Itsuki Sakon

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Infrared Camera (IRC) for AKARI–Design and Imaging Performance

The Infrared Camera (IRC) is one of two focal-plane instruments on the AKARI satellite. It is designed for wide-field deep imaging and low-resolution spectroscopy in the nearto mid-infrared (1.8–26.5 m) in the pointed observation mode of AKARI. The IRC is also operated in the survey mode to make an All-Sky Survey at 9 and 18 m. It comprises three channels. The NIR channel (1.8–5.5 m) employs a 512 412 InSb array, whereas both the MIR-S (4.6–13.4 m) and MIR-L (12.6–26.5 m) channels use 256 256 Si:As impurity band conduction arrays. Each of the three channels has a field-of-view of about 100 100, and they are operated simultaneously. The NIR and MIR-S share the same field-of-view by virtue of a beam splitter. The MIR-L observes the sky about 250 away from the NIR/MIR-S field-of-view. The IRC gives us deep insights into the formation and evolution of galaxies, the evolution of planetary disks, the process of star-formation, the properties of interstellar matter under various physical conditions, and the nature and evolution of solar system objects. The in-flight performance of the IRC has been confirmed to be in agreement with the pre-flight expectation. This paper summarizes the design and the in-flight operation and imaging performance of the IRC.

AKARI Near-Infrared Spectroscopic Survey for CO2 in 18 Comets

We conducted a spectroscopic survey of cometary volatiles with the Infrared Camera on board the Japanese infrared satellite AKARI in the wavelength range from 2.5 to 5 μm. In our survey, 18 comets, including both the Oort cloud comets and the Jupiter-family comets, were observed in the period from 2008 June to 2010 January, most of which were observed at least twice. The prominent emission bands in the observed spectra are the fundamental vibrational bands of water (H2O) at 2.7 μm and carbon dioxide (CO2) at 4.3 μm. The fundamental vibrational band of carbon monoxide (CO) around 4.7 μm and the broad emission feature, probably related to carbon-hydrogen-bearing molecules, can also be recognized around the 3.3-3.5-μm region in some of the comets. With respect to H2O, gas production rate ratios of CO2 have been derived in 17 comets, except for the comet 29P/Schwassmann-Wachmann 1. Our data set provides the largest homogeneous database of CO2/H2O production rate ratios in comets obtained so far. The CO2/H2O production rate ratios are considered to reflect the composition of cometary ice when a comet is observed at a heliocentric distance within ~2.5 AU, since H2O ice fully sublimates there. The CO2/H2O ratio in cometary ice spans from several to ~30% among the comets observed at <2.5 AU (13 out of the 17 comets). Alternatively, the ratio of CO/CO2 in the comets seems to be smaller than unity based on our observations, although we only obtain upper limits for CO in most of the comets.

An early extrasolar planetary system revealed by planetesimal belts in β Pictoris

β Pictoris (β Pic) is a main-sequence star with an edge-on dust disk that might represent a state of the early Solar System. The dust does not seem to be a remnant from the original protoplanetary disk, but rather is thought to have been generated from large bodies like planetesimals and/or comets. The history and composition of the parent bodies can therefore be revealed by determining the spatial distribution, grain size, composition and crystallinity of the dust through high-resolution mid-infrared observations. Here we report that the sub-micrometre amorphous silicate grains around β Pic have peaks in their distribution around 6, 16 and 30 au (1 au is the Sun–Earth distance), whereas the crystalline and micrometre-sized amorphous silicate grains are concentrated in the disk centre. As sub-micrometre grains are blown quickly out from the system by radiation pressure from the central star, the peaks indicate the locations of ongoing dust replenishment, which originates from ring-like distributions of planetesimals or ‘planetesimal belts’.

Properties of Polycyclic Aromatic Hydrocarbons in Local Elliptical Galaxies Revealed by the Infrared Spectrograph on Spitzer

We performed mid-infrared spectroscopic observations of 18 local dusty elliptical galaxies by using the Infrared Spectrograph (IRS) on board Spitzer. We have significantly detected polycyclic aromatic hydrocarbon (PAH) features from 14 out of the 18 galaxies and, thus, found that the presence of PAHs is not rare but rather common in dusty elliptical galaxies. Most of these galaxies show an unusually weak 7.7 ?m emission feature relative to 11.3 and 17 ?m emission features. A large fraction of the galaxies also exhibit H2 rotational line and ionic fine-structure line emissions, which have no significant correlation with the PAH emissions. The PAH features are well correlated with the continuum at 35 ?m, whereas they are not correlated with the continuum at 6 ?m. We conclude that the PAH emission of the elliptical galaxies is mostly of interstellar origin rather than of stellar origin, and that the unusual PAH interband strength ratios are likely to be due to a large fraction of neutral to ionized PAHs.

Early Formation of Dust in the Ejecta of Type Ib SN 2006jc and Temperature and Mass of the Dust

SN 2006jc is a peculiar supernova (SN), in which the formation of dust has been confirmed at an early epoch of ~50 days after the explosion. We investigate the possibility of such an early formation of dust grains in the expanding ejecta of SN 2006jc, applying the Type Ib SN model that is developed to reproduce the observed light curve. We find that the rapid decrease of the gas temperature in SN 2006jc enables the condensation of C grains in the C-rich layer at 40-60 days after the explosion, which is followed by the condensation of silicate and oxide grains until ~200 days. The average radius of each grain species is confined to be less than 0.01 -->μ m due to the low gas density at the condensation time. The calculated total dust mass reaches 1.5 -->M☉, of which C dust shares 0.7 -->M☉. On the other hand, based on the calculated dust temperature, we show that the dust species and mass evaluated to reproduce the spectral energy distribution observed by AKARI and MAGNUM at day 200 are different from those obtained by the dust formation calculations; the dust species contributing to the observed flux are hot C and FeS grains with masses of -->5.6 × 10−4 and -->2.0 × 10−3 -->M☉, respectively, although we cannot defy the presence of a large amount of cold dust such as silicate and oxide grains up to 0.5 -->M☉. One of the physical processes responsible for the difference between calculated and evaluated masses of C and FeS grains could be considered to be the destruction of small-sized clusters by energetic photons and electrons prevailing within the ejecta at the earlier epoch.

Detection of PAH Emission Features from Nearby Elliptical Galaxies with the Spitzer Infrared Spectrograph

According to the current understanding, the presence of a considerable amount of dust in elliptical galaxies is quite common. Recent studies with ISO and Spitzer even suggest the presence of polycyclic aromatic hydrocarbon (PAH) emission features in the spectral energy distributions of several elliptical galaxies. Hot ionized gas filling the interstellar space of elliptical galaxies, however, is expected to easily destroy such very small grains through sputtering by plasma ions. Here we present the results of mid-IR spectroscopic observations of four elliptical galaxies with the Spitzer Infrared Spectrograph (IRS). We succeeded in detecting PAH emission features from elliptical galaxies. The observed spectra seem to be quite unusual; the PAH features at 6.2, 7.7, and 8.6 ?m are very faint or even absent, in contrast to prominent emission features at 11.3 and 12.7 ?m, which may reflect peculiar physical conditions of the interstellar medium. The detection of the PAHs provides strong constraints on evolution scenarios for the interstellar medium of elliptical galaxies.

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

Aims. 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). Methods. We performed near-infrared (2.5-5 μm) 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 H 2 O, CO 2 , and CO ices based on their 3.05, 4.27, and 4.67 μm absorption features, and we investigated the correlation between ice abundances and physical properties of YSOs. Results. The ice absorption features of H 2 O, CO 2 , 13 CO 2 , CO, CH 3 0H, 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 CO 2 /H 2 O ice ratio of our samples (~0.36 ± 0.09) is greater than that of Galactic massive YSOs (~0.17 ± 0.03), while the CO/H 2 O ice ratio is comparable. It is shown that the CO 2 ice abundance does not correlate with the observed characteristics of YSOs: the strength of hydrogen recombination line and the total luminosity. Likewise, clear correlation is not 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. Conclusions. The systematic difference in the C02 ice abundance around the LMCs massive YSOs, which was suggested by previous studies, is confirmed with the new near-IR 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 CO 2 ice. It is suggested that the internal stellar radiation does not play an important role in the evolution of the CO 2 ice around a massive YSO, while more volatile molecules like CO are susceptible to the effect of the stellar radiation.

The asymmetric thermal emission of the 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 μm with the Subaru/COMICS. Bright extended arclike emission (outer disk) is recognized at r = 085 together with a strong central source (inner disk) and a gap around r = 06 in both images. The thermal emission on the eastern side is much brighter than that on the western side in the MIR. We estimate the dust size to be a few microns from the observed color of the extended emission and the distance from the star. The dust temperature T and the optical depth τ of the MIR-emitting dust are also derived from the two images as T = 82 ± 1 K, τ = 0.052 ± 0.001 for the eastern side and T = 85 ± 3 K, τ = 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 on the far side while the inner rim of the outer disk on the eastern side is directly exposed to us. The proposed model can successfully account for the MIR observations as well as the near-infrared images of the scattering light, in which the asymmetry is seen in the opposite sense and in which the forward scattering light (near side-western side) is brighter.

THE UNIDENTIFIED INFRARED BANDS IN THE DIFFUSE INTERSTELLAR MEDIUM ACROSS THE GALAXY BASED ON THE INFRARED TELESCOPE IN SPACE MID-INFRARED SPECTROMETER OBSERVATION

We present the results of observations of the unidentified infrared (UIR) bands in the diffuse Galactic emission across the Galaxy by the Mid-Infrared Spectrometer (MIRS) on board the Infrared Telescope in Space (IRTS). While previous studies on the UIR bands in the Milky Way were limited to the inner Galactic plane, we extend the observing area to the outer Galactic plane. In this paper we analyze the data of four areas of 8° × 8° around the Galactic plane (|b| ≤ 4°; -12° ≤ l ≤ -4°, 44° ≤ l ≤ 52°, -136° ≤ l ≤ -128°, and 168° ≤ l ≤ 176°) and investigate the UIR band intensity relative to the far-infrared (FIR) intensity, as well as the variation in the band profile. Together with the good correlation between the UIR band and the FIR intensities in the four regions, we have found a systematic variation in the UIR-to-FIR ratio such that the ratio becomes larger in the outer Galactic plane than in the inner Galactic plane. In addition, the 8.6 and 11.3 μm UIR bands were found to be stronger relative to the 6.2 and 7.7 μm bands in the outer Galactic plane, which may be related to differences in the structure or physical conditions of the band carriers. We have also found small shifts (~0.1 μm) in the peak wavelength of each UIR band to shorter wavelengths from the inner Galactic plane to the outer Galactic plane. Possible interpretations of these variations are discussed.