arXiv subjects

Takashi Onaka

Publications and source records attributed to Takashi Onaka.

156 records · Page 9Linked to original sources

arXiv2007

AKARI Detection of the Infrared-Bright Supernova Remnant B0104-72.3 in the Small Magellanic Cloud

We present a serendipitous detection of the infrared-bright supernova remnant (SNR) B0104-72.3 in the Small Magellanic Cloud by the Infrared Camera (IRC) onboard AKARI. An elongated, partially complete shell is detected in all four observed IRC bands covering 2.6-15 um. The infrared shell surrounds radio, optical, and X-ray emission associated with the SNR and is probably a radiative SNR shell. This is the first detection of a SNR shell in this near/mid-infrared waveband in the Small Magellanic Cloud. The IRC color indicates that the infrared emission might be from shocked H2 molecules with some possible contributions from ionic lines. We conclude that B0104-72.3 is a middle-aged SNR interacting with molecular clouds, similar to the Galactic SNR IC 443. Our results highlight the potential of AKARI IRC observations in studying SNRs, especially for diagnosing SNR shocks.

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arXiv2007

The Asymmetric Thermal Emission of Protoplanetary Disk Surrounding HD 142527 Seen by Subaru/COMICS

Mid-infrared (MIR) images of the Herbig Ae star HD 142527 were obtained at 18.8 and 24.5 micron with the Subaru/COMICS. Bright extended arc-like emission (outer disk) is recognized at r=0.85" together with a strong central source (inner disk) and a gap around r=0.6" in the both images. Thermal emission of the eastern side is much brighter than that of the western side in the MIR. We estimate the dust size as a few micron from the observed color of the extended emission and the distance from the star. The dust temperature T and the optical depth tau of the MIR emitting dust are also derived from the two images as T=82+/-1K, tau=0.052+/-0.001 for the eastern side and T=85+/-3K, tau=0.018+/-0.001 for the western side. The observed asymmetry in the brightness can be attributed to the difference in the optical depth of the MIR emitting dust. To account for the present observations, we propose an inclined disk model, in which the outer disk is inclined along the east-west direction with the eastern side being in the far side and the inner rim of the outer disk in the eatern side is exposed directly to us. The proposed model can successfully account for the MIR observations as well as near-infrared (NIR) images of the scattering light, in which the asymmetry is seen in the opposite sense and the forward scattering light (near side -- western side) is brighter.

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arXiv2007

Ionization of Polycyclic Aromatic Hydrocarbon Molecules around the Herbig Ae/Be environment

We present the results of mid-infrared N-band spectroscopy of the Herbig Ae/Be system MWC1080 using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on board the 8m Subaru Telescope. The MWC1080 has a geometry such that the diffuse nebulous structures surround the central Herbig B0 type star. We focus on the properties of polycyclic aromatic hydrocarbons (PAHs) and PAH-like species, which are thought to be the carriers of the unidentified infrared (UIR) bands in such environments. A series of UIR bands at 8.6, 11.0, 11.2, and 12.7um is detected throughout the system and we find a clear increase in the UIR 11.0um/11.2um ratio in the vicinity of the central star. Since the UIR 11.0um feature is attributed to a solo-CH out-of-plane wagging mode of cationic PAHs while the UIR 11.2um feature to a solo-CH out-of-plane bending mode of neutral PAHs, the large 11.0um/11.2um ratio directly indicates a promotion of the ionization of PAHs near the central star.

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arXiv2006

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and High-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II]1.644 micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0 micron, resembles the general morphology of visible H-alpha and near-IR [Fe II] line emission, while the 3.6 and 4.5 micron images are dominated by continuum synchrotron emission. The 24 micron and 70 micron images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum jet and torus with the steeper diffuse nebula, and suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and HST images (Hester et al. 2002) can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of approximately 1264 km/s.

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arXiv2005

Large Silicon Abundance in Photodissociation Regions

We have made one-dimensional raster-scan observations of the rho Oph and sigma Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the rho Oph region, [SiII] 35um, [OI] 63um, 146um, [CII] 158um, and the H2 pure rotational transition lines S(0) to S(3) are detected, and the PDR properties are derived as the radiation field scaled by the solar neighborhood value G_0~30-500, the gas density n~250--2500 /cc, and the surface temperature T~100-400 K. The ratio of [SiII] 35um to [OI] 146um indicates that silicon of 10--20% of the solar abundance must be in the gaseous form in the photodissociation region (PDR), suggesting that efficient dust destruction is undergoing even in the PDR and that part of silicon atoms may be contained in volatile forms in dust grains. The [OI] 63um and [CII] 158um emissions are too weak relative to [OI] 146um to be accounted for by standard PDR models. We propose a simple model, in which overlapping PDR clouds along the line of sight absorb the [OI] 63um and [CII] 158um emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the sigma Sco region, we have detected 3 fine-structure lines, [OI] 63um, [NII] 122um, and [CII] 158um, and derived that 30-80% of the [CII] emission comes from the ionized gas. The upper limit of the [SiII] 35um is compatible with the solar abundance relative to nitrogen and no useful constraint on the gaseous Si is obtained for the sigma Sco region.

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arXiv2005

The ASTRO-F Mission : Large Area Infrared Survey

ASTRO-F is the first Japanese satellite mission dedicated for large area surveys in the infrared. The 69cm aperture telescope and scientific instruments are cooled to 6K by liquid Helium and mechanical coolers. During the expected mission life of more than 500 days, ASTRO-F will make the most advanced all-sky survey in the mid- to far-infrared since the Infrared astronomical Satellite (IRAS). The survey will be made in 6 wavebands and will include the first all sky survey at >100-160(mu)m. Deep imaging and spectroscopic surveys with pointed observations will also be carried out in 13 wavelength bands from 2-160(mu)m. ASTRO-F should detect more than a half million galaxies tracing the large-scale structure of the Universe out to redshifts of unity, detecting rare, exotic extraordinarily luminous objects at high redshift, numerous brown dwarfs, Vega-like stars, protostars, and will reveal the large-scale structure of nearby galactic star forming regions. ASTRO-F is a perfect complement to Spitzer Space Telescope in respect of its wide sky and wavelength coverage. Approximately 30 percent of pointed observations will be allocated to an open-time opportunity. Updated pre-flight ensitivities as well as the observation plan including the large-area surveys are described.

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arXiv2004

Crystalline Silicate Feature of the Vega-like star HD145263

We have observed the 8-13 $μ$m spectrum (R$\sim$250) of the Vega-like star candidate HD145263 using Subaru/COMICS. The spectrum of HD145263 shows the broad trapezoidal silicate feature with the shoulders at 9.3 $μ$m and 11.44 $μ$m, indicating the presence of crystalline silicate grains. This detection implies that crystalline silicate may also be commonly present around Vega-like stars. The 11.44 $μ$m feature is slightly shifted to a longer wavelength compared to the usual 11.2-3 $μ$m crystalline forsterite feature detected toward Herbig Ae/Be stars and T Tauri stars. Although the peak shift due to the effects of the grain size can not be ruled out, we suggest that Fe-bearing crystalline olivine explains the observed peak wavelength fairly well. Fe-bearing silicates are commonly found in meteorites and most interplanetary dust particles, which originate from planetesimal-like asteroids. According to studies of meteorites, Fe-bearing silicate must have been formed in asteroidal planetesimals, supporting the scenario that dust grains around Vega-like stars are of planetesimal origin, if the observed 11.44 $μ$m peak is due to Fe-bearing silicates.

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arXiv2003

Detection of crystalline silicates around the T Tauri star Hen 3-600A

We have carried out mid-infrared N-band spectroscopic observations of the T Tauri star Hen 3-600A in the TW Hydra association with the COMICS on the 8.2m Subaru Telescope and found structured features in its spectrum. These structured features are well explained by a combination of crystalline forsterite, crystalline enstatite, silica and glassy olivine grains. Among intermediate-mass young stellar objects (YSOs), crystalline silicates have already been detected, but no firm detection has been reported so far for low-mass YSOs such as T Tauri stars. This is the first clear detection of crystalline silicates in low-mass YSOs and shows that the crystallization event occurs even in the protoplanetary disk of low-mass YSOs in the T Tauri phase. The physical processes leading to the inferred dust composition in the Hen3-600A system may be analogous to those occured in the early epoch of the Solar system.

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arXiv2002

Detection of Far-Infrared Features in Star-Forming Regions

We report the detection of a feature at 65um and a broad feature around 100um in the far-infrared spectra of the diffuse emission from two active star-forming regions, the Carina nebula and Sharpless 171. The features are seen in the spectra over a wide area of the observed regions, indicating that the carriers are fairly ubiquitous species in the interstellar medium. A similar 65um feature has been detected in evolved stars and attributed to diopside, a Ca-bearing crystalline silicate. The present observations indicate the first detection of a crystalline silicate in the interstellar medium if this identification holds true also for the interstellar feature. A similar broad feature around 90um reported in the spectra of evolved stars has been attributed to calcite, a Ca-bearing carbonate mineral. The interstellar feature seems to be shifted to longer wavelengths and have a broader width although the precise estimate of the feature profile is difficult. As a carrier for the interstellar 100um feature, we investigate the possibility that the feature originates from carbon onions, grains consisting of curved graphitic shells. Because of the curved graphitic sheet structure, the optical properties in the direction parallel to the graphitic plane interacts with those in the vertical direction in carbon onion grains. This effect enhances the interband transition feature in the direction parallel to the graphitic plane in carbon onions, which is suppressed in graphite particles. Simple calculations suggest that carbon onion grains are a likely candidate for the observed 100um feature carrier, but the appearance of the feature is sensitive to the assumed optical properties.

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arXiv2002

O and Ne K absorption edge structures and interstellar abundance towards Cyg X-2

We have studied the O and Ne absorption features in the X-ray spectrum of Cyg X-2 observed with the Chandra LETG. The O absorption edge is represented by the sum of three absorption-edge components within the limit of the energy resolution and the photon counting statistics. Two of them are due to the atomic O; their energies correspond to two distinct spin states of photo-ionized O atoms. The remaining edge component is considered to represent compound forms of oxide dust grains. Since Cyg X-2 is about 1.4 kpc above the galactic disk, the H column densities can be determined by radio (21 cm and CO emission line) and H alpha observations with relatively small uncertainties. Thus the O abundance relative to H can be determined from the absorption edges. We found that the dust scattering can affect the apparent depth of the edge of the compound forms. We determined the amplitude of the effect, which we consider is the largest possible correction factor. The ratio of column densities of O in atomic to compound forms and the O total abundance were respectively determined to be in the range 1.7^{+3.0}_{-0.9} to 2.8^{+5.1}_{-1.5} (ratio), and 0.63 +/- 0.12 solar to 0.74 +/- 0.14 solar (total), taking into account the uncertainties in the dust-scattering correction and in the ionized H column density. We also determined the Ne abundance from the absorption edge to be 0.75 +/- 0.20 solar. These abundance values are smaller than the widely-used solar values but consistent with the latest estimates of solar abundance.

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arXiv2002

A New Empirical Method of Estimation of the Far-Infrared Flux of Galaxies

We propose a new empirical method to estimate the total far-infrared flux of galaxies from the spectral energy distribution (SED) at wavelengths lambda shorter than 100 microns. It is difficult to derive the total far-infrared luminosity from only the IRAS data, though it is one of the most important properties of galaxies. Observations by Infrared Telescope in Space (IRTS) indicate that the SED of the diffuse emission from the Galactic plane in this wavelength region can be derived from the 60 microns to 100 microns color. This empirical SED relation was improved in order to obtain a better fit to the Galactic plane data for I(60 microns)/I(100 microns) > 0.6, and applied to 96 IRAS galaxies for which ISOPHOT and KAO data are available at lambda > 100 microns. As a result, the empirical relation describes well the far-infrared (FIR) SED for a majority of the galaxies. Additionally, the total FIR flux for lambda longer than 40 microns is derived from the flux densities at 60 and 100 microns by using this model. For the 96 IRAS galaxies, the uncertainty in the total far-infrared flux of the present method is 26%. The present method is more accurate than the previous one widely used to derive the total infrared flux from the IRAS 60 and 100 microns data.

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arXiv1999

Polarization of Thermal Emission from Aligned Dust Grains Under an Anisotropic Radiation Field

If aspherical dust grains are immersed in an anisotropic radiation field, their temperature depends on the cross-sections projected in the direction of the anisotropy.It was shown that the temperature difference produces polarized thermal emission even without alignment, if the observer looks at the grains from a direction different from the anisotropic radiation. When the dust grains are aligned, the anisotropy in the radiation makes various effects on the polarization of the thermal emission, depending on the relative angle between the anisotropy and alignment directions. If the both directions are parallel, the anisotropy produces a steep increase in the polarization degree at short wavelengths. If they are perpendicular, the polarization reversal occurs at a wavelength shorter than the emission peak. The effect of the anisotropic radiation will make a change of more than a few % in the polarization degree for short wavelengths and the effect must be taken into account in the interpretation of the polarization in the thermal emission. The anisotropy in the radiation field produces a strong spectral dependence of the polarization degree and position angle, which is not seen under isotropic radiation. The dependence changes with the grain shape to a detectable level and thus it will provide a new tool to investigate the shape of dust grains. This paper presents examples of numerical calculations of the effects and demonstrates the importance of anisotropic radiation field on the polarized thermal emission.

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