Masashi Omiya

National Astronomical Observatory of Japan, Edo, Tokyo, Japan

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Publications (32)76.64 Total impact

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    ABSTRACT: 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.
    No preview · Article · Jan 2016
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    ABSTRACT: 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.
    Full-text · Article · Mar 2015 · The Astrophysical Journal
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    ABSTRACT: Because of their large numbers, red dwarfs may be the most abundant planet hosts in our Galaxy. In order to detect Earth-like planets around nearby red dwarfs (in particular late-M stars), it is crucial to conduct the precise radial velocity (RV) measurements at near-infrared wavelengths where these stars emit most of light. We report the development of the Infrared Doppler (IRD) spectrometer for the Subaru telescope. IRD is a fiber-fed, high-precision, near infrared spectrometer with a spectral resolution of R~70,000 covering from 0.97 to 1.75 μm. To achieve 1m/s RV measurement precision, we employ our original laser frequency comb of a widewavelength coverage as an extremely stable wavelength standard in the near-infrared. The spectrometer optics is composed of a new wide-pitch Echelle-grating and Volume-Phase Holographic gratings. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench. The spectrometer will utilize a 4096x4096-pixel HgCdTe array.
    No preview · Article · Mar 2015
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    ABSTRACT: 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.
    Full-text · Article · Jan 2015 · The Astrophysical Journal
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    Yoichi Takeda · Bun'ei Sato · Masashi Omiya · Hiroki Harakawa
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    ABSTRACT: Extensive oxygen abundance determinations were carried out for 239 late-G/early-K giant stars of 1.5–5 M⊙ 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 the 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) cannot be explained 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 re-analyzing the previous equivalent-width data of C i 5052/5380 and Na i 6160 lines.
    Preview · Article · Dec 2014 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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 $D\gtrsim 20$.
    Full-text · Article · Nov 2014 · The Astrophysical Journal
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    ABSTRACT: We report the detection of a substellar companion orbiting an evolved intermediate-mass ($M_\star=2.4\,M_\odot$) star HD 14067 (G9 III) using precise Doppler technique. Radial velocities of this star can be well fitted either by a periodic Keplerian variation with a decreasing linear velocity trend (P=1455 days, $K_1=92.2$ m s$^{-1}$, $e=0.533$, and $\dot{\gamma}=-22.4$ m s$^{-1}$ yr$^{-1}$) or a single Keplerian orbit without linear trend (P=2850 days, $K_1=100.1$ m s$^{-1}$, and $e=0.697$). The minimum mass ($m_2\sin{i}=7.8\,M_{\rm J}$ for the model with a linear trend, or $m_2\sin{i}=9.0\,M_{\rm J}$ for the model without a linear trend) suggests a long-period giant planet around an evolved intermediate-mass star. The eccentricity of the orbit is among the highest known for planets ever detected around evolved stars.
    Preview · Article · Sep 2014 · Publications- Astronomical Society of Japan
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    ABSTRACT: We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R∼70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: We have a plan to conduct a Doppler planet search for low-mass planets around nearby middle-to-late M dwarfs using IRD. IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. We expect to achieve the accuracy of the radial velocity measurements of 1 m/s using IRD with a frequency comb as a wavelengh calibrator. Thus, we would detect super-Earths in habitable zone and low-mass rocky planets in close-in orbits around late-M dwarfs. In this survey, we aim to understand and discuss statistical properties of low-mass planets around low-mass M dwarfs compared with those derived from theoretical simulations.
    Preview · Article · Mar 2014 · Proceedings of the International Astronomical Union
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    ABSTRACT: We report on the detections of planetary companions orbiting around three evolved intermediate-mass stars based on precise radial velocity measurements at Okayama Astrophysical Observatory. HD2952 (K0III, 2.5M) and ω Ser (G8III, 2.2M) host a relatively low-mass planet with minimum masses of m 2sini = 1.6MJ and 1.7MJ in nearly circular orbits with periods of P = 312 and 277 d, respectively. HD120084 (G7 III, 2.4M) hosts an eccentric planet with m2sini = 4.5MJ in an orbit with P = 2082 d and an 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 an eccentricity smaller than 0.4. We also show that the radial-velocity variations of stellar oscillations for G giants can be averaged out below a level of a few ms -1, at least on a timescale of a week by high-cadence observations. This enables us to detect a super-Earth and a Neptune-mass planet in short-period orbits even around such giant stars.
    No preview · Article · Aug 2013 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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.
    Preview · Article · Apr 2013 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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.
    Preview · Article · Oct 2012 · The Astrophysical Journal
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    ABSTRACT: IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (similar to 1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s(-1). Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.
    No preview · Conference Paper · Sep 2012
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    ABSTRACT: We report on the detections of substellar companions orbiting around seven evolved intermediate-mass stars from precise Doppler measurements at Okayama Astrophysical Observatory. oUMa (G4 II-III) is a giant with a mass of 3.1M, and hosts a planet with a minimum mass of m2sin i = 4.1MJ in an orbit with a period P = 1630 d and an eccentricity e = 0.13. This is the first planet candidate (<13MJ) ever discovered around a star more massive than 3M. o CrB (K0 III) is a 2.1M giant, and has a planet of m2sin i = 1.5MJ in a 187.8 d orbit with e = 0.19. This is one of the least-massive planets ever discovered around an ̃2M star. HD5608 (K0 IV) is an 1.6M subgiant hosting a planet of m2sin i = 1.4MJ 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 2.5M giant hosting a planet of m2sin i = 3.0MJ in a 692 d orbit with e = 0.12. The star also shows a possible additional periodicity of about 200 d and 1880 d with a velocity amplitude of ̃7-10ms-1, although these are not significant at this stage. Oph (K0 III) is a 3.0M giant, and has two brown-dwarf companions of m2sin i = 24MJ and 27MJ, in orbits with P = 530.3 d and 3190 d, and e = 0.126 and 0.17, respectively, which were independently announced by Quirrenbach, Reffert, and Bergmann (2011, AIP Conf. Proc. 1331, 102). 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 CrB (K0 III-IV) and HD210702 (K1 IV), which were announced by Johnson et al. (2008, ApJ, 675, 784) and Johnson et al. (2007a, ApJ, 665, 785), respectively. All of the orbital parameters we obtained are consistent with the previous results.
    Preview · Article · Jul 2012 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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.
    Full-text · Article · Nov 2011 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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.
    Full-text · Article · Oct 2011 · Research in Astronomy and Astrophysics
  • Masashi Omiya · I. Han · H. Izumiura · B. Lee · B. Sato
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    ABSTRACT: In the framework of a Korean-Japanese planet search program, we have been carrying out a precise Doppler survey of about 190 G or K type giants to search for planets around intermediate-mass giants. The main purpose of this program is to show the properties of planetary systems around intermediate-mass stars by the survey using the 1.8m telescope at Bohyunsan Optical Astronomy Observatory in Korea, and the 1.88m telescope at Okayama Astrophysical Observatory in Japan. In our presentation, we report a new planetary companion with a minimum mass of 1.8 MJupiter orbiting the giant star with 2.4 M⊙ at a semi-major axis of 0.77 AU. The planet is the lowest-mass planetary companion among those discovered around clump giant stars with masses of >1.9 M⊙. Plotting this system and other known systems on semi-major axis vs. stellar mass diagram, it seems like that the distribution of planetary systems around 1.9-2.5 M⊙ stars may differ from those around other intermediate-mass stars. Almost all the planets orbiting 1.5-1.9 M⊙ stars are normal giant planets (1-6 MJupiter), and are located on orbits with semi-major axes of >1 AU. However, planets orbiting 1.9-2.5 M⊙ stars seem to be classified in two groups: normal giant planets at inner orbits (0.6-1.3 AU) and superplanets (6-13 MJupiter) at outer orbits (1.9-3 AU). Moreover, all planet-mass companions orbiting 2.5-3 M⊙ stars reside at semi-major axes larger than 1.9 AU, while all brown dwarf-mass companions are orbiting at semi-major axes less than 1.9 AU. In order to verify these properties and examine roles of the Type-II migration and other typical mechanisms on planet formation and evolution around intermediate-mass giant starss, it would be required to evaluate the semi-major axes distribution by further Doppler surveys.
    No preview · Article · Sep 2011
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    ABSTRACT: A Korean-Japanese planet search program has been carried out using the 1.8m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea, and the 1.88m telescope at Okayama Astrophysical Observatory (OAO) in Japan to search for planets around intermediate-mass giant stars. The program aims to show the properties of planetary systems around such stars by precise Doppler survey of about 190 G or K type giants together with collaborative surveys of the East-Asian Planet Search Network. So far, we detected two substellar companions around massive intermediate-mass giants in the Korean-Japanese planet search program. One is a brown dwarf-mass companion with 37.6 $M_{\mathrm{J}}$ orbiting a giant HD 119445 with 3.9 $M_{\odot}$, which is the most massive brown dwarf companion among those found around intermediate-mass giants. The other is a planetary companion with 1.8 $M_{\mathrm{J}}$ orbiting a giant star with 2.4 $M_{\odot}$, which is the lowest-mass planetary companion among those detected around giant stars with $>$ 1.9 $M_{\odot}$. Plotting these systems on companion mass vs. stellar mass diagram, there seem to exist two unpopulated regions of substellar companions around giants with 1.5--3 $M_{\odot}$ and planetary companions orbiting giants with 2.4--4 $M_{\odot}$. The existence of these possible unpopulated regions supports a current characteristic view that more massive substellar companions tend to exist around more massive stars.
    Full-text · Article · Jan 2011
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    ABSTRACT: We report the detections of two substellar companions orbiting around evolved intermediate-mass stars from precise Doppler measurements at Subaru Telescope and Okayama Astrophysical Observatory. HD 145457 is a K0 giant with a mass of 1.9 M_sun and has a planet of minimum mass m_2sini=2.9 M_J orbiting with period of P=176 d and eccentricity of e=0.11. HD 180314 is also a K0 giant with 2.6 M_sun and hosts a substellar companion of m_2sin i=22 M_J, which falls in brown-dwarf mass regime, in an orbit with P=396 d and e=0.26. HD 145457 b is one of the innermost planets and HD 180314 b is the seventh candidate of brown-dwarf-mass companion found around intermediate-mass evolved stars.
    Full-text · Article · May 2010 · Publications- Astronomical Society of Japan
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    ABSTRACT: 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 $1.36M_{\odot}$ and 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\mps, which yield a minimum-mass for the companion of $10.7\mjup$ and semimajor axis of 1.71 AU. Such super-massive objects with very low-eccentricities and hundreds of days period are uncommon among the ensemble of known exoplanets.
    Preview · Article · Apr 2010 · The Astrophysical Journal

Publication Stats

427 Citations
76.64 Total Impact Points

Institutions

  • 2014-2015
    • National Astronomical Observatory of Japan
      • Okayama Astrophysical Observatory
      Edo, Tokyo, Japan
  • 2012-2014
    • Tokyo Institute of Technology
      • Earth and Planetary Sciences Department
      Edo, Tōkyō, Japan
  • 2011
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
  • 2010-2011
    • Korea Astronomy and Space Science Institute
      Daiden, Daejeon, South Korea
  • 2008-2010
    • Tokai University
      • Department of Physics
      Hiratuka, Kanagawa, Japan