Takashi Onaka

The University of Tokyo, Tōkyō, Japan

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Publications (506)984.43 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The near-infrared (NIR) spectral range (2-5 μm) contains a number of interesting features for the study of the interstellar medium. In particular, the aromatic and aliphatic components in carbonaceous dust can be investigated most efficiently with the NIR spectroscopy. We analyze NIR spectra of the diffuse Galactic emission taken with the Infrared Camera onboard AKARI and find that the aliphatic to aromatic emission band ratio decreases toward the ionized gas, which suggests processing of the band carriers in the ionized region.
    Proceedings of the International Astronomical Union 08/2015; 10(H16):703-704. DOI:10.1017/S1743921314012976
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    ABSTRACT: We performed near-infrared line mapping of the supernova remnant IC 443 with the IRSF 1.4-m telescope, using the narrow-band filters tuned for [Fe II] 1.257 μm, [Fe II] 1.644 μm, and Paβ. We detected the [Fe II] and Paβ emissions from a very wide area of the observed region ( ). These line intensity maps reveal a global correlation between [Fe II] and Paβ. Kokusho et al. (2013) found that the [Fe II] emission is notably strong while dust emission is faint in inner regions of IC 443. These results indicate that the abundant Fe+ is likely to be of interstellar origins through shock destruction of dust, rather than of ejecta origins. From the X-ray intensity map of the 6.7 keV Fe-K line obtained with Suzaku, we find that highly-ionized He-like Fe ions exist in the inner regions of IC 443, toward which Fe+ is detected. The presence of the He-like Fe ions and the faint dust emission indicates that Fe is likely to be interacting with X-ray plasma for a time long enough to be highly ionized. We discuss the implications of the detection of the lowly-ionized Fe ions in such regions for the processing and composition of dust in the interstellar environment around IC 443.
    Planetary and Space Science 04/2015; DOI:10.1016/j.pss.2015.04.009 · 1.63 Impact Factor
  • Takashi Onaka, Tamami I. Mori, Yoko Okada
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    ABSTRACT: Dust processing in the Carina nebula is investigated based on mid- to far-infrared spectroscopy with Infrared Space Observatory (ISO). Mapping observations over a central area of the nebula with PHT-S, SWS, and LWS onboard ISO not only reveal spectroscopically that the mid-infrared unidentified infrared (UIR) bands at 6.2, 7.7, 8.6, and 11.3 μm are absent in the ionized region, but also indicate that the 11.3 μm may behave differently from the other three UIR bands near the edge of the ionized region, suggesting a variation either in the size distribution or in the ionization fraction of the band carriers. The correlation of [NII]122 μm and [SiII]35 μm line emissions observed with SWS and LWS are reinvestigated based on the recent atomic data as well as the latest cosmic abundance, suggesting that a large fraction ( %), if not all, of silicon returns to the gas phase in the Carina nebula, suggesting that silicates cannot survive in harsh conditions, such as massive star-forming regions. The present observations clearly show dust processing taking place in active regions in the Galaxy.
    Planetary and Space Science 04/2015; DOI:10.1016/j.pss.2015.03.025 · 1.63 Impact Factor
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    ABSTRACT: In the evolutionary path of interstellar medium inquiry, many new species of interstellar dust have been modeled and discovered. The modes by which these species interact and evolve are beginning to be understood, but in recent years a peculiar new feature has appeared in microwave surveys. Anomalous microwave emission (AME), appearing between 10 and 90 GHz, has been correlated with thermal dust emission, leading to the popular suggestion that this anomaly is electric dipole emission from spinning dust. The observed frequencies suggest that spinning grains should be on the order of 10nm in size, hinting at poly-cyclic aromatic hydrocarbon molecules. We present data from AKARI/Infrared Camera (IRC), due to the effective PAH/Unidentified Infrared Band (UIR) coverage of its 9 micron survey to investigate their role within a few regions showing strong AME in the Planck low frequency data. We include the well studied Perseus and rho Ophiuchi clouds . We use the IRAS/IRIS 100 micron data to account for the overall dust temperature. We present our results as abundance maps for dust emitting around 9 micron, and 100 micron. Part of the AME in these regions may actually be attributed to thermal dust emission, or the star forming nature of these targets is masking the vibrational modes of PAHs which should be present there, suggesting further investigation for various galactic environments.
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    ABSTRACT: We imaged circumstellar disks around 22 Herbig Ae/Be stars at 25 \mu m using Subaru/COMICS and Gemini/T-ReCS. Our sample consists of equal numbers of objects belonging to the two categories defined by Meeus et al. (2001); 11 group I (flaring disk) and II (at disk) sources. We find that group I sources tend to show more extended emission than group II sources. Previous studies have shown that the continuous disk is hard to be resolved with 8 meter class telescopes in Q-band due to the strong emission from the unresolved innermost region of the disk. It indicates that the resolved Q-band sources require a hole or gap in the disk material distribution to suppress the contribution from the innermost region of the disk. As many group I sources are resolved at 25 \mu m, we suggest that many, not all, group I Herbig Ae/Be disks have a hole or gap and are (pre-)transitional disks. On the other hand, the unresolved nature of many group II sources at 25 \mu m supports that group II disks have continuous at disk geometry. It has been inferred that group I disks may evolve into group II through settling of dust grains to the mid-plane of the proto-planetary disk. However, considering growing evidence for the presence of a hole or gaps in the disk of group I sources, such an evolutionary scenario is unlikely. The difference between groups I and II may reflect different evolutionary pathways of protoplanetary disks.
    The Astrophysical Journal 03/2015; 804(2). DOI:10.1088/0004-637X/804/2/143 · 6.28 Impact Factor
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    ABSTRACT: In photon-dominated regions (PDRs), UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of gas-phase polycyclic aromatic hydrocarbons (PAHs). Our goal is to get further insights into the composition and evolution of evaporating very small grains (eVSGs) and PAHs through the analysis of the infrared (IR) aliphatic and aromatic emission bands. We combine spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023. We use near-IR spectra obtained with AKARI to trace the evolution of the 3.3$\mu$m and 3.4$\mu$m bands that are associated with aromatic and aliphatic C-H bonds on PAHs, respectively. The spectral fitting involves an additional broad feature centred at 3.45$\mu$m. Mid-IR observations obtained with Spitzer are used to discriminate the signatures of eVSGs, neutral and cationic PAHs. We correlate the spatial evolution of all these bands with the intensity of the UV field to explore the processing of their carriers. The intensity of the 3.45$\mu$m plateau shows an excellent correlation with that of the 3.3$\mu$m aromatic band (correlation coefficient R = 0.95), indicating that the plateau is dominated by the emission from aromatic bonds. The ratio of the 3.4$\mu$m and 3.3$\mu$m band intensity ($I_{3.4}/I_{3.3}$) decreases by a factor of 4 at the PDR interface from the more UV-shielded to the more exposed layers. The transition region between the aliphatic and aromatic material corresponds spatially with the transition zone between neutral PAHs and eVSGs. We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4$\mu$m emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic/aliphatic composition in PDRs.
    Astronomy and Astrophysics 02/2015; 577. DOI:10.1051/0004-6361/201425590 · 4.48 Impact Factor
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    ABSTRACT: A model is described that can be used to estimate the bulk polarization of large rotating meteoroids in the magnetic field of a neutron star. The results of this model are applicable to the Supernova Neutrino Amino Acid Processing model, which describes on possible way in which the amino acids, known in nearly all cases to exhibit supramolecular chirality, could have become enantiomeric.
    Symmetry 11/2014; Volume 6, p. 909. DOI:10.3390/sym6040909 · 0.92 Impact Factor
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    ABSTRACT: The infrared (IR) emission from interstellar dust grains is a powerful tool to trace star-formation activities in galaxies. Beyond such star-formation tracers, spectral information on polycyclic aromatic hydrocarbons (PAHs) and large grains, or even their photometric intensity ratios, has deep physical implications for understanding the properties of the interstellar medium. With the AKARI satellite launched in 2006, we have performed a systematic study of interstellar dust grains in various environments of galaxies including our Galaxy. Because of its unique capabilities, such as mid-/far-IR all-sky surveys and near-/far-IR spectroscopy, AKARI has provided new knowledge on the processing of dust, particularly carbonaceous grains including PAHs, in the interstellar space. For example, the near-IR spectroscopy has revealed structural changes of hydrocarbon grains in harsh environments of galaxies. In this paper, we focus on the properties of the PAH emission obtained by the AKARI mid-IR all-sky survey and near-IR spectroscopy.
    Planetary and Space Science 10/2014; 100. DOI:10.1016/j.pss.2014.01.017 · 1.63 Impact Factor
  • The Astrophysical Journal 08/2014; 792(1):80. DOI:10.1088/0004-637X/792/1/80 · 6.28 Impact Factor
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    ABSTRACT: The Japanese SPace Infrared telescope for Cosmology and Astrophysics, SPICA, aims to provide astronomers with a truly new window on the universe. With a large -3 meter class- cold -6K- telescope, the mission provides a unique low background environment optimally suited for highly sensitive instruments limited only by the cosmic background itself. SAFARI, the SpicA FAR infrared Instrument SAFARI, is a Fourier Transform imaging spectrometer designed to fully exploit this extremely low far infrared background environment. The SAFARI consortium, comprised of European and Canadian institutes, has established an instrument reference design based on a Mach-Zehnder interferometer stage with outputs directed to three extremely sensitive Transition Edge Sensor arrays covering the 35 to 210 μm domain. The baseline instrument provides R > 1000 spectroscopic imaging capabilities over a 2’ by 2’ field of view. A number of modifications to the instrument to extend its capabilities are under investigation. With the reference design SAFARI’s sensitivity for many objects is limited not only by the detector NEP but also by the level of broad band background radiation – the zodiacal light for the shorter wavelengths and satellite baffle structures for the longer wavelengths. Options to reduce this background are dedicated masks or dispersive elements which can be inserted in the optics as required. The resulting increase in sensitivity can directly enhance the prime science goals of SAFARI; with the expected enhanced sensitivity astronomers would be in a better position to study thousands of galaxies out to redshift 3 and even many hundreds out to redshifts of 5 or 6. Possibilities to increase the wavelength resolution, at least for the shorter wavelength bands, are investigated as this would significantly enhance SAFARI’s capabilities to study star and planet formation in our own galaxy.
    SPIE Astronomical Telescopes + Instrumentation; 08/2014
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    ABSTRACT: We present the current status of SPICA (Space Infrared Telescope for Cosmology and Astrophysics), which is a mission optimized for mid- and far-infrared astronomy with a cryogenically cooled 3.2 m telescope. SPICA is expected to achieve high spatial resolution and unprecedented sensitivity in the mid- and far-infrared, which will enable us to address a number of key problems in present-day astronomy, ranging from the star-formation history of the universe to the formation of planets. We have carried out the “Risk Mitigation Phase” activity, in which key technologies essential to the realization of the mission have been extensively developed. Consequently, technical risks for the success of the mission have been significantly mitigated. Along with these technical activities, the international collaboration framework of SPICA had been revisited, which resulted in maintenance of SPICA as a JAXA-led mission as in the original plan but with larger contribution of ESA than that in the original plan. To enable the ESA participation, a SPICA proposal to ESA is under consideration as a medium-class mission under the framework of the ESA Cosmic Vision. The target launch year of SPICA under the new framework is FY2025.
    SPIE Astronomical Telescopes + Instrumentation; 08/2014
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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: 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.
    The Astrophysical Journal 01/2014; 784(1). DOI:10.1088/0004-637X/784/1/53 · 6.28 Impact Factor
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    Ji Yeon Seok, Bon-Chul Koo, Takashi Onaka
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    ABSTRACT: We present a comprehensive infrared study of supernova remnants (SNRs) in the Large Magellanic Cloud (LMC) using near- to mid-infrared images taken by Infrared Array Camera (IRAC; 3.6, 4.5, 5.8, and 8 {\mu}m) and Multiband Imaging Photometer (MIPS; 24 and 70 {\mu}m) onboard the Spitzer Space Telescope. Among the 47 bona fide LMC SNRs, 29 were detected in infrared, giving a high detection rate of 62%. All 29 SNRs show emission at 24 {\mu}m, and 20 out of 29 show emission in one or several IRAC bands. We present their 4.5, 8, 24, and 70 {\mu}m images and a table summarizing their Spitzer fluxes. We find that the LMC SNRs are considerably fainter than the Galactic SNRs, and that, among the LMC SNRs, Type Ia SNRs are significantly fainter than core-collapse SNRs. We conclude that the MIPS emission of essentially all SNRs origi- nates from dust emission, whereas their IRAC emissions originate from ionic/molecular lines, polycyclic aromatic hydrocarbons emission, or synchrotron emission. The infrared fluxes show correlation with radio and X-ray fluxes. For SNRs that have similar mor- phology in infrared and X-rays, the ratios of 24 to 70 {\mu}m fluxes have good correlation with the electron density of hot plasma. The overall correlation is explained well by the emission from collisionally-heated silicate grains of 0.1 {\mu}m size, but for mature SNRs with relatively low gas temperatures, the smaller-sized grain population is favored more. For those that appear different between infrared and X-rays, the emission in the MIPS bands is probably from dust heated by shock radiation.
    The Astrophysical Journal 12/2013; 779(2). DOI:10.1088/0004-637X/779/2/134 · 6.28 Impact Factor
  • Proceedings of The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments (LCDU2013); 11/2013
  • Proceedings of The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments (LCDU2013); 11/2013
  • Proceedings of The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments (LCDU2013); 11/2013
  • Proceedings of The Life Cycle of Dust in the Universe: Observations, Theory, and Laboratory Experiments (LCDU2013); 11/2013
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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). DOI:10.1088/0004-637X/780/2/114 · 6.28 Impact Factor

Publication Stats

3k Citations
984.43 Total Impact Points

Institutions

  • 2–2015
    • The University of Tokyo
      • Department of Astronomy
      Tōkyō, Japan
  • 2013
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 1996–2011
    • Nagoya University
      • • Graduate School of Science
      • • Division of Cell Science
      Nagoya, Aichi, Japan
  • 2010
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
  • 1984–2010
    • National Astronomical Observatory of Japan
      • Division of Optical and Infrared Astronomy
      Edo, Tōkyō, Japan
  • 2006–2009
    • Ibaraki University
      • College of Science
      Mito-shi, Ibaraki, Japan
  • 2003–2008
    • University of California, Berkeley
      • Radio Astronomy Laboratory
      Berkeley, California, United States
    • Imperial College London
      Londinium, England, United Kingdom
  • 2003–2004
    • Kitasato University
      • Center for Natural Sciences
      Edo, Tōkyō, Japan
  • 1999
    • Netherlands Institute for Space Research, Utrecht
      Utrecht, Utrecht, Netherlands
  • 1998
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 1989–1998
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdam, North Holland, Netherlands
  • 1991–1994
    • Seikei University
      Edo, Tōkyō, Japan
  • 1984–1987
    • The University of Electro-Communications
      • Department of Applied Physics and Chemistry
      Edo, Tōkyō, Japan