T. Yamashita

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (241)476.41 Total impact

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    ABSTRACT: We performed optical and near-infrared multi-band linear polarimetry for highly reddened Type Ia SN 2014J appeared in M82. SN 2014J exhibits large polarization at shorter wavelengths, reaching $p\simeq 4.8$\% in $B$ band and steeply decreasing with wavelength, while it has almost constant position angle $\sim 40^{\circ}$ over the observed wavelength range. No significant temporal variation is found. Since intrinsic polarization of continuum light from a normal Type Ia supernova is generally weak ($\lesssim 0.3$\%) and the Galactic interstellar polarization component is likely negligibly small, the observed polarization is likely predominantly caused by the interstellar media within M82; however, we cannot completely exclude the possibility that it is caused by circumstellar media. The wavelength dependence of polarization can be explained by the empirical Serkowski-law at wavelengths shorter than $1 \mu$m and by an inverse power-law at wavelengths longer than $0.5 \mu$m. The peak polarization wavelength $\lambda_{\rm max}$ is quite short, $\lesssim 0.4\ \mu$m, suggesting the mean radius of polarizing dust grains is small ($< 0.1 \mu$m). The empirical law between $K$ and $\lambda_{\rm max}$ for the Galactic interstellar polarization is apparently broken, although the positive correlation between $R_{V}=A_{V}/E_{B-V}$ and $\lambda_{\rm max}$ seems to still hold. These facts suggest the nature of the dust grains in M82 is different from that in our Galaxy. These observed properties are similar to those in the other highly reddened Type Ia SNe 1986G and 2006X that have ever been polarimetrically observed, and this high probability suggests that such properties of dust grains are rather common in extragalaxies.
    07/2014;
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    ABSTRACT: We report that the optical polarization in the afterglow of GRB 091208B is measured at t = 149 - 706 s after the burst trigger, and the polarization degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward shock synchrotron emission, and thus this is the first detection of the early-time optical polarization in the forward shock (rather than that in the reverse shock reported by Steele et al. (2009). This detection disfavors the afterglow model in which the magnetic fields in the emission region are random on the plasma skin depth scales, such as amplified by the plasma instabilities, e.g., Weibel instability. We suggest that the fields are amplified by the magnetohydrodynamic instabilities, which would be tested by future observations of the temporal changes of the polarization degrees and angles for other bursts.
    The Astrophysical Journal Letters 05/2012; 752(1). · 6.35 Impact Factor
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    ABSTRACT: The disk around the Herbig Ae star HD\,169142 was imaged and resolved at 18.8 and 24.5\,$\mu$m using Subaru/COMICS. We interpret the observations using a 2D radiative transfer model and find evidence for the presence of a large gap. The MIR images trace dust that emits at the onset of the strong rise in the spectral energy distribution (SED) at 20\,$\mu$m, therefore are very sensitive to the location and characteristics of the inner wall of the outer disk and its dust. We determine the location of the wall to be 23$^{+3}_{-5}$\,AU from the star. An extra component of hot dust must exist close to the star. We find that a hydrostatic optically thick inner disk does not produce enough flux in the NIR and an optically thin geometrically thick component is our solution to fit the SED. Considering the recent findings of gaps and holes in a number of Herbig Ae/Be group I disks, we suggest that such disk structures may be common in group I sources. Classification as group I should be considered a support for classification as a transitional disk, though improved imaging surveys are needed to support this speculation.
    The Astrophysical Journal 04/2012; 752(2). · 6.73 Impact Factor
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    ABSTRACT: We present near-infrared imaging polarimetry of the silhouette young stellar object M17-SO1. When the continuum and the scattering components are separated, the 2.17-μm Brγ image reveals the absorptive polarization vectors (i.e., magnetic field lines) exiting the mid-plane of the circumstellar envelope at relatively wide angles. Such a configuration may imply a slightly pulled-in, co-rotating frozen-in poloidal magnetic field.
    11/2011;
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    ABSTRACT: We present the SPICA Coronagraphic Instrument (SCI), which has been designed for a concentrated study of extra-solar planets (exoplanets). SPICA mission provides us with a unique opportunity to make high contrast observations because of its large telescope aperture, the simple pupil shape, and the capability for making infrared observations from space. The primary objectives for the SCI are the direct coronagraphic detection and spectroscopy of Jovian exoplanets in infrared, while the monitoring of transiting planets is another important target. The specification and an overview of the design of the instrument are shown. In the SCI, coronagraphic and non-coronagraphic modes are applicable for both an imaging and a spectroscopy. The core wavelength range and the goal contrast of the coronagraphic mode are 3.5--27$\mu$m, and 10$^{-6}$, respectively. Two complemental designs of binary shaped pupil mask coronagraph are presented. The SCI has capability of simultaneous observations of one target using two channels, a short channel with an InSb detector and a long wavelength channel with a Si:As detector. We also give a report on the current progress in the development of key technologies for the SCI.
    Advances in Space Research - ADV SPACE RES. 08/2011; 48(2).
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    ABSTRACT: A mid-infrared (MIR) imager and spectrometer is under consideration for construction in the first decade of the Thirty Meter Telescope's (TMT) operation. When combined with a MIR adaptive optics system, the instrument will afford 15 times higher sensitivity, 4 times better spatial resolution (0.08") than 8m-class telescopes, and 4.5 times better spatial resolution than the JWST. Additionally, its huge light gathering power opens a new window of high-dispersion spectroscopy in the MIR. We discuss the key science drivers, from star and planet formation to galaxies and black holes and cosmology; science drivers which are in close synergy with the recent Astro 2010 Decadal Survey report. We flow down our science cases to produce fundamental and optional instrument capabilities, including imaging, long-slit and IFU spectroscopy, and polarimetry.
    01/2011;
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    ABSTRACT: Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was ∼16.4 hr. Starting in 2010 August the period changed from 16.6 hr to near 19 hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO2-driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production
    apjl. 01/2011; 734:L1.
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    ABSTRACT: The disk around AB Aur was imaged and resolved at 24.6 μm using the Cooled Mid-infrared Camera and Spectrometer on the 8.2 m Subaru Telescope. The Gaussian full width at half-maximum of the source size is estimated to be 90 ± 6 AU, indicating that the disk extends further out at 24.6 μm than at shorter wavelengths. In order to interpret the extended 24.6 μm image, we consider a disk with a reduced surface density within a boundary radius Rc , which is motivated by radio observations that suggest a reduced inner region within about 100 AU from the star. Introducing the surface density reduction factor fc for the inner disk, we determine that the best match with the observed radial intensity profile at 24.6 μm is achieved with Rc = 88 AU and fc = 0.01. We suggest that the extended emission at 24.6 μm is due to the enhanced emission from a wall-like structure at the boundary radius (the inner edge of the outer disk), which is caused by a jump in the surface density at Rc . Such a reduced inner disk and geometrically thick outer disk structure can also explain the more point-like nature at shorter wavelengths. We also note that this disk geometry is qualitatively similar to a pre-transitional disk, suggesting that the AB Aur disk is in a pre-transitional disk phase.
    The Astrophysical Journal Letters 07/2010; 718(2):L199. · 6.35 Impact Factor
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    ABSTRACT: We investigate the origin of short X-ray flares which are occasionally observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed two events, GRB 071112C and GRB 080506, before the start of X-ray flares in the optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In conjunction with published X-ray and optical data, we analyzed densely sampled light curves of the early afterglows and spectral energy distributions (SEDs) in the NIR-X-ray ranges. We found that the SEDs had a break between the optical and X-ray bands in the normal decay phases of both GRBs regardless of the model for the correction of the interstellar extinction in host galaxies of GRBs. In the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB 071112C and 080506, respectively, and the X-ray spectra became harder than those in the normal decay phases. No significant variation in the optical-NIR range was detected together with the X-ray flares. These results suggest that the X-ray flares were associated with either late internal shocks or external shocks from two-component jets. Comment: 10 pages, 5 figures, accepted to Astronomy and Astrophysics
    Astronomy and Astrophysics 06/2010; · 5.08 Impact Factor
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    ABSTRACT: The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a rapid (~5 days duration), high-energy (E > 100 MeV) gamma-ray outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z = 1.839) starting on 2008 August 5 (~23 UTC, MJD 54683.95), and followed by bright and variable flux over the next few months. Results on the gamma-ray localization and identification, as well as spectral and temporal behavior during the first months of the Fermi all-sky survey, are reported here in conjunction with a multiwaveband characterization as a result of one of the first Fermi multifrequency campaigns. The campaign included a Swift ToO (followed up by a 16 day observation on August 7-22, MJD 54685-54700), VLBA (within the MOJAVE program), Owens Valley Radio Observatory (OVRO) 40 m, Effelsberg-100 m, Metsähovi-14 m, RATAN-600, and Kanata-Hiroshima radio/optical observations. Results from the analysis of archival observations by INTEGRAL, XMM-Newton, and Spitzer space telescopes are reported for a more complete picture of this new gamma-ray blazar. PKS 1502+106 is a sub-GeV peaked, powerful flat spectrum radio quasar (luminosity at E > 100 MeV, L_γ, is about 1.1 × 10^(49) erg s^(–1), and black hole mass likely close to 10^9 M_⊙), exhibiting marked gamma-ray bolometric dominance, in particular during the asymmetric outburst (L_γ/L_(opt) ~ 100, and 5 day averaged flux F_(E > 100 MeV) = 2.91 ± 1.4 × 10^(–6) ph cm^(–2) s^(–1)), which was characterized by a factor greater than 3 of flux increase in less than 12 hr. The outburst was observed simultaneously from optical to X-ray bands (F_(0.3 – 10 keV) = 2.18^(+0.15)_(–0.12) × 10^(–12) erg cm^(–2) s^(–1), and hard photon index ~1.5, similar to past values) with a flux increase of less than 1 order of magnitude with respect to past observations, and was likely controlled by Comptonization of external-jet photons produced in the broad-line region (BLR) in the gamma-ray band. No evidence of a possible blue bump signature was observed in the optical-UV continuum spectrum, while some hints for a possible 4 day time lag with respect to the gamma-ray flare were found. Nonetheless, the properties of PKS 1502+106 and the strict optical/UV, X-, and gamma-ray cross-correlations suggest the contribution of the synchrotron self-Compton (SSC), in-jet, process should dominate from radio to X-rays. This mechanism may also be responsible for the consistent gamma-ray variability observed by the LAT on longer timescales, after the ignition of activity at these energies provided by the BLR-dissipated outburst. Modulations and subsequent minor, rapid flare events were detected, with a general fluctuation mode between pink-noise and a random-walk. The averaged gamma-ray spectrum showed a deviation from a simple power law, and can be described by a log-parabola curved model peaking around 0.4-0.5 GeV. The maximum energy of photons detected from the source in the first four months of LAT observations was 15.8 GeV, with no significant consequences on extragalactic background light predictions. A possible radio counterpart of the gamma-ray outburst can be assumed only if a delay of more than three months is considered on the basis of opacity effects at cm and longer wavelengths. The rotation of the electric vector position angle observed by VLBA from 2007 to 2008 could represent a slow field ordering and alignment with respect to the jet axis, likely a precursor feature of the ejection of a superluminal radio knot and the high-energy outburst. This observing campaign provides more insight into the connection between MeV-GeV flares and the moving, polarized structures observed by the VLBI.
    01/2010;
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    Astrophysical Journal. 01/2010; 710(1):810-827.
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    ABSTRACT: We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is Δm 15(B) = 0.65 ± 0.03, which is one of the slowest among SNe Ia. The peak V-band absolute magnitude is estimated to be MV = –19.90 ± 0.15 mag if no host extinction is assumed. It reaches MV = –20.19 ± 0.19 mag if we assume the host extinction of AV = 0.29 mag. SN 2009dc belongs to the most luminous class of SNe Ia, like SNe 2003fg and 2006gz. Our JHKs -band photometry shows that this SN is also one of the most luminous SNe Ia in near-infrared wavelengths. We estimate the ejected 56Ni mass of 1.2 ± 0.3 M ☉ for the no host extinction case (and of 1.6 ± 0.4 M ☉ for the host extinction of AV = 0.29 mag). The C II λ6580 absorption line remains visible until a week after the maximum brightness, in contrast to its early disappearance in SN 2006gz. The line velocity of Si II λ6355 is about 8000 km s–1 around the maximum, being considerably slower than that of SN 2006gz. The velocity of the C II line is similar to or slightly less than that of the Si II line around the maximum. The presence of the carbon line suggests that the thick unburned C+O layer remains after the explosion. Spectropolarimetric observations by Tanaka et al. indicate that the explosion is nearly spherical. These observational facts suggest that SN 2009dc is a super-Chandrasekhar mass SN Ia.
    The Astrophysical Journal 12/2009; 707(2):L118. · 6.73 Impact Factor
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    ABSTRACT: We made mid-infrared (MIR) observations of the 10 M ☉ Herbig Be star HD200775 with the Cooled Mid-Infrared Camera and Spectrometer on the 8.2 m Subaru Telescope. We discovered diffuse emission of an elliptical shape extended in the north-south direction in an ~1000 AU radius around unresolved excess emission. The diffuse emission is perpendicular to the cavity wall formed by the past outflow activity and is parallel to the projected major axis of the central close binary orbit. The centers of the ellipse contours of the diffuse emission are shifted from the stellar position, and the amount of the shift increases as the contour brightness level decreases. The diffuse emission is well explained in all of geometry (the shape and the shift), size, and configuration by an inclined flared disk where only its surface emits the MIR photons. Our results give the first well-resolved infrared disk images around a massive star and strongly support that HD200775 is formed through the disk accretion. The disk survives the main accretion phase and shows a structure similar to that around lower mass stars with "disk atmosphere." At the same time, the disk also shows properties characteristic of massive stars such as photoevaporation traced by the 3.4 mm free-free emission and unusual silicate emission with a peak at 9.2 μm, which is shorter than that of many astronomical objects. It provides a good place to compare the disk properties between massive and lower mass stars.
    The Astrophysical Journal 11/2009; 706(1):665. · 6.73 Impact Factor
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    ABSTRACT: We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And during a superoutburst in 2007. These observations were performed with the KANATA (V, J, and K_s bands) and MITSuME (g', Rc, and Ic bands) telescopes. Our 6-band simultaneous observations allowed us to investigate the temporal variation of the temperature and the size of the emitting region associated with the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K) accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably from the disk, still remained dominant in the optical region with a moderately high temperature (~8000 K). This indicates that a substantial amount of gas was stored in the disk even after the outburst. This remnant matter may be a sign of an expected mass-reservoir which can trigger echo outbursts observed in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a low-temperature region made the maximum. The color variation of early superhumps was totally different from that of superhumps: the object was bluest at the early superhump minimum. The temperature of the early superhump light source was lower than that of an underlying component, indicating that the early superhump light source was a vertically expanded low-temperature region at the outermost part of the disk. Comment: 14 pages, 12 figures, PASJ accepted
    Publications- Astronomical Society of Japan 08/2009; · 2.44 Impact Factor
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    ABSTRACT: We present the early results of mid-infrared (MIR) pointed observations of nearby mainsequence stars with AKARI. Out of 117 AFGKM stars, 7 AF-type stars show the large (>7% of the photospheric level) infrared excesses at 24 µm, in which three sources are newly identified with AKARI.
    08/2009;
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    ABSTRACT: We report our optical and near-infrared observations of 4 classical novae at the Higashi-Hiroshima Observatory, Japan. These novae show unusual features in their light curves. V2362 Cyg experienced a rebrightening about 240 days after the maximum. The behavior is similar to V1493 Aql. After the rebrightening maximum, we obtained the strong evidence of a dust formation. V1280 Sco is a dust formation nova. The dust formation was found at its early phase of the outburst. We found that the object did not show any declines in Ks-band. The result indicates that the dust component remains in outer shells of V1280 Sco. We also observed V458 Vul and V5558 Sgr which experienced rebrightenings or flares, but their durations and amplitudes were shorter and smaller than V2362 Cyg. V5558 Sgr is a very slow nova and showed many some striking rebrightenings which was similar to V723 Cas which was also very slow nova. In V458 Vul, our spectral observation indicates that the P-Cyg profiles reappeared during rebrightenings.
    07/2009; 404:90.
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    ABSTRACT: Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.
    Physical Review Letters 02/2009; 102(1):017006. · 7.94 Impact Factor
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    ABSTRACT: We present the SPICA Coronagraph Instrument (SCI) for the direct imaging and spectroscopy of exo-planets. The SPICA mission gives us a unique opportunity for high-contrast observations because of the large telescope aperture, the simple pupil shape, and the capability for infrared observations from space. The primary target of SCI is Jovian exo-planets. Using the spectroscopy mode of SCI, we will try the detection and the characterization of mid-infrared line features of the atmosphere of exoplanets. The specifications, performance and the design of the instrument are shown. The main wavelengths and the contrast required for the observations are 3.5–27 μm, and 10-6 , respectively. We also show the progress of the development of key technology to realize SCI. Laboratory demonstration of the principle of coronagraph, realistic design and fabrication of masks, and the development of cryogenic active optics have been carried out, or are successfully ongoing. We are preparing a cryogenic chamber for the tests of the whole infrared coronagraph. A potentially important by-product of the instrument, transit monitoring for characterization of exo-planets, is also described. We expect that SCI will provide drastic progress in the understanding of various planetary systems and will be a unique capability in the SPICA era.
    http://dx.doi.org/10.1051/spica/200901004. 01/2009;
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    ABSTRACT: SPICA will provide the best sensitivity and image quality than ever at 5-210 mum. This will revolutionize our understanding of exoplanets, protoplanetary disks, debris disks, and Solar system small bodies. This paper summarizes such key sciences with SPICA discussed so far among the Japanese SPICA Science Working Group, stressing on the planetary formation, and exoplanet detection and characterization.
    01/2009;

Publication Stats

2k Citations
476.41 Total Impact Points

Institutions

  • 2007–2012
    • Hiroshima University
      • • Hiroshima Astrophysical Science Center (HASC)
      • • Division of Physical Sciences
      Hirosima, Hiroshima, Japan
  • 1997–2012
    • National Astronomical Observatory of Japan
      Edo, Tōkyō, Japan
  • 2003–2008
    • The University of Tokyo
      • Department of Astronomy
      Tokyo, Tokyo-to, Japan
    • Hirosaki University
      • Faculty of Science and Technology
      Khirosaki, Aomori Prefecture, Japan
  • 2004–2006
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan
  • 1968–2006
    • Tohoku University
      • • Institute for Materials Research
      • • Research Institute of Electrical Communication
      Sendai, Kagoshima-ken, Japan
  • 2005
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, HI, United States
    • Nanjing University
      • School of Electronic Science & Engineering
      Nanjing, Jiangsu Sheng, China
  • 1999–2002
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
    • Tohoku Gakuin University
      Japan
  • 2001
    • Sendai National College of Technology
      Sendai, Kagoshima, Japan
  • 2000
    • Los Alamos National Laboratory
      • Superconductivity Technology Center
      Los Alamos, California, United States
  • 1995–1999
    • National Defense Medical College
      • Department of Internal Medicine
      Tokorozawa, Saitama-ken, Japan
  • 1987–1999
    • Kyoto University
      • Department of Astronomy
      Kioto, Kyōto, Japan
  • 1991–1998
    • RIKEN
      Вако, Saitama, Japan
  • 1992
    • Rensselaer Polytechnic Institute
      Troy, New York, United States
  • 1989
    • The Royal Observatory, Edinburgh
      Edinburgh, Scotland, United Kingdom
  • 1974–1977
    • University of Lausanne
      Lausanne, Vaud, Switzerland