Itsuki Sakon

The University of Tokyo, Edo, Tōkyō, Japan

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Publications (177)404.17 Total impact

  • The Astrophysical Journal 08/2014; 792(1):80. · 6.73 Impact Factor
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    ABSTRACT: We have carried out the trial production of small format (n=5) image slicer aiming to obtain the technical verification of the Integral Field Unit (IFU) that can be equipped to the next generation infrared instruments such as TMT/MICHI and SPICA/SMI. Our goal is to achieve stable pseudo slit image with high efficiency. Here we report the results of the assembly of the image slicer unit and the non-cryogenic evaluation system of the pseudo slit image quality in the infrared.
    SPIE Astronomical Telescopes + Instrumentation; 08/2014
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    ABSTRACT: We report the restraint deformation and the corrosion protection of gold deposited aluminum mirrors for mid-infrared instruments. To evaluate the deformation of the aluminum mirrors by thermal shrinkage, monitoring measurement of the surface of a mirror has been carried out in the cooling cycles from the room temperature to 100 K. The result showed that the effect of the deformation was reduced to one fourth if the mirror was screwed with spring washers. We have explored an effective way to prevent the mirror from being galvanically corroded. A number of samples have been prepared by changing the coating conditions, such as inserting an insulation layer, making a multi-layer and overcoating water blocking layer, or carrying out precision cleaning before coating. Precision cleaning before the deposition and protecting coat with SiO over the gold layer seemed to be effective in blocking corrosion of the aluminum. The SiO over-coated mirror has survived the cooling test for the mid-infrared use and approximately 1 percent decrease in the reflectance has been detected at 6-25 microns compared to gold deposited mirror without coating.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
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    ABSTRACT: We analyzed the continuous 3-µm spectra of dark asteroids using PCA, suggesting that the shape of absorption band around 2.7 µm may be controlled by serpentine.
    02/2014;
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    ABSTRACT: Using a large collection of near-infrared spectra (2.5-5.4 um) of Galactic HII regions and HII region-like objects, we perform a systematic investigation of the astronomical polycyclic aromatic hydrocarbon (PAH) features. 36 objects were observed by the use of the infrared camera onboard the AKARI satellite as a part of a directer's time program. In addition to the well-known 3.3-3.6 um features, most spectra show a relatively-weak emission feature at 5.22 um with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 um band previously reported. By careful analysis, we find good correlations between the 5.25 um band and both the aromatic hydrocarbon feature at 3.3 um and the aliphatic ones at around 3.4-3.6 um. The present results give us convincing evidence that the astronomical 5.25 um band is associated with C-H vibrations as suggested by previous studies and show its potential to probe the PAH size distribution. The analysis also shows that the aliphatic to aromatic ratio of I(3.4-3.6)/I(3.3) decreases against the ratio of the 3.7 um continuum intensity to the 3.3 um band, I(3.7 cont)/I(3.3), which is an indicator of the ionization fraction of PAHs. The mid-infrared color of I(9)/I(18) also declines steeply against the ratio of the hydrogen recombination line Bralpha at 4.05 um to the 3.3 um band, I(Bralpha)/I(3.3). These facts indicate possible dust processing inside or at the boundary of ionized gas.
    01/2014; 784(1).
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    ABSTRACT: We report the results of a search for emission features from interstellar deuterated polycyclic aromatic hydrocarbons (PAHs) in the 4um region with the Infrared Camera (IRC) onboard AKARI. No significant excess emission is seen in 4.3-4.7um in the spectra toward the Orion Bar and M17 after the subtraction of line emission from the ionized gas. A small excess of emission remains at around 4.4 and 4.65um, but the ratio of their intensity to that of the band emission from PAHs at 3.3-3.5um is estimated as 2-3%. This is an order of magnitude smaller than the values previously reported and also those predicted by the model of deuterium depletion onto PAHs. Since the subtraction of the ionized gas emission introduces an uncertainty, the deuterated PAH features are also searched for in the reflection nebula GN 18.14.0, which does not show emission lines from ionized gas. We obtain a similar result that excess emission in the 4um region, if present, is about 2% of the PAH band emission in the 3um region. The present study does not find evidence for the presence of the large amount of deuterated PAHs that the depletion model predicts. The results are discussed in the context of deuterium depletion in the interstellar medium.
    The Astrophysical Journal 11/2013; 780(2). · 6.73 Impact Factor
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    ABSTRACT: For the last few years, it has been pioneer days for infrared spectroscopic studies of extragalactic young stellar objects (YSOs). A number of embedded YSOs are spectroscopically identified in the Magellanic Clouds with AKARI and Spitzer, and their near- to mid-infrared spectral features are investigated in detail. In this proceeding, we are going to present our current research on the infrared spectroscopic observations of ices and dust around embedded YSOs in the Magellanic Clouds with AKARI.
    10/2013;
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    ABSTRACT: Using mid-infrared (MIR) images of four photometric bands of the Infrared Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um), and L24 (24 um), we investigate the interstellar dust properties of the nearby pair of galaxies M51 with respect to its spiral arm structure. The arm and interarm regions being defined based on a spatially filtered stellar component model image, we measure the arm-to-interarm contrast for each band. The contrast is lowest in the S11 image, which is interpreted as that among the four AKARI MIR bands the S11 image best correlates with the spatial distribution of dust grains including colder components, while the L24 image with the highest contrast traces warmer dust heated by star forming activities. The surface brightness ratio between the bands, i.e. color, is measured over the disk of the main galaxy, M51a, at 300 pc resolution. We find that the distribution of S7/S11 is smooth and well traces the global spiral arm pattern while L15/S11 and L24/S11 peak at individual HII regions. This result indicates that the ionization state of PAHs is related to the spiral structure. Comparison with observational data and dust models also supports the importance of the variation in the PAH ionization state within the M51a disk. However, the mechanism driving this variation is not yet clear from currently available data sets. Another suggestion from the comparison with the models is that the PAH fraction to the total dust mass is higher than previously estimated.
    The Astrophysical Journal 09/2013; 778(1). · 6.73 Impact Factor
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    ABSTRACT: An image slicer is highly in demand for an integral field unit (IFU) spectrograph of the next generation infrared telescopes. This paper reports the results of the trial production of three key optical elements for a small format (number of slice; n=5) image slicer, i.e. monolithic slice mirrors, monolithic pupil mirrors and monolithic pseudo slit mirrors. We have demonstrated that sufficiently high processing accuracy and mirror surface accuracy for infrared observations are achieved for each optical element based on our super precision cutting techniques.
    SPIE Optical Engineering + Applications; 09/2013
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    ABSTRACT: We present near-infrared spectroscopic observations of 19 molecular clouds made using the AKARI satellite, and the data reduction pipeline written to analyze those observations. The 2.5-5 μm spectra of 30 objects—22 field stars behind quiescent molecular clouds and 8 low-mass young stellar objects in cores—were successfully extracted using the pipeline. Those spectra are further analyzed to calculate the column densities of key solid phase molecular species, including H2O, CO2, CO, and OCN–. The profile of the H2O ice band is seen to vary across the objects observed and we suggest that the extended red wing may be an evolutionary indicator of both dust and ice mantle properties. The observation of 22 spectra with fluxes as low as < 5 mJy toward background stars, including 15 where the column densities of H2O, CO, and CO2 were calculated, provides valuable data that could help to benchmark the initial conditions in star-forming regions prior to the onset of star formation.
    The Astrophysical Journal 09/2013; 775(2):85. · 6.73 Impact Factor
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    ABSTRACT: We first obtained the spectrum of the diffuse Galactic light (DGL) at general interstellar space in 1.8-5.3 um wavelength region with the low-resolution prism spectroscopy mode of the AKARI Infra-Red Camera (IRC) NIR channel. The 3.3 um PAH band is detected in the DGL spectrum at Galactic latitude |b| < 15 deg, and its correlations with the Galactic dust and gas are confirmed. The correlation between the 3.3 um PAH band and the thermal emission from the Galactic dust is expressed not by a simple linear correlation but by a relation with extinction. Using this correlation, the spectral shape of DGL at optically thin region (5 deg < |b| < 15 deg) was derived as a template spectrum. Assuming that the spectral shape of this template spectrum is uniform at any position, DGL spectrum can be estimated by scaling this template spectrum using the correlation between the 3.3 um PAH band and the thermal emission from the Galactic dust.
    Publications- Astronomical Society of Japan 07/2013; 65(6). · 2.44 Impact Factor
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    ABSTRACT: The Extragalactic Background Light (EBL) as an integrated light from outside of our Galaxy includes information of the early universe and the Dark Ages. We analyzed the spectral data of the astrophysical diffuse emission obtained with the low-resolution spectroscopy mode on the AKARI Infra-Red Camera (IRC) in 1.8-5.3 um wavelength region. Although the previous EBL observation in this wavelength region is restricted to the observations by DIRBE and IRTS, this study adds a new independent result with negligible contamination of Galactic stars owing to higher sensitivity for point sources. Other two major foreground components, the zodiacal light (ZL) and the diffuse Galactic light (DGL), were subtracted by taking correlations with ZL brightness estimated by the DIRBE ZL model and with the 100 um dust thermal emission, respectively. The isotropic emission was obtained as EBL, which shows significant excess over integrated light of galaxies at <4 um. The obtained EBL is consistent with the previous measurements by IRTS and DIRBE.
    Publications- Astronomical Society of Japan 07/2013; 65(6). · 2.44 Impact Factor
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    ABSTRACT: We present near infrared spectroscopic observations of 19 molecular clouds made using the AKARI satellite, and the data reduction pipeline written to analyse those observations. The 2.5 --~5 $\mu$m spectra of 30 objects -- 22 field stars behind quiescent molecular clouds and eight low mass YSOs in cores -- were successfully extracted using the pipeline. Those spectra are further analysed to calculate the column densities of key solid phase molecular species, including H$_2$O, CO$_2$, CO, and OCN$^-$. The profile of the H$_2$O ice band is seen to vary across the objects observed and we suggest that the extended red wing may be an evolutionary indicator of both dust and ice mantle properties. The observation of 22 spectra with fluxes as low as $<$~5 mJy towards background stars, including 15 where the column densities of H$_2$O, CO and CO$_2$ were calculated, provides valuable data that could help to benchmark the initial conditions in star-forming regions prior to the onset of star formation.
    07/2013;
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    ABSTRACT: We present the near- and mid-infrared zodiacal light spectrum obtained with the AKARI Infra-Red Camera (IRC). A catalog of 278 spectra of the diffuse sky covering a wide range of Galactic and ecliptic latitudes was constructed. The wavelength range of this catalog is 1.8-5.3 {\mu}m with wavelength resolution of \lambda /\Delta \lambda ~20. Advanced reduction methods specialized for the slit spectroscopy of diffuse sky spectra are developed for constructing the spectral catalog. Based on the comparison analysis of the spectra collected in different seasons and ecliptic latitudes, we confirmed that the spectral shape of the scattered component and the thermal emission component of the zodiacal light in our wavelength range does not show any dependence on location and time, but relative brightness between them varies with location. We also confirmed that the color temperature of the zodiacal emission at 3-5 {\mu}m is 300+/-10 K at any ecliptic latitude. This emission is expected to be originated from sub-micron dust particles in the interplanetary space.
    Publications- Astronomical Society of Japan 06/2013; · 2.44 Impact Factor
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    ABSTRACT: We performed a near-infrared spectroscopic survey toward an area of ~10 deg2 of the Large Magellanic Cloud (LMC) with the infrared satellite AKARI. Observations were carried out as part of the AKARI Large-area Survey of the Large Magellanic Cloud (LSLMC). The slitless multi-object spectroscopic capability of the AKARI/IRC enabled us to obtain low-resolution (R ~ 20) spectra in 2-5 μm for a large number of point sources in the LMC. As a result of the survey, we extracted about 2000 infrared spectra of point sources. The data are organized as a near-infrared spectroscopic catalog. The catalog includes various infrared objects such as young stellar objects (YSOs), asymptotic giant branch (AGB) stars, supergiants, and so on. It is shown that 97% of the catalog sources have corresponding photometric data in the wavelength range from 1.2 to 11 μm, and 67% of the sources also have photometric data up to 24 μm. The catalog allows us to investigate near-infrared spectral features of sources by comparison with their infrared spectral energy distributions. In addition, it is estimated that about 10% of the catalog sources are observed at more than two different epochs. This enables us to study a spectroscopic variability of sources by using the present catalog. Initial results of source classifications for the LSLMC samples are presented. We classified 659 LSLMC spectra based on their near-infrared spectral features by visual inspection. As a result, it is shown that the present catalog includes 7 YSOs, 160 C-rich AGBs, 8 C-rich AGB candidates, 85 O-rich AGBs, 122 blue and yellow supergiants, 150 red super giants, and 128 unclassified sources. Distributions of the classified sources on the color-color and color-magnitude diagrams are discussed in the text. Continuous wavelength coverage and high spectroscopic sensitivity in 2-5 μm can only be achieved by space observations. This is an unprecedented large-scale spectroscopic survey toward the LMC in the near-infrared. A large number of near-infrared spectral data provided by the survey possess scientific potential that can be applied to various studies. In this paper, we present the details of the spectroscopic survey and the catalog, and discuss its scientific applications.
    The Astronomical Journal 01/2013; 145(2):32. · 4.97 Impact Factor
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    ABSTRACT: Formation of new stars requires the condensation of ambient neutral atomic hydrogen into the molecular phase. It is well known that stars form from molecular hydrogen clouds; less understood is how molecular clouds themselves begin to form. We use AKARI (MIR and FIS), HIRES (IRAS) and Spitzer (IRAC and MIPS) imaging photometry to probe thermal dust emission in a target cloud in the Perseus spiral arm, where the H1 to H2 transition appears to be underway. H2 formation in this region is indicated by strong HI self-absorption, variable CO emission, and significant ``excess'' infrared emission. We have sampled the dust spectral energy distribution (SED) at many positions on and off this cloud using AKARI, IRAS, and SST data. We present these SED plots with fits generated by the DustEM modeling program to provide some insight as to the nature of the dust composition, size distribution, and dominant heating mechanisms within this target and possibly other H2 forming regions.
    American Astronomical Society Meeting Abstracts; 01/2013
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    ABSTRACT: We investigate the polycyclic aromatic hydrocarbon features in the young Galactic planetary nebula PN G095.2+00.7 based on mid-infrared observations. The near- to mid-infrared spectra obtained with the AKARI/IRC and the Spitzer/IRS show the PAH features as well as the broad emission feature at 12 {\mu}m usually seen in proto-planetary nebulae (pPNe). The spatially resolved spectra obtained with Subaru/COMICS suggest that the broad emission around 12 {\mu}m is distributed in a shell-like structure, but the unidentified infrared band at 11.3 {\mu}m is selectively enhanced at the southern part of the nebula. The variation can be explained by a difference in the amount of the UV radiation to excite PAHs, and does not necessarily require the chemical processing of dust grains and PAHs. It suggests that the UV self-extinction is important to understand the mid-infrared spectral features. We propose a mechanism which accounts for the evolutionary sequence of the mid-infrared dust features seen in a transition from pPNe to PNe.
    The Astrophysical Journal Letters 11/2012; 760(2). · 6.35 Impact Factor
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    ABSTRACT: With AKARI, we have performed a systematic study of interstellar dust grains in various environments of galaxies. In many cases, the IR emission of dust is an important tool to trace star-forming activities in galaxies. However it is much more than just star-formation tracers. AKARI has revealed the detailed properties of dust grains in regions not relevant to star formation as well, some of which are found not to follow our old empirical knowledge. Because of its unique capabilities, such as near- and far-IR spectroscopy, and all-sky coverage, AKARI has provided new knowledge on the processing of carbonaceous grains including polycyclic aromatic hydrocarbons. We present the latest results obtained from our AKARI observations of the ISM in our Galaxy and nearby galaxies.
    Proceedings of the International Astronomical Union 11/2012; 27(4):237-242.
  • Publication of Korean Astronomical Society. 11/2012; 27:195-200.
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    ABSTRACT: Spectroscopic studies of extragalactic YSOs have shown a great progress in the last few years. Infrared observations with AKARI made significant contributions to that progress. In this proceeding, we are going to introduce our current research on the infrared observations of ices and dust around embedded YSOs in the Magellanic Clouds.
    Publication of Korean Astronomical Society. 11/2012; 27(4):171-175.

Publication Stats

1k Citations
404.17 Total Impact Points

Institutions

  • 2004–2014
    • The University of Tokyo
      • • Department of Astronomy
      • • Institute for the Physics and Mathematics of the Universe (IPMU)
      Edo, Tōkyō, Japan
    • Kitasato University
      • Center for Natural Sciences
      Edo, Tōkyō, Japan
  • 2013
    • Kobe University
      • Department of Earth and Planetary Sciences
      Kōbe, Hyōgo, Japan
  • 2012
    • National Astronomical Observatory of Japan
      Edo, Tōkyō, Japan
  • 2010
    • Nagoya University
      • Graduate School of Science
      Nagoya, Aichi, Japan
  • 2009
    • Scottish Universities Physics Alliance
      Glasgow, Scotland, United Kingdom
  • 2005
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, HI, United States