arXiv subjects

Bun'ei Sato

Publications and source records attributed to Bun'ei Sato.

At least 55 records · Page 3Linked to original sources

arXiv2016

A Pair of Giant Planets around the Evolved Intermediate-Mass Star HD 47366: Multiple Circular Orbits or a Mutually Retrograde Configuration

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.

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arXiv2015

Five New Exoplanets Orbiting Three Metal-Rich, Massive Stars: Two-Planet Systems Including Long-Period Planets, and an Eccentric Planet

We report detections of new exoplanets from a radial velocity (RV) survey of metal-rich FGK stars by using three telescopes. By optimizing our RV analysis method to long time-baseline observations, we have succeeded in detecting five new Jovian-planets around three metal-rich stars HD 1605, HD 1666, and HD 67087 with the masses of $1.3 M_{\odot}$, $1.5 M_{\odot}$, and $1.4 M_{\odot}$, respectively. A K1 subgiant star HD 1605 hosts two planetary companions with the minimum masses of $ M_p \sin i = 0.96 M_{\mathrm{JUP}}$ and $3.5 M_{\mathrm{JUP}}$ in circular orbits with the planets' periods $P = 577.9$ days and $2111$ days, respectively. HD 1605 shows a significant linear trend in RVs. Such a system consisting of Jovian planets in circular orbits has rarely been found and thus HD 1605 should be an important example of a multi-planetary system that is likely unperturbed by planet-planet interactions. HD 1666 is a F7 main sequence star which hosts an eccentric and massive planet of $ M_p \sin i = 6.4 M_{\mathrm{JUP}}$ in the orbit with $a_{\rm p} = 0.94$ AU and an eccentricity $e=0.63$. Such an eccentric and massive planet can be explained as a result of planet-planet interactions among Jovian planets. While we have found the large residuals of $\mathrm{rms} = 35.6\ \mathrm{m\ s^{-1}}$, the periodogram analysis does not support any additional periodicities. Finally, HD 67087 hosts two planets of $ M_p \sin i = 3.1 M_{\mathrm{JUP}}$ and $4.9 M_{\mathrm{JUP}}$ in orbits with $P=352.2$ days and $2374$ days, and $e=0.17$ and $0.76$, respectively. Although the current RVs do not lead to accurate determinations of its orbit and mass, HD 67087 c can be one of the most eccentric planets ever discovered in multiple systems.

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arXiv2015

HAT-P-50b, HAT-P-51b, HAT-P-52b, and HAT-P-53b: Three Transiting Hot Jupiters and a Transiting Hot Saturn From the HATNet Survey

We report the discovery and characterization of four transiting exoplanets by the HATNet survey. The planet HAT-P-50b has a mass of 1.35 M_J and a radius of 1.29 R_J, and orbits a bright (V = 11.8 mag) M = 1.27 M_sun, R = 1.70 R_sun star every P = 3.1220 days. The planet HAT-P-51b has a mass of 0.31 M_J and a radius of 1.29 R_J, and orbits a V = 13.4 mag, M = 0.98 M_sun, R = 1.04 R_sun star with a period of P = 4.2180 days. The planet HAT-P-52b has a mass of 0.82 M_J and a radius of 1.01 R_J, and orbits a V = 14.1 mag, M = 0.89 M_sun, R = 0.89 R_sun star with a period of P = 2.7536 days. The planet HAT-P-53b has a mass of 1.48 M_J and a radius of 1.32 R_J, and orbits a V = 13.7 mag, M = 1.09 M_sun, R = 1.21 R_sun star with a period of P = 1.9616 days. All four planets are consistent with having circular orbits and have masses and radii measured to better than 10% precision. The low stellar jitter and favorable R_P/R_star ratio for HAT-P-51 make it a promising target for measuring the Rossiter-McLaughlin effect for a Saturn-mass planet.

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arXiv2015

Precise Radial Velocity Measurements for Kepler Giants Hosting Planetary Candidates: Kepler-91 and KOI-1894

We present results of radial-velocity follow-up observations for the two Kepler evolved stars Kepler-91 (KOI-2133) and KOI-1894, which had been announced as candidates to host transiting giant planets, with the Subaru 8.2m telescope and the High Dispersion Spectrograph (HDS). By global modeling of the high-precision radial-velocity data taken with Subaru/HDS and photometric ones taken by Kepler mission taking account of orbital brightness modulations (ellipsoidal variations, reflected/emitted light, etc.) of the host stars, we independently confirmed that Kepler-91 hosts a transiting planet with a mass of 0.66 M_Jup (Kepler-91b), and newly detected an offset of ~20 m s$^{-1}$ between the radial velocities taken at ~1-yr interval, suggesting the existence of additional companion in the system. As for KOI-1894, we detected possible phased variations in the radial velocities and light curves with 2--3 sigma confidence level which could be explained as a reflex motion and ellipsoidal variation of the star caused by the transiting sub-saturn-mass (~0.18 M_Jup) planet.

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arXiv2014

Can surface oxygen abundances of red giants be explained by the canonical mixing theory?

Extensive oxygen abundance determinations were carried out for 239 late-G/early-K giant stars of 1.5-5 M_sun by applying the spectrum-fitting technique to O I 7771-5 and [O I] 6300/6363 lines based on the high-dispersion spectra in the red region newly obtained at Okayama Astrophysical Observatory. Our main purpose was to clarify whether any significantly large (<~0.4-0.5 dex) O-deficit really exists in these evolved stars, which was once suspected by Takeda et al. (2008, PASJ, 60, 781) from the analysis of the [O I] 5577 line, since it (if real) is inexlainable by the current theory and may require the necessity of special non-canonical deep mixing in the envelope. We found, however, that the previous [O/H]_5577 results (differential abundances relative to the sun) were systematically underestimated compared to the more reliable [O/H]_7773 (from O I 7771-5 triplet lines) or [O/H]_6300 (from [O I] 6300 line) obtained in this study. Comparing the updated [O/Fe] ratios with the theoretically predicted surface abundance changes caused by mixing of nuclear-processed products dredged-up from the interior, we concluded that the oxygen deficiency in these red giants is insignificantly marginal (only by <~0.1 dex), which does not contradict the expectation from the recent theoretical simulation. This consequence of reasonable consistency between theory and observation also applies to the extent of peculiarity in [C/Fe] and [Na/Fe], which were also examined by reanalyzing the previous equivalent-width data of C I 5052/5380 and Na I 6160 lines.

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arXiv2014

Global Analysis of KOI-977: Spectroscopy, Asteroseismology, and Phase-curve Analysis

We present a global analysis of KOI-977, one of the planet host candidates detected by {\it Kepler}. Kepler Input Catalog (KIC) reports that KOI-977 is a red giant, for which few close-in planets have been discovered. Our global analysis involves spectroscopic and asteroseismic determinations of stellar parameters (e.g., mass and radius) and radial velocity (RV) measurements. Our analyses reveal that KOI-977 is indeed a red giant in the red clump, but its estimated radius ($\gtrsim 20R_\odot=0.093$ AU) is much larger than KOI-977.01's orbital distance ($\sim 0.027$ AU) estimated from its period ($P_\mathrm{orb}\sim 1.35$ days) and host star's mass. RV measurements show a small variation, which also contradicts the amplitude of ellipsoidal variations seen in the light-curve folded with KOI-977.01's period. Therefore, we conclude that KOI-977.01 is a false positive, meaning that the red giant, for which we measured the radius and RVs, is different from the object that produces the transit-like signal (i.e., an eclipsing binary). On the basis of this assumption, we also perform a light-curve analysis including the modeling of transits/eclipses and phase-curve variations, adopting various values for the dilution factor $D$, which is defined as the flux ratio between the red giant and eclipsing binary. Fitting the whole folded light-curve as well as individual transits in the short cadence data simultaneously, we find that the estimated mass and radius ratios of the eclipsing binary are consistent with those of a solar-type star and a late-type star (e.g., an M dwarf) for $D\gtrsim 20$.

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arXiv2014

Solar rotation inferred from radial velocities of the sun-as-a-star during the 2012 May 21 eclipse

With an aim to examine how much information of solar rotation can be obtained purely spectroscopically by observing the sun-as-a-star during the 2012 May 21 eclipse at Okayama Astrophysical Observatory, we studied the variation of radial velocities (V_r), which were derived by using the iodine-cell technique based on a set of 184 high-dispersion spectra consecutively obtained over the time span of ~4 hours. The resulting V_r(t) was confirmed to show the characteristic variation (Rossiter-McLaughlin effect) caused by time-varying visibility of the solar disk. By comparing the observed V_r(t) curve with the theoretical ones, which were simulated with the latitude (psi) dependent solar rotation law omega(psi) = A + B sin^2(psi) (deg/day), we found that the relation B = -5.5 A + 77 gives the best fit, though separate determinations of A and B were not possible. Since this relationship is consistent with the real values known for the sun (A = 14.5, B = -2.8), we may state that our analysis yielded satisfactory results. This consequence may provide a prospect of getting useful information on stellar rotation of eclipsing binaries from radial-velocity studies during eclipse, if many spectra of sufficiently high time-resolution are available.

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arXiv2014

The Transiting Exoplanet Survey Satellite

The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I = 4-13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.

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arXiv2014

The lithium abundances for a large sample of red giants

The lithium abundances for 378 G/K giants are derived with non-LTE correction considered. Among these, there are 23 stars that host planetary systems. The lithium abundance is investigated, as a function of metallicity, effective temperature, and rotational velocity, as well as the impact of a giant planet on G/K giants. The results show that the lithium abundance is a function of metallicity and effective temperature. The lithium abundance has no correlation with rotational velocity at vsini $<$ 10 km s$^{-1}$. Giants with planets present lower lithium abundance and slow rotational velocity (vsini $<$ 4 km s$^{-1}$). Our sample includes three Li-rich G/K giants, 36 Li-normal stars and 339 Li-depleted stars. The fraction of Li-rich stars in this sample agrees with the general rate of less than 1$\%$ in literature, and the stars that show normal amounts of Li are supposed to possess the same abundance at the current interstellar medium. For the Li-depleted giants, Li deficiency may have already taken place at the main sequence stage for many intermediate-mass (1.5-5 M$_{\odot}$) G/K giants. Finally, we present the lithium abundance and kinematic parameters for an enlarged sample of 565 giants using a compilation of literature, and confirm that the lithium abundance is a function of metallicity and effective temperature. With the enlarged sample, we investigate the differences between the lithium abundance in thin-/thick-disk giants, which indicate that the lithium abundance in thick-disk giants is more depleted than that in thin-disk giants.

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arXiv2013

A Discovery of a Candidate Companion to a Transiting System KOI-94: A Direct Imaging Study for a Possibility of a False Positive

We report a discovery of a companion candidate around one of {\it Kepler} Objects of Interest (KOIs), KOI-94, and results of our quantitative investigation of the possibility that planetary candidates around KOI-94 are false positives. KOI-94 has a planetary system in which four planetary detections have been reported by {\it Kepler}, suggesting that this system is intriguing to study the dynamical evolutions of planets. However, while two of those detections (KOI-94.01 and 03) have been made robust by previous observations, the others (KOI-94.02 and 04) are marginal detections, for which future confirmations with various techniques are required. We have conducted high-contrast direct imaging observations with Subaru/HiCIAO in $H$ band and detected a faint object located at a separation of $\sim0.6''$ from KOI-94. The object has a contrast of $\sim 1\times 10^{-3}$ in $H$ band, and corresponds to an M type star on the assumption that the object is at the same distance of KOI-94. Based on our analysis, KOI-94.02 is likely to be a real planet because of its transit depth, while KOI-94.04 can be a false positive due to the companion candidate. The success in detecting the companion candidate suggests that high-contrast direct imaging observations are important keys to examine false positives of KOIs. On the other hand, our transit light curve reanalyses lead to a better period estimate of KOI-94.04 than that on the KOI catalogue and show that the planetary candidate has the same limb darkening parameter value as the other planetary candidates in the KOI-94 system, suggesting that KOI-94.04 is also a real planet in the system.

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arXiv2013

Planetary Companions to Three Evolved Intermediate-Mass Stars: HD 2952, HD 120084, and omega Serpentis

We report the detections of planetary companions orbiting around three evolved intermediate-mass stars from precise radial velocity measurements at Okayama Astrophysical Observatory. HD 2952 (K0III, 2.5 M_sun) and omega Ser (G8III, 2.2 M_sun) host a relatively low mass planet with minimum mass of m_2sin i=1.6 M_J and 1.7 M_J in nearly circular orbits with period of P=312 and 277 d, respectively. HD 120084 (G7 III, 2.4 M_sun) hosts an eccentric planet with m_2sin i=4.5 M_J in an orbit with P=2082 d and eccentricity of e=0.66. The planet has one of the largest eccentricities among those ever discovered around evolved intermediate-mass stars, almost all of which have eccentricity smaller than 0.4. We also show that radial velocity variations of stellar oscillations for G giants can be averaged out below a level of a few m/s at least in timescale of a week by high cadence observations, which enables us to detect a super-Earth and a Neptune-mass planet in short-period orbits even around such giant stars.

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arXiv2012

A Double Planetary System around the Evolved Intermediate-Mass Star HD 4732

We report the detection of a double planetary system orbiting around the evolved intermediate-mass star HD 4732 from precise Doppler measurements at Okayama Astrophysical Observatory (OAO) and Anglo-Australian Observatory (AAO). The star is a K0 subgiant with a mass of 1.7 M_sun and solar metallicity. The planetary system is composed of two giant planets with minimum mass of msini=2.4 M_J, orbital period of 360.2 d and 2732 d, and eccentricity of 0.13 and 0.23, respectively. Based on dynamical stability analysis for the system, we set the upper limit on the mass of the planets to be about 28 M_J (i>5 deg) in the case of coplanar prograde configuration.

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arXiv2012

Planet-Planet Eclipse and the Rossiter-McLaughlin Effect of a Multiple Transiting System: Joint Analysis of the Subaru Spectroscopy and the Kepler Photometry

We report a joint analysis of the Rossiter-McLaughlin (RM) effect with Subaru and the Kepler photometry for Kepler Object of Interest (KOI) 94 system. The system comprises four transiting planet candidates with orbital periods of 22.3 (KOI-94.01), 10.4 (KOI-94.02), 54.3 (KOI-94.03), and 3.7 (KOI-94.04) days from the Kepler photometry. We performed the radial velocity (RV) measurement of the system with the Subaru 8.2 m telescope on August 10, 2012 (UT), covering a complete transit of KOI-94.01 for $\sim 6.7$ hours. The resulting RV variation due to the RM effect spectroscopically confirms that KOI-94.01 is indeed the transiting planet, and implies that its orbital axis is well aligned with the stellar spin axis; the projected spin-orbit angle $λ$ is estimated as $-6_{-11}^{+13}$ deg. This is the first measurement of the RM effect for a multiple transiting system. Remarkably, the archived Kepler lightcurve around BJD=2455211.5 (date in UT January 14/15, 2010) indicates a "ouble transit" event of KOI-94.01 and KOI-94.03, in which the two planets transit the stellar disk simultaneously. Moreover, the two planets partially overlap each other, and exhibit a "planet-planet eclipse" around the transit center. This provides a rare opportunity to put tight constraints on the configuration of the two transiting planets by joint analysis with our Subaru RM measurement. Indeed, we find that the projected mutual inclination of KOI-94.01 and KOI-94.03 is estimated to be $δ= -1.15 \pm 0.55$ deg. Implications for the migration model of multiple planet systems are also discussed.

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arXiv2012

A Common Proper Motion Stellar Companion to HAT-P-7

We report that HAT-P-7 has a common proper motion stellar companion. The companion is located at $\sim3.9$ arcsec to the east and estimated as an M5.5V dwarf based on its colors. We also confirm the presence of the third companion, which was first reported by Winn et al. (2009), based on long-term radial velocity measurements. We revisit the migration mechanism of HAT-P-7b given the presence of those companions, and propose sequential Kozai migration as a likely scenario in this system. This scenario may explain the reason for an outlier in the discussion of the spin-orbit alignment timescale for HAT-P-7b by Albrecht et al. (2012).

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arXiv2012

Substellar Companions to Seven Evolved Intermediate-Mass Stars

We report the detections of substellar companions orbiting around seven evolved intermediate-mass stars from precise Doppler measurements at Okayama Astrophysical Observatory. o UMa (G4 II-III) is a giant with a mass of 3.1 M_sun and hosts a planet with minimum mass of m_2sini=4.1 M_J in an orbit with a period P=1630 d and an eccentricity e=0.13. This is the first planet candidate (< 13 M_J) ever discovered around stars more massive than 3 M_sun. o CrB (K0 III) is a 2.1 M_sun giant and has a planet of m_2sini=1.5 M_J in a 187.8 d orbit with e=0.19. This is one of the least massive planets ever discovered around ~2 M_sun stars. HD 5608 (K0 IV) is an 1.6 M_sun subgiant hosting a planet of m_2sini=1.4 M_J in a 793 d orbit with e=0.19. The star also exhibits a linear velocity trend suggesting the existence of an outer, more massive companion. 75 Cet (G3 III:) is a 2.5 M_sun giant hosting a planet of m_2sini=3.0 M_J in a 692 d orbit with e=0.12. The star also shows possible additional periodicity of about 200 d and 1880 d with velocity amplitude of ~7--10 m/s, although these are not significant at this stage. nu Oph (K0 III) is a 3.0 M_sun giant and has two brown-dwarf companions of m_2sini= 24 M_J and 27 M_J, in orbits with P=530.3 d and 3190 d, and e=0.126 and 0.17, respectively, which were independently announced by Quirrenbach et al. (2011). The ratio of the periods is close to 1:6, suggesting that the companions are in mean motion resonance. We also independently confirmed planets around k CrB (K0 III-IV) and HD 210702 (K1 IV), which had been announced by Johnson et al. (2008) and Johnson et al. (2007a), respectively. All of the orbital parameters we obtained are consistent with the previous results.

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arXiv2011

A Planetary Companion to the Intermediate-Mass Giant HD 100655

A precise radial velocity survey conducted by a Korean-Japanese planet search program revealed a planetary companion around the intermediate-mass clump giant HD 100655. The radial velocity of the star exhibits a periodic Keplerian variation with a period, semi-amplitude and eccentricity of 157.57 d, 35.2 m s^-1 and 0.085, respectively. Adopting an estimated stellar mass of 2.4 M_Sun, we confirmed the presence of a planetary companion with a semi-major axis of 0.76 AU and a minimum mass of 1.7 M_Jup. The planet is the lowest-mass planet yet discovered around clump giants with masses greater than 1.9 M_Sun.

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arXiv2011

XO-2b: a Prograde Planet with a Negligible Eccentricity, and an Additional Radial Velocity Variation

We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2$σ$ ($e=0.045\pm0.024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $λ=10^{\circ}\pm72^{\circ}$). The poor constraint of $λ$ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90$^{\circ}$). The data also provide an improved estimate of the mass of XO-2b as $0.62\pm0.02$ $M_{\rm Jup}$. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.

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arXiv2011

Further Observations of the Tilted Planet XO-3: A New Determination of Spin-Orbit Misalignment, and Limits on Differential Rotation

We report on observations of the Rossiter-McLaughlin (RM) effect for the XO-3 exoplanetary system. The RM effect for the system was previously measured by two different groups, but their results were statistically inconsistent. To obtain a decisive result we observed two full transits of XO-3b with the Subaru 8.2-m telescope. By modeling these data with a new and more accurate analytic formula for the RM effect, we find the projected spin-orbit angle to be λ=37.3 deg \pm 3.0 deg, in good agreement with the previous finding by Winn et al. (2009). In addition, an offset of ~22 m/s was observed between the two transit datasets. This offset could be a signal of a third body in the XO-3 system, a possibility that should be checked with future observations. We also attempt to search for a possible signature of the stellar differential rotation in the RM data for the first time, and put weak upper limits on the differential rotation parameters.

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