Fumihiko Usui

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 Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 mu m, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

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.

First Detection of an Optical Dust Trail along the Orbit of 22P/Kopff

We present the first evidence of a cometary dust trail in optical wavelengths along the orbit of 22P/Kopff, observed when the parent comet was at a heliocentric distance of 3.01 AU. We find that the surface brightness and the width of the trail become, respectively, fainter and wider as the distance from the comet nucleus increases, except for a region with delta mean anomaly ΔMA ≤ 002. This suggests that the majority of the centimeter-sized dust particles were ejected before the comets previous perihelion passage and that they spread due to their initial velocity with respect to the comet. By comparing this trail with the IRAS data at wavelengths of 12 and 25 μm, we infer that the trail is composed of very low albedo particles (~0.01).

Physical properties of asteroids in comet-like orbits in infrared asteroid survey catalogs

We investigated the population of asteroids in comet-like orbits using available asteroid size and albedo catalogs of data taken with the Infrared Astronomical Satellite, AKARI, and the Wide-field Infrared Survey Explorer on the basis of their orbital properties (i.e., the Tisserand parameter with respect to Jupiter, TJ, and the aphelion distance, Q). We found that (i) there are 123 asteroids in comet-like orbits by our criteria (i.e., Q 0.1. We noticed that such high-albedo objects mostly consist of small (D < 3 km) bodies distributed in near-Earth space (with perihelion distance of q < 1.3 AU). We suggest that such high-albedo, small objects were susceptible to the Yarkovsky effect and drifted into comet-like orbits via chaotic resonances with planets.

(25143) Itokawa: The power of radiometric techniques for the interpretation of remote thermal observations in the light of the Hayabusa rendezvous results

The near-Earth asteroid (25143) Itokawa was characterized in great detail by the Japanese Hayabusa mission. We revisited the available thermal observations in the light of the true asteroid properties with the goal of evaluating the possibilities and limitations of thermal model techniques. In total, we used 25 published ground-based mid-infrared photometric observations and five so far unpublished measurements from the Japanese infrared astronomical satellite AKARI in combination with improved H–G values (absolute magnitude and slope parameter). Our thermophysical model (TPM) approach allowed us to determine correctly the sense of rotation, to estimate the thermal inertia and to derive robust effective size and albedo values by only using a simple spherical shape model. A more complex shape model, derived from light-curve inversion techniques, improved the quality of the predictions considerably and made the interpretation of the thermal light curve possible. The radiometrically derived effective diameter value agrees within 2% with the true Itokawa size value. The combination of our TPM and the final (25143) Itokawa in-situ shape model was then used as a benchmark for deriving and testing radiometric solutions. The consolidated value for the surface-averaged thermal inertia is = 700±200 J m−2 s−0.5 K−1. We found that even the high-resolution shape models still require additional small-scale roughness in order to explain the disk-integrated infrared measurements. Our description of the thermal effects as a function of wavelengths, phase angle, and rotational phase facilitates the planning of crucial thermal observations for sophisticated characterization of small bodies, including other potentially hazardous asteroids. Our analysis shows the power of radiometric techniques to derive the size, ∗Based on observations with AKARI, a JAXA project with the participation of ESA. C

AKARI Observations of Circumstellar Dust in the Globular Clusters NGC 104 and NGC 362

We report on preliminary results of AKARI observations of two globular clusters, NGC 104 and NGC 362. Imaging data covering areas of about 10×10 arcmin 2 centered on the two clusters were obtained with the Infrared Camera (IRC) at 2.4, 3.2, 4.1, 7.0, 9.0, 11.0, 15.0, 18.0, and 24.0 μm. We used IRC 11.0νm to 2MASS K s 2 μm flux-density ratio (F 11/F 2) and IRC 24.0 μm to 7.0 μm flux-density ratio (F 24/F 7) as diagnostics of circumstellar dust emission. Dust emissions are mainly detected from variable stars, obviously on the asymptotic giant branch, but some variable stars that reside below the tip of the first-ascending giant branch also show dust emissions. We found eight red sources with the F 24/F 7 ratio being greater than unity in NGC 362. Six out of the eight have no 2MASS counterparts. However, we found no such source in NGC 104.

AKARI Infrared Camera Survey of the Large Magellanic Cloud. I. Point-source Catalog

We present a near- to mid-infrared point-source catalog of five photometric bands at 3.2, 7, 11, 15, and 24 ?m for a 10?deg2 area of the Large Magellanic Cloud (LMC) obtained with the Infrared Camera on board the AKARI satellite. To cover the survey area the observations were carried out at three separate seasons from 2006 May to June, 2006 October to December, and 2007 March to July. The 10? limiting magnitudes of the present survey are 17.9, 13.8, 12.4, 9.9, and 8.6?mag at 3.2, 7, 11, 15, and 24 ?m, respectively. The photometric accuracy is estimated to be about 0.1?mag at 3.2 ?m and 0.06-0.07?mag in the other bands. The position accuracy is 03 at 3.2, 7, and 11 ?m and 10 at 15 and 24 ?m. The sensitivities at 3.2, 7, and 24??m are roughly comparable to those of the Spitzer SAGE LMC point-source catalog, while the AKARI catalog provides the data at 11 and 15??m, covering the mid-infrared spectral range contiguously. Two types of catalog are provided: a Catalog and an Archive. The Archive contains all the detected sources, while the Catalog only includes the sources that have a counterpart in the Spitzer SAGE point-source catalog. The Archive contains about 650,000, 140,000, 97,000, 43,000, and 52,000 sources at 3.2, 7, 11, 15, and 24??m, respectively. Based on the catalog, we discuss the luminosity functions at each band, the color-color diagram, and the color-magnitude diagram using the 3.2, 7, and 11??m band data. Stars without circumstellar envelopes, dusty C-rich and O-rich stars, young stellar objects, and background galaxies are located at distinct regions in the diagrams, suggesting that the present catalog is useful for the classification of objects toward the LMC.

AKARI IRC Survey of the Large Magellanic Cloud: Outline of the Survey and Initial Results

We observed an area of 10 deg^2 of the Large Magellanic Cloud using the Infrared Camera on board AKARI. The observations were carried out using five imaging filters (3, 7, 11, 15, and 24 micron) and a dispersion prism (2 -- 5 micron,

DETECTION OF PARENT H2O AND CO2 MOLECULES IN THE 2.5-5 μm SPECTRUM OF COMET C/2007 N3 (LULIN) OBSERVED WITH AKARI

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