_{\odot}$, a radius of $10.64\,R_{\odot}$, and a metallicity of ${\rm [Fe/H]}=-0.77$. The star hosts one planet with a minimum mass of $0.91\,M_{\rm Jup}$ and an orbital period of $30.35{\rm d}$. The planet has one of the shortest orbital periods among those ever found around evolved stars by radial-veloocity methods. The stellar radial velocities show additional periodicity with $150{\rm d}$, which are probably attributed to stellar activity. The star is one of the lowest-metallicity stars orbited by planets currently known. $\gamma$ Lib (K0III) is also a metal-poor giant with a mass of $1.47\,M_{\odot}$, a radius of $11.1\,R_{\odot}$, and ${\rm [Fe/H]}=-0.30$. The star hosts two planets with minimum masses of $1.02M_{\rm Jup}$ and $4.58\,M_{\rm Jup}$, and periods of $415{\rm d}$ and $964{\rm d}$, respectively. The star has the second lowest metallicity among the giant stars hosting more than two planets. Dynamical stability analysis for the $\gamma$ Lib system sets a minimum orbital inclination angle to be about $70^{\circ}$ and suggests that the planets are in 7:3 mean-motion resonance, though the current best-fitted orbits to the radial-velocity data are not totally regular., Comment: 29 pages, 19 figures, accepted for publication in PASJ

During the next closest approach of the orbiting star S2/S0-2 to the Galactic supermassive black hole (SMBH), it is estimated that radial velocity (RV) measurements with an uncertainty of a few 10 km s −1 will allow us to detect post-Newtonian effects throughout 2018. To evaluate the best achievable uncertainty in RV and its stability, we carried out near-infrared, high-resolution (R ∼ 20000) spectroscopic monitoring observations of S2 using the Subaru telescope and its near-infrared spectrograph IRCS from 2014 to 2016. Br-γ absorption lines have been detected in the 2015 and 2016 spectra, but have not been clearly detected in the 2014 spectrum. The detected Br-γ absorption lines are used to determine the RVs of S2. The statistical uncertainties are derived using the jackknife analysis, and spectra combined from divided subdata sets. The wavelength calibrations in our three-year monitoring are stable: short-term (hours to days) uncertainties in RVs are 0.5 km s −1 , and the long-term (three years) uncertainty is 1.2 km s −1 . We thoroughly analyzed possible sources of systematic uncertainties, such as the incomplete subtraction of OH skylines. The relevant uncertainties are estimated to be less than several km s −1 . The final results using the Br-γ line are 877 ± 25 km s −1 in 2015, and 1109 ± 14 km s −1 in 2016. When we use two He I lines at 2.113 μm in addition to Br-γ , the mean RV and its standard error are 1114 km s −1 and 5 km s −1 , respectively, in 2016. However, we have found a larger scatter around the expected RV curve with the best-fitting orbiting parameters of S2, implying additional uncertainties not yet considered. The difference between the RVs estimated by Newtonian mechanics and general relativity will reach about 200 km s −1 near the next pericenter passage in 2018. Therefore, in addition to astrometric and spectroscopic data obtained with other telescopes, RV measurements with Subaru in 2018 will form important data sets with which to detect general relativistic effects from the SMBH. © The Author(s) 2018. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved., We thank the Subaru Telescope staff for the support for our observations. This work was supported by JSPS KAKENHI, Grant-in-Aid for Young Scientists (A) 25707012, Grant-in-Aid for Challenging Exploratory Research 15K13463. H. S. was supproted by KAKENHI Grant-in-Aid for Challenging Exploratory Research 26610050. Y.T. was supproted by KAKENHI Grant-in-Aid for Young Scientists (B) 26800150. M.T. was supported by KAKENHI Grant Number 17K05439 and DAIKO FOUNDATION. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no[614922]. FN acknowledges financial support through Spanish grants ESP2015-65597-C4-1-R and ESP2017-86582-C4-1-R (MINECO/FEDER).

Aims. The purpose of this paper is to detect and investigate the nature of long-term radial velocity (RV) variations of K-type giants and to confirm planetary companions around the stars. Methods. We have conducted two planet search programs by precise RV measurement using the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) and the 1.88 m telescope at Okayama Astrophysical Observatory (OAO). The BOAO program searches for planets around 55 early K giants. The OAO program is looking for 190 G-K type giants. Results. In this paper, we report the detection of long-period RV variations of three K giant stars, HD 40956, HD 111591, and HD 113996. We investigated the cause of the observed RV variations and conclude the substellar companions are most likely the cause of the RV variations. The orbital analyses yield P = 578.6 $\pm$ 3.3 d, $m$ sin $i$ = 2.7 $\pm$ 0.6 $M_{\rm{J}}$, $a$ = 1.4 $\pm$ 0.1 AU for HD 40956; P = 1056.4 $\pm$ 14.3 d, $m$ sin $i$ = 4.4 $\pm$ 0.4 $M_{\rm{J}}$, $a$ = 2.5 $\pm$ 0.1 AU for HD 111591; P = 610.2 $\pm$ 3.8 d, $m$ sin $i$ = 6.3 $\pm$ 1.0 $M_{\rm{J}}$, $a$ = 1.6 $\pm$ 0.1 AU for HD 113996., Comment: 10 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics

Sulfur and zinc are chemically volatile elements, which play significant roles as depletion-free tracers in studying galactic chemical evolution. However, regarding red giants having evolved off the main sequence, reliable abundance determinations of S and Zn seem to be difficult despite that a few studies have been reported so far. Given this situation, we tried to establish the abundances of these elements for an extensive sample of 239 field GK giants (-0.8 < [Fe/H] < +0.2), by applying the spectrum-fitting technique to S I 8694-5, S I 6757, and Zn I 6362 lines and by taking into account the non-LTE effect. Besides, similar abundance analysis was done for 160 FGK dwarfs to be used for comparison. The non-LTE corrections for the S and Zn abundances derived from these lines turned out < 0.1(-0.2) dex for most cases and not very significant. It revealed that the S I 6757 feature is more reliable as an abundance indicator than S I 8694-5 for the case of red giants, because the latter suffers blending of unidentified lines. The finally resulting [S/Fe]-[Fe/H] and [Zn/Fe]-[Fe/H] relations for GK giants were confirmed to be in good agreement with those for FGK dwarfs, indicating that S and Zn abundances of red giants are reliably determinable from the S I 6757 and Zn I 6362 lines. Accordingly, not only main-sequence stars but also evolved red giant stars are usable for tracing the chemical evolution history of S and Zn in the regime of disk metallicity by using these lines., 20 pages, 8 figures, 3 tables (along with 2 electronic data tables), accepted for publication in Publ. Astron. Soc. Japan

A radial velocity (RV) survey for intermediate-mass giants has been operated for over a decade at Okayama Astrophysical Observatory (OAO). The OAO survey has revealed that some giants show long-term linear RV accelerations (RV trends), indicating the presence of outer companions. Direct imaging observations can help clarify what objects generate these RV trends. We present the results of high-contrast imaging observations or six intermediate-mass giants with long-term RV trends using the Subaru Telescope and HiCIAO camera. We detected co-moving companions to $\gamma$ Hya B ($0.61^{+0.12}_{-0.14} M_\odot$), HD 5608 B ($0.10 \pm 0.01 M_\odot$), and HD 109272 B ($0.28 \pm 0.06 M_\odot$). For the remaining targets($\iota$ Dra, 18 Del, and HD 14067) we exclude companions more massive than 30-60 $M_\mathrm{Jup}$ at projected separations of 1arcsec-7arcsec. We examine whether these directly imaged companions or unidentified long-period companions can account for the RV trends observed around the six giants. We find that the Kozai mechanism can explain the high eccentricity of the inner planets $\iota$ Dra b, HD 5608 b, and HD 14067 b., Comment: 33 pases, 9 figures, accepted to ApJ

We report the discovery of a substellar companion around the intermediatemass giant HD 175679. Precise radial velocity data of the star from Xinglong Station and Okayama Astrophysical Observatory (OAO) revealed a Keplerian velocity variation with an orbital period of 1366.8 \pm 5.7 days, a semiamplitude of 380.2 \pm 3.2m s.1, and an eccentricity of 0.378 \pm 0.008. Adopting a stellar mass of 2.7 \pm 0.3 M\odot, we obtain the minimum mass of the HD 175679 b is 37.3 \pm 2.8 MJ, and the semimajor axis is 3.36 \pm 0.12 AU. This discovery is the second brown dwarf companion candidate from a joint planet-search program between China and Japan., 10 pages, 3 figures, RAA accepted

We report the detection of a large mass planet orbiting around the K0 metal-rich subgiant HD38801 (V = 8.26) by precise radial velocity (RV) measurements from the Subaru Telescope and the Keck Telescope. The star has a mass of 1.36 M_☉ and a metallicity of [Fe/H] = +0.26. The RV variations are consistent with a circular orbit with a period of 696.0 days and a velocity semiamplitude of 200.0 m s^(–1), which yield a minimum mass for the companion of 10.7 M JUP and a semimajor axis of 1.71 AU. Such super-massive objects with very low eccentricities and periods of hundreds of days are uncommon among the ensemble of known exoplanets.

We report the detection of a substellar companion orbiting the G5 dwarf HD 16760 from the N2K sample. Precise Doppler measurements of the star from Subaru and Keck revealed a Keplerian velocity variation with a period of 466.47+-0.35 d, a semiamplitude of 407.71+-0.84 m/s, and an eccentricity of 0.084+-0.003. Adopting a stellar mass of 0.78+-0.05 M_Sun, we obtain a minimum mass for the companion of 13.13+-0.56 M_JUP, which is close to the planet/brown-dwarf transition, and the semimajor axis of 1.084+-0.023 AU. The nearly circular orbit despite the large mass and intermediate orbital period makes this companion unique among known substellar companions., Comment: 14 pages, 2 figures, accepted for publication in ApJ

We report on the detection of four extrasolar planets orbiting evolved intermediate-mass stars from a precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. All of the host stars are considered to be formerly early F-type or A-type dwarfs when they were on the main sequence. 14 And (K0 III) is a clump giant with a mass of 2.2 M_solar and has a planet of minimum mass m_2sin i=4.8 M_Jup in a nearly circular orbit with a 186 day period. This is one of the innermost planets around evolved intermediate-mass stars and such planets have only been discovered in clump giants. 81 Cet (G5 III) is a clump giant with 2.4 M_solar hosting a planet of m_2sin i=5.3 M_Jup in a 953 day orbit with an eccentricity of e=0.21. 6 Lyn (K0 IV) is a less evolved subgiant with 1.7 M_solar and has a planet of m_2sin i=2.4 M_Jup in a 899 day orbit with e=0.13. HD 167042 (K1 IV) is also a less evolved star with 1.5 M_solar hosting a planet of m_2sin i=1.6 M_Jup in a 418 day orbit with e=0.10. This planet was independently announced by Johnson et al. (2008, ApJ, 675, 784). All of the host stars have solar or sub-solar metallicity, which supports the lack of metal-rich tendency in planet-harboring giants in contrast to the case of dwarfs., Comment: 23 pages, 6 figures, accepted for publication in PASJ

We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at Okayama Astrophysical Observatory, Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81+-0.13M_sun, a radius of 7.30+-0.33R_sun, and solar metallicity. The planetary system is composed of two giant planets with minimum mass of 1.75^{+0.20}_{-0.17}Mjup and 1.86^{+0.16}_{-0.15}Mjup, orbital period of 363.3^{+2.5}_{-2.4} d and 684.7^{+5.0}_{-4.9} d, and eccentricity of 0.089^{+0.079}_{-0.060} and 0.278^{+0.067}_{-0.094}, respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ~0.15, which is just within 1.4sigma of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160 degree., 11 pages, 9 figures, accepted for publication in ApJ