arXiv subjects

Takashi Onaka

Publications and source records attributed to Takashi Onaka.

At least 127 records · Page 7Linked to original sources

arXiv2011

Characterization and Improvement of the Image Quality of the Data Taken with the Infrared Camera (IRC) Mid-Infrared Channels onboard AKARI

Mid-infrared images frequently suffer artifacts and extended point spread functions (PSFs). We investigate the characteristics of the artifacts and the PSFs in images obtained with the Infrared Camera (IRC) onboard AKARI at four mid-infrared bands of the S7 (7μm), S11 (11μm), L15 (15μm), and L24 (24 μm). Removal of the artifacts significantly improves the reliability of the ref- erence data for flat-fielding at the L15 and L24 bands. A set of models of the IRC PSFs is also constructed from on-orbit data. These PSFs have extended components that come from diffraction and scattering within the detector arrays. We estimate the aperture correction factors for point sources and the surface brightness correction factors for diffuse sources. We conclude that the surface brightness correction factors range from 0.95 to 0.8, taking account of the extended component of the PSFs. To correct for the extended PSF effects for the study of faint structures, we also develop an image reconstruction method, which consists of the deconvolution with the PSF and the convolution with an appropriate Gaussian. The appropriate removal of the artifacts, improved flat-fielding, and image reconstruction with the extended PSFs enable us to investigate de- tailed structures of extended sources in IRC mid-infrared images.

astro-ph.IM↗

arXiv2011

A temporal study of oxygen-rich pulsating variable AGB star, T Cep: Investigation on dust formation and dust properties

Pulsation is believed to be the leading cause of dusty mass loss from Asymptotic Giant Branch (AGB) stars. We present a temporal study of T Cep, a long-period Mira variable, using seven ISO SWS spectra, covering a 16-month period over a single pulsation cycle. The observed spectral dust features change over the pulsation cycle of this Mira. In general, the overall apparent changes in spectral features can be attributed to changes in the dust temperature, resulting from the intrinsic pulsation cycle of the central star. However, not all feature changes are so easily explained. Based on direct comparison with laboratory spectra of several potential minerals, the dust is best explained by crystalline iron-rich silicates. These findings contradict the currently favored dust formation hypotheses.

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arXiv2011

Properties of dust and PAHs in the hot plasma of the elliptical galaxy NGC4125 revealed with AKARI and Spitzer

We present the spatial distributions of dust and polycyclic aromatic hydrocarbons (PAHs) in the elliptical galaxy NGC4125, revealed by AKARI and Spitzer. NGC4125 is relatively bright in the dust and the PAH emision for elliptical galaxies, although it certainly possesses diffuse interstellar hot plasma indicated by the high spatial resolution X-ray data of Chandra. We investigate how the dust and PAHs interact with the X-ray plasma or avoid the interaction by comparing their spatial distributions. We find that the distributions of the PAHs and dust are different from each other, both showing a significant deviation from a smooth stellar distribution. The PAH emission predominantly comes from a dust lane, a compact dense molecular gas region in the galactic center, where the PAHs are likely to have been protected from the interaction with the X-ray plasma. The dust emission has more extended structures similar to the distribution of the X-ray plasma, suggesting their interaction to some extent. We also discuss a possible origin of the dust and PAHs in the galaxy.

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arXiv2011

Far-infrared emission from intergalactic medium in Stephan's Quintet revealed by AKARI

The Stephan's Quintet (SQ, HCG92) was observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared (IR) bands at 65, 90, 140, and 160 um. The AKARI four-band images of the SQ show far-IR emission in the intergalactic medium (IGM) of the SQ. In particular, the 160 um band image shows single peak emission in addition to the structure extending in the North-South direction along the shock ridge as seen in the 140 um band, H2 emission and X-ray emission. Whereas most of the far-IR emission in the shocked region comes from the cold dust component, shock-powered [CII]158um emission can significantly contribute to the emission in the 160 um band that shows a single peak at the shocked region. In the shocked region, the observed gas-to-dust mass ratio is in agreement with the Galactic one. The color temperature of the cold dust component (~20 K) is lower than that in surrounding galaxies (~30 K). We discuss a possible origin of the intergalactic dust emission.

astro-ph.CO↗

arXiv2011

IRAS 15099-5856: Remarkable Mid-Infrared Source with Prominent Crystalline Silicate Emission Embedded in the Supernova Remnant MSH15-52

We report new mid-infrared observations of the remarkable object IRAS 15099-5856 using the space telescopes AKARI and Spitzer, which demonstrate the presence of prominent crystalline silicate emission in this bright source. IRAS 15099-5856 has a complex morphology with a bright central compact source (IRS1) surrounded by knots, spurs, and several extended (~4') arc-like filaments. The source is seen only at >= 10 um. The Spitzer MIR spectrum of IRS1 shows prominent emission features from Mg-rich crystalline silicates, strong [Ne II] 12.81 um and several other faint ionic lines. We model the MIR spectrum as thermal emission from dust and compare with the Herbig Be star HD 100546 and the luminous blue variable R71, which show very similar MIR spectra. Molecular line observations reveal two molecular clouds around the source, but no associated dense molecular cores. We suggest that IRS1 is heated by UV radiation from the adjacent O star Muzzio 10 and that its crystalline silicates most likely originated in a mass outflow from the progenitor of the supernova remnant (SNR) MSH 15-52. IRS1, which is embedded in the SNR, could have been shielded from the SN blast wave if the progenitor was in a close binary system with Muzzio 10. If MSH15-52 is a remnant of Type Ib/c supernova (SN Ib/c), as has been previously proposed, this would confirm the binary model for SN Ib/c. IRS1 and the associated structures may be the relics of massive star death, as shaped by the supernova explosion, the pulsar wind and the intense ionizing radiation of the embedded O star.

astro-ph.GA↗

arXiv2011

Effects of high-energy ionizing particles on the Si:As mid-infrared detector array on board the AKARI satellite

We evaluate the effects of high-energy ionizing particles on the Si:As impurity band conduction (IBC) mid-infrared detector on board AKARI, the Japanese infrared astronomical satellite. IBC-type detectors are known to be little influenced by ionizing radiation. However we find that the detector is significantly affected by in-orbit ionizing radiation even after spikes induced by ionizing particles are removed. The effects are described as changes mostly in the offset of detector output, but not in the gain. We conclude that the changes in the offset are caused mainly by increase in dark current. We establish a method to correct these ionizing radiation effects. The method is essential to improve the quality and to increase the sky coverage of the AKARI mid-infrared all-sky-survey map.

astro-ph.IM↗

arXiv2011

AKARI Near-Infrared Spectroscopic Observations of Interstellar Ices in Edge-on Starburst Galaxy NGC253

We present the spatially-resolved near-infrared (2.5-5.0 um) spectra of the edge-on starburst galaxy NGC253 obtained with the Infrared Camera onboard AKARI. Near the center of the galaxy, we clearly detect the absorption features of interstellar ices (H_2O: 3.05 um, CO_2: 4.27 um, and XCN: 4.62 um) and the emission of polycyclic aromatic hydrocarbons (PAHs) at 3.29 um and hydrogen recombination line Br alpha at 4.05 um. We find that the distributions of the ices differ from those of the PAH and gas. We calculate the column densities of the ices and derive the abundance ratios of N(CO_2)/N(H_2O) = 0.17 +- 0.05. They are similar to those obtained around the massive young stellar objects in our Galaxy (0.17 +- 0.03), although much stronger interstellar radiation field and higher dust temperature are expected near the center of NGC253.

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arXiv2011

Detection of the detached dust shell of U Antliae at mid-infrared wavelengths with AKARI/IRC

We report mid-infrared (MIR) imaging observations of the carbon star U Ant made with the Infrared Camera (IRC) on board AKARI. Subtraction of the artifacts and extended PSF of the central star reveals the detached dust shell around the carbon star at MIR wavelengths (15 and 24μm) for the first time. The observed radial brightness profiles of the MIR emission are well explained by two shells at 43" and 50" from the central star detected in optical scattered light observations. Combining Herschel/PACS, AKARI/FIS, and AKARI/IRC data, we obtain the infrared spectral energy distribution (SED) of the thermal emission from the detached shell of U Ant in a wide infrared spectral range of 15- 160 μm. Thermal emission of amorphous carbon grains with a single temperature cannot account for the observed SED from 15 to 160 μm: it underestimates the emission at 15 μm. Alternatively, the observed SED is fitted by the model that amorphous carbon grains in the two shells have different temperatures of 60 and 104 K, which allocates most dust mass in the shell at 50". This supports previous suggestion that the 43" shell is gas-rich and the 50" one is dust-rich. We suggest a possibility that the segregation of the gas and dust resulting from the drift motion of submicron-sized dust grains relative to the gas and that the hot dust component associated with the gas-rich shell is composed of very small grains that are strongly coupled with the gas.

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arXiv2010

Origin of the dust emission from Tycho's SNR

Aims: We investigate the spatial distribution of dust emission around Tycho's SNR to understand its origin. We distinguish the dust associated with the SNR from that of the surrounding ISM. Methods: We performed mid- to far-infrared imaging observations of the remnant at wavelengths of 9, 15, 18, 24, 65, 90, 140, and 160um using the Infrared Camera and the Far-Infrared Surveyor onboard AKARI. We compared the AKARI images with the Suzaku X-ray image and the 12CO image of Tycho's SNR. Results: All the AKARI images except the 9, 140, and 160um band images show a shell-like emission structure with brightness peaks at the north east (NE) and north west (NW) boundaries, sharply outlining part of the X-ray shell. The 140 and 160um bands are dominated by cold dust emission from the surrounding ISM near the NE boundary. Conclusion: We conclude that the dust emission at the NE boundary comes from the ambient cloud interacting with the shock front, while the origin of the dust emission at the NW boundary is rather unclear because of the absence of prominent interstellar clouds near the corresponding region. We cannot rule out the possibility that the latter is mostly of an SN ejecta origin.

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arXiv2010

AKARI IRC infrared 2.5-5 micron spectroscopy of a large sample of luminous infrared galaxies

We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

astro-ph.CO↗

arXiv2010

Detection of parent H2O and CO2 molecules in the 2.5--5 micron spectrum of comet C/2007 N3 (Lulin) observed with AKARI

Comet C/2007 N3 (Lulin) was observed with the Japanese infrared satellite AKARI in the near-infrared at a post-perihelion heliocentric distance of 1.7 AU. Observations were performed with the spectroscopic (2.5--5.0 micron) and imaging (2.4, 3.2, and 4.1 micron) modes on 2009 March 30 and 31 UT, respectively. AKARI images of the comet exhibit a sunward crescent-like shape coma and a dust tail extended toward the anti-solar direction. The 4.1 micron image (CO/CO2 and dust grains) shows a distribution different from the 2.4 and 3.2 micron images (H2O and dust grains). The observed spectrum shows distinct bands at 2.66 and 4.26 micron, attributed to H2O and CO2, respectively. This is the fifth comet in which CO2 has been directly detected in the near-infrared spectrum. In addition, CO at 4.67 micron and a broad 3.2--3.6 micron emission band from C-H bearing molecules were detected in the AKARI spectrum. The relative abundance ratios CO2/H2O and CO/H2O derived from the molecular production rates are \sim 4%--5% and < 2%, respectively. Comet Lulin belongs to the group that has relatively low abundances of CO and CO2 among the comets observed ever.

astro-ph.EP↗

arXiv2010

Cepheid Period-Luminosity Relation from the AKARI Observations

In this paper, we derive the period-luminosity (P-L) relation for Large Magellanic Cloud (LMC) Cepheids based on mid-infrared AKARI observations. AKARI's IRC sources were matched to the OGLE-III LMC Cepheid catalog. Together with the available I band light curves from the OGLE-III catalog, potential false matches were removed from the sample. This procedure excluded most of the sources in the S7 and S11 bands: hence only the P-L relation in the N3 band was derived in this paper. Random-phase corrections were included in deriving the P-L relation for the single epoch AKARI data, even though the derived P-L relation is consistent with the P-L relation without random-phase correction, though there is a \sim 7 per-cent improvement in the dispersion of the P-L relation. The final adopted N3 band P-L relation is N3 = -3.246 log(P) + 15.844, with a dispersion of 0.149.

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arXiv2010

AKARI infrared observations of edge-on spiral galaxy NGC 3079

We present AKARI near- to far-infrared images of the nearby edge-on spiral galaxy NGC 3079 in 10 photometric bands. The spectral energy distribution consists of continuum emission from dust with a single temperature of 28-33 K together with strong mid-infrared emission features from polycyclic aromatic hydrocarbons (PAHs). We derive the dust masses of 5.6x10^6 M_sun and 1.4x10^7 M_sun for the central 4 kpc region and the whole galaxy, respectively, and find that a gas-to-dust mass ratio is unusually high in the central region (\sim 1100) and even for the whole galaxy (\sim 860). The ratio of the surface brightness distribution at the wavelength of 7 um to that at 11 um suggests that the properties of PAHs have spatial variations. Emission from ionized and neutral PAHs is relatively strong in the center and the disk regions, respectively, suggesting stronger radiation field and thus relatively active star formation in the center. Yet the total infrared luminosities of the galaxy indicate rather low star formation rates. These results suggest that NGC 3079 is in an early-phase starburst stage.

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arXiv2010

PAH emission from the dust lane of an elliptical galaxy with the Spitzer IRS

Spitzer and AKARI observations have found that polycyclic aromatic hydrocarbons (PAHs) are present in nearby elliptical galaxies, but their spatial distributions are still unknown. In order to investigate their distributions, we performed deep spectral mapping observations of the PAH-detected elliptical galaxy NGC4589, a merger remnant with a minor-axis optical dust lane. As a result, we obtain clear evidence that the PAH 11.3 um emission comes predominantly from the dust lane of the galaxy. We also detect molecular hydrogen line emissions from the dust lane. The PAH 17 um emission is distributed differently from the PAH 11.3 um emission, and more similarly to the dust continuum emission. From their distinctive distributions, we suggest that the PAHs responsible for the 11.3 um feature are secondary products through the evolution of the ISM brought in by the merger.

astro-ph.CO↗

arXiv2010

Enstatite-rich Warm Debris Dust around HD165014

We present the Spitzer/Infrared Spectrograph spectrum of the main-sequence star HD165014, which is a warm (>~ 200 K) debris disk candidate discovered by the AKARI All-Sky Survey. The star possesses extremely large excess emission at wavelengths longer than 5 \mum. The detected flux densities at 10 and 20 \mum are ~ 10 and ~ 30 times larger than the predicted photospheric emission, respectively. The excess emission is attributable to the presence of circumstellar warm dust. The dust temperature is estimated as 300-750 K, corresponding to the distance of 0.7-4.4 AU from the central star. Significant fine-structured features are seen in the spectrum and the peak positions are in good agreement with those of crystalline enstatite. Features of crystalline forsterite are not significantly seen. HD165014 is the first debris disk sample that has enstatite as a dominant form of crystalline silicate rather than forsterite. Possible formation of enstatite dust from differentiated parent bodies is suggested according to the solar system analog. The detection of an enstatite-rich debris disk in the current study suggests the presence of large bodies and a variety of silicate dust processing in warm debris disks.

astro-ph.EP↗

arXiv2010

Spectroscopic observations of ices around embedded young stellar objects in the Large Magellanic Cloud with AKARI

The aim of this study is to understand the chemical conditions of ices around embedded young stellar objects (YSOs) in the metal-poor Large Magellanic Cloud (LMC). We performed near-infrared (2.5-5 micron) spectroscopic observations toward 12 massive embedded YSOs and their candidates in the LMC using the Infrared Camera (IRC) onboard AKARI. We estimated the column densities of the H2O, CO2, and CO ices based on their 3.05, 4.27, and 4.67 micron absorption features, and we investigated the correlation between ice abundances and physical properties of YSOs.The ice absorption features of H2O, CO2, 13CO2, CO, CH3OH, and possibly XCN are detected in the spectra. In addition, hydrogen recombination lines and PAH emission bands are detected toward the majority of the targets. The derived typical CO2/H2O ice ratio of our samples (~0.36 +- 0.09) is greater than that of Galactic massive YSOs (~0.17 +- 0.03), while the CO/H2O ice ratio is comparable. It is shown that the CO2 ice abundance does not correlate with the observed characteristics of YSOs; the strength of hydrogen recombination line and the total luminosity. Likewise, clear no correlation is seen between the CO ice abundance and YSO characteristics, but it is suggested that the CO ice abundance of luminous samples is significantly lower than in other samples.The systematic difference in the CO2 ice abundance around the LMC's massive YSOs, which was suggested by previous studies, is confirmed with the new near-infrared data. We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC are responsible for the observed high abundance of the CO2 ice. It is suggested that the internal stellar radiation does not play an important role in the evolution of the CO2 ice around a massive YSO, while more volatile molecules like CO are susceptible to the effect of the stellar radiation.

astro-ph.SR↗

arXiv2010

The AKARI/IRC Mid-Infrared All-Sky Survey

Context : AKARI is the first Japanese astronomical satellite dedicated to infrar ed astronomy. One of the main purposes of AKARI is the all-sky survey performed with six infrared bands between 9 and 200um during the period from 2006 May 6 to 2007 August 28. In this paper, we present the mid-infrared part (9um and 18um b ands) of the survey carried out with one of the on-board instruments, the Infrar ed Camera (IRC). Aims : We present unprecedented observational results of the 9 and 18um AKARI al l-sky survey and detail the operation and data processing leading to the point s ource detection and measurements. Methods : The raw data are processed to produce small images for every scan and point sources candidates, above the 5-sigma noise level per single scan, are der ived. The celestial coordinates and fluxes of the events are determined statisti cally and the reliability of their detections is secured through multiple detect ions of the same source within milli-seconds, hours, and months from each other. Results : The sky coverage is more than 90% for both bands. A total of 877,091 s ources (851,189 for 9um, 195,893 for 18um) are confirmed and included in the cur rent release of the point source catalogue. The detection limit for point source s is 50mJy and 90mJy for the 9um and 18um bands, respectively. The position accu racy is estimated to be better than 2". Uncertainties in the in-flight absolute flux calibration are estimated to be 3% for the 9um band and 4% for the 18um ban d. The coordinates and fluxes of detected sources in this survey are also compar ed with those of the IRAS survey and found to be statistically consistent.

astro-ph.IM↗

arXiv2010

Detection of unidentified infrared bands in a Halpha filament in the dwarf galaxy NGC1569 with AKARI

We performed observations of NGC1569 for 6 infrared bands (3.2, 4.1, 7, 11, 15, and 24 micron) with the Infrared Camera (IRC) onboard AKARI. Near- to mid-infrared (2--13 micron) spectroscopy of a Halpha filament was also carried out with the IRC. The extended structure associated with a Halpha filament appears bright at 7 micron, suggesting that the filament is bright at the UIR band emission. Follow-up spectroscopic observations with the IRC confirm the presence of 6.2, 7.7, and 11.3 micron emission in the filament. The filament spectrum exhibits strong 11.3 micron UIR band emission relative to the 7.7 micron band compared to the galaxy disk observed with the Infrared Spectrograph on Spitzer. The near-infrared spectrum also suggests the presence of excess continuum emission in 2.5--5 micron in the filament. The Halpha filament is thought to have been formed by the galactic outflow originating from the star-formation activity in the disk of NGC1569. The destruction timescale of the UIR band carriers in the outflow is estimated to be much shorter (~ 1.3 x 10^3 yr) than the timescale of the outflow (~ 5.3 Myr). Thus it is unlikely that the band carriers survive the outflow environment. Alternatively, we suggest that the band carriers in the filaments may be produced by the fragmentation of large carbonaceous grains in shocks, which produces the Halpha emission. The NIR excess continuum emission cannot be accounted for by free-free emission alone and a hot dust contribution may be needed, although the free-free emission intensity estimated from HI recombination lines has a large uncertainty.

astro-ph.CO↗