Masashi Omiya

Tokyo Institute of Technology, Edo, Tōkyō, Japan

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Publications (21)58.82 Total impact

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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.
    Publications- Astronomical Society of Japan 04/2013; · 2.44 Impact Factor
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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.
    The Astrophysical Journal 10/2012; 762(1). · 6.73 Impact Factor
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    ABSTRACT: 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.
    Publications- Astronomical Society of Japan 07/2012; · 2.44 Impact Factor
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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.
    Publications- Astronomical Society of Japan 11/2011; · 2.44 Impact Factor
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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.
    Research in Astronomy and Astrophysics 10/2011; 12(1). · 1.35 Impact Factor
  • 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.
    09/2011;
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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.
    01/2011;
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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. Comment: 16 pages, 4 figures, accepted for publication in PASJ
    Publications- Astronomical Society of Japan 05/2010; · 2.44 Impact Factor
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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 1.36 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.
    The Astrophysical Journal 04/2010; 715(1):550. · 6.73 Impact Factor
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    ABSTRACT: 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–1, and an eccentricity of 0.084 ± 0.003. Adopting a stellar mass of 0.78 ± 0.05 M ☉, 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.
    The Astrophysical Journal 08/2009; 703(1):671. · 6.73 Impact Factor
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    ABSTRACT: We report the detection of a new large-mass planet orbiting around a K0 IV(V = 8.26) star which has a minimum mass Mpsin i = 10.70+/-0.50 MJup in a 696.0+/-2.6-day orbit. It was detected in precise radial velocity (RV) measurements from Subaru and Keck. The derived orbital parameters, based on a kappa2 which minimized by Downhill Simplex algorithm, suggests that these radial velocity variations are consistent with an almost circular planetary orbit and a Mars-like semimajor axis(e~0.0, a = 1.70+/-0.03 AU). Extra-solar planets that have several times the mass of Jupiter orbiting in periods of hundreds or thousands of days, with very low eccentricities(e
    08/2009;
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    ABSTRACT: We detected a brown dwarf-mass companion around the intermediate-mass giant star HD 119445 (G6III) using the Doppler technique. This discovery is the first result from a Korean-Japanese planet search program based on precise radial velocity measurements. The radial velocity of this star exhibits a periodic Keplerian variation with a period, semi-amplitude and eccentricity of 410.2 days, 413.5 m/s and 0.082, respectively. Adopting a stellar mass of 3.9 M_solar, we were able to confirm the presence of a massive substellar companion with a semimajor axis of 1.71 AU and a minimum mass of 37.6 M_Jup, which falls in the middle of the brown dwarf-mass region. This substellar companion is the most massive ever discovered within 3 AU of a central intermediate-mass star. The host star also ranks among the most massive stars with substellar companions ever detected by the Doppler technique. This result supports the current view of substellar systems that more massive substellar companions tend to exist around more massive stars, and may further constrain substellar system formation mechanisms. Comment: 17 pages, 5 figures, PASJ accepted
    Publications- Astronomical Society of Japan 06/2009; · 2.44 Impact Factor
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    ABSTRACT: We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong Station and Okayama Astrophysical Observatory (OAO) reveal Keplerian velocity variations with an orbital period of 326.03 ± 0.32 days, a semiamplitude of 302.8 ± 2.6 m s−1, and an eccentricity of 0.231 ± 0.005. Adopting a stellar mass of 2.7 ± 0.3 M☉, the minimum mass of the companion is 19.4 ± 1.5 MJ, well above the deuterium-burning limit, and the semimajor axis is 1.29 ± 0.05 AU. This is the first result from a joint planet-search program between China and Japan aimed at revealing the statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet-search program at OAO. The program's current detection rate of brown dwarf candidates seems to be comparable to the rate of such detections around solar-type stars with orbital separations of 3 AU.
    The Astrophysical Journal 12/2008; 672(1):553. · 6.73 Impact Factor
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    ABSTRACT: We report the detection of five Jovian-mass planets orbiting high-metallicity stars. Four of these stars were first observed as part of the N2K program, and exhibited low rms velocity scatter after three consecutive observations. However, follow-up observations over the last 3 years now reveal the presence of longer period planets with orbital periods ranging from 21 days to a few years. HD 11506 is a G0 V star with a planet of M sin i = 4.74 MJup in a 3.85 yr orbit. HD 17156 is a G0 V star with a 3.12 MJup planet in a 21.2 day orbit. The eccentricity of this orbit is 0.67, one of the highest known for a planet with a relatively short period. The orbital period for this planet places it in a region of parameter space where relatively few planets have been detected. HD 125612 is a G3 V star with a planet of M sin i = 3.5 MJup in a 1.4 yr orbit. HD 170469 is a G5 IV star with a planet of M sin i = 0.67 MJup in a 3.13 year orbit. HD 231701 is an F8 V star with planet of 1.08 MJup in a 142 day orbit. All of these stars have supersolar metallicity. Three of the five stars were observed photometrically, but showed no evidence of brightness variability. A transit search conducted for HD 17156 was negative, but covered only 25% of the search space, and so is not conclusive.
    The Astrophysical Journal 12/2008; 669(2):1336. · 6.73 Impact Factor
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    ABSTRACT: We report the detection of an extrasolar planet orbiting Tau, one of the giant stars in the Hyades open cluster. This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594.9 ± 5.3 days, a semiamplitude of 95.9 ± 1.8 m s-1, and an eccentricity of 0.151 ± 0.023. The minimum mass of the companion is 7.6 ± 0.2 MJ, and the semimajor axis is 1.93 ± 0.03 AU adopting a stellar mass of 2.7 ± 0.1 M☉. The age of 625 Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 M☉ for the host star is robustly determined by isochrone fitting, which makes the star the heaviest among planet-harboring stars. Putting together the fact that no planets have been found around about 100 low-mass dwarfs in the cluster, the frequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones.
    The Astrophysical Journal 12/2008; 661(1):527. · 6.73 Impact Factor
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    ABSTRACT: We report on the detection of four extrasolar planets orbiting evolved intermediate-mass stars from a precise Doppler survey of G-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 Modot and has a planet of minimum mass m2 sin i = 4.8 MJ in a nearly circular orbit with a 186d 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 Modot hosting a planet of m2 sin i = 5.3 MJ in a 953d orbit with an eccentricity of e = 0.21. 6 Lyn (K0 IV) is a less-evolved subgiant with 1.7 Modot, and has a planet of m2 sin i = 2.4 MJ in a 899d orbit with e = 0.13. HD167042 (K1 IV) is also a less-evolved star with 1.5 Modot hosting a planet of m2 sin i = 1.6 MJ in a 418d 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 a metal-rich tendency in planet-harboring giants in contrast to the case of dwarfs.
    Publications- Astronomical Society of Japan 12/2008; · 2.44 Impact Factor
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    ABSTRACT: We report the detection of 3 new extrasolar planets from the precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, namely, 18 Del (G6 III), xi Aql (K0 III) and HD 81688 (K0 III-IV), are located at the clump region on the HR diagram with estimated masses of 2.1-2.3 M_solar. 18 Del b has a minimum mass of 10.3 M_Jup and resides in a nearly circular orbit with period of 993 days, which is the longest one ever discovered around evolved stars. xi Aql b and HD 81688 b have minimum masses of 2.8 and 2.7 M_Jup, and reside in nearly circular orbits with periods of 137 and 184 days, respectively, which are the shortest ones among planets around evolved stars. All of the substellar companions ever discovered around possible intermediate-mass (1.7-3.9 M_solar) clump giants have semimajor axes larger than 0.68 AU, suggesting the lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5 AU (even up to 1 AU depending on the metallicity and adopted models) around 2-3 M_solar stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars. Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial or due to engulfment by central stars. Deriving reliable mass and evolutionary status for evolved stars is highly required for further investigation of formation and evolution of planetary systems around intermediate-mass stars.
    03/2008;
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    ABSTRACT: We have been carrying out precise Doppler surveys of G, K giants to search for planets around intermediate-mass (1.5--5M&sun;) stars within the framework of international collaboration between China, Korea, and Japan. The number of targets at these sites is now about 1000 in total. We here describe the current status of our project and discuss the properties of planets around evolved stars, especially around clump giants. All of the substellar companions ever discovered around possible intermediate-mass clump giants have semimajor axes larger than 0.7 AU. When we assume that most of the clump giants are post-RGB stars, short-period planets around them might have been engulfed by the central stars at the tip of RGB due to tidal force.
    01/2008;
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    ABSTRACT: We report on the detection of 3 new extrasolar planets from a precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, 18Del (G6III), xiAql (K0III) and HD81688 (K0III-IV), are located in the clump region on the HRdiagram with estimated masses of 2.1-2.3Modot. 18Del b has a minimum mass of 10.3MJ and resides in a nearly circular orbit with period of 993d, which is the longest one around evolved stars. xiAqlb and HD81688b have minimum masses of 2.8 and 2.7MJ, and reside in nearly circular orbits with periods of 137 and 184d, respectively, which are the shortest ones around evolved stars. All of the substellar companions ever discovered around intermediate-mass (1.7-3.9Modot) clump giants have semimajor axes larger than 0.68AU, suggesting a lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5AU (even up to 1AU, depending on the metallicity and adopted models) around 2-3Modot stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars. Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial, or due to engulfment by central stars.
    Publications- Astronomical Society of Japan 01/2008; 60. · 2.44 Impact Factor
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    ABSTRACT: Since 2005, we have been carrying out a precise radial velocity survey of about 190 intermediate-mass (1.5-5 M) G and K giants at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea and Okayama Astrophysical Observatory (OAO) in Japan, which aims to reveal statistical properties of planetary systems around intermediate-mass stars. We have finished the first screening of 120 stars so far and have identified 5 candidates with large periodic radial velocity variations. One of the candidates turned out to be orbited by a brown dwarf mass companion with minimum mass of 37.6 MJup and semimajor axis of 1.71 AU. The primary star has a mass of 3.9 M, which ranks among the most massive stars with substellar companions. Our discovery may support the current view obtained from results of planet searches around intermediate-mass stars that massive substellar companions tend to form around massive stars.
    Proceedings of the International Astronomical Union 01/2008; 249:53-56.

Publication Stats

127 Citations
152 Downloads
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58.82 Total Impact Points

Institutions

  • 2012
    • 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
      Sŏul, Seoul, South Korea
  • 2008–2010
    • Tokai University
      • Department of Physics
      Hiratsuka, Kanagawa-ken, Japan