Takafumi Ootsubo

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.

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.

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.

Planetary Formation Scenarios Revisited: Core-Accretion versus Disk Instability

The core-accretion and disk instability models have so far been used to explain planetary formation. These models have different conditions, such as planet mass, disk mass, and metallicity for formation of gas giants. The core-accretion model has a metallicity condition ([Fe/H] > -1.17 in the case of G-type stars), and the mass of planets formed is less than 6 times that of the Jupiter mass MJ. On the other hand, the disk instability model does not have the metallicity condition, but requires the disk to be 15 times more massive than the minimum mass solar nebulae model. The mass of planets formed is more than 2 MJ. These results are compared to the 161 detected planets for each spectral type of the central stars. The results show that 90% of the detected planets are consistent with the core-accretion model regardless of the spectral type. The remaining 10% are not in the region explained by the core-accretion model, but are explained by the disk instability model. We derived the metallicity dependence of the formation probability of gas giants for the core-accretion model. Comparing the result with the observed fraction having gas giants, they are found to be consistent. On the other hand, the observation cannot be explained by the disk instability model, because the condition for gas giant formation is independent of the metallicity. Consequently, most of planets detected so far are thought to have been formed by the core-accretion process, and the rest by the disk instability process.

Evidence of Icy Grains in Comet C/2002 T7 (LINEAR) at 3.52 AU

We present evidence of icy grains in the coma of comet C/2002 T7 (LINEAR) at 3.52 AU from the Sun. This comet will approach the Sun in the spring of 2004, and it is expected to be very bright near its perihelion passage. The comet was observed using the Subaru Telescope with the Cooled Infrared Spectrograph and Camera for OHS (CISCO) on 2003 September 14.6 (UT). The near-infrared (J, H, K) spectrum was extracted from the near-nucleus region (1250 km × 1250 km at the comet), and it showed clear absorption features at 1.5 and 2.05 μm that originated from water ice grains. The calculated reflectance spectrum, based on the intimate mixture model for water ice grains and astronomical silicate grains (the diameters are 5 and 0.5 μm, respectively), can reproduce the observed reflectance spectrum of the comet up to 2.1 μm. The poor fit for the wavelength region longer than 2.1 μm is probably indicative of other grain species to be included in the model. Furthermore, the absence of the 1.65 μm feature of crystalline water ice may indicate that the water ice was in an amorphous state during the observation.

SiO MASER SURVEY OF THE GALACTIC DISK IRAS SOURCES. I. 15¡ \ l \ 25¡, NEAR END OF THE GALACTIC BAR

A survey has been made in the SiO J = 1-0, v = 1 and 2 transitions (~43 GHz) for the color-selected IRAS sources in the Galactic disk area of 15? < l < 25? and |b| < 3? with the Nobeyama 45 m telescope. We have detected 67 out of 119 observed sources in SiO masers. Distances to the sources are deduced by the IRAS 12 and 25 ?m flux densities and range approximately from 2 to 12 kpc. The lines of sight of this region cross the location of the near end of the bar in the Galaxy. Radial velocities of the detected sources spread between -100 and 200 km s-1. This range slightly exceeds the possible limits expected from Galactic rotation. A comparison of the SiO with the CO and H I velocity-longitude (v-l) diagrams reveals that the overall distribution of the SiO maser sources on the v-l diagram resembles the molecular ring feature. Observational data are compared with the theoretical results of the bar models.

AKARI/IRC 18 μm survey of warm debris disks

Context . Little is known about the properties of the warm ( T dust ≳ 150 K) debris disk material located close to the central star, which has a more direct link to the formation of terrestrial planets than does the low-temperature debris dust that has been detected to date. Aims : To discover new warm debris disk candidates that show large 18 μm excess and estimate the fraction of stars with excess based on the AKARI/IRC Mid-Infrared All-Sky Survey data. Methods : We searched for point sources detected in the AKARI/IRC All-Sky Survey, which show a positional match with A-M dwarf stars in the Tycho-2 Spectral Type Catalogue and exhibit excess emission at 18 μm compared to what is expected from the K S magnitude in the 2MASS catalogue. Results : We find 24 warm debris candidates including 8 new candidates among A-K stars. The apparent debris disk frequency is estimated to be 2.8 ± 0.6%. We also find that A stars and solar-type FGK stars have different characteristics of the inner component of the identified debris disk candidates. While debris disks around A stars are cooler and consistent with steady-state evolutionary model of debris disks, those around FGK stars tend to be warmer and cannot be explained by the steady-state model.

SiO Maser Survey of the Galactic Disk IRAS Sources. II. |l| ≤ 3° and |b| ≤ 3°, the Galactic Center Area

A survey has been made in the SiO J = 1-0, v = 1 and 2 transitions (~43 GHz) for color-selected IRAS sources in the Galactic-center region of |l| ≤ 3° and |b| ≤ 3° using the Nobeyama 45 m telescope. We have detected 86 of 176 observed sources in the SiO masers. Distances to the sources are deduced from IRAS 12 and 25 μm flux densities and range approximately from 5 to 12 kpc. Radial velocities of the detected sources spread between -300 and 310 km s-1. The longitude-velocity (l-v) diagram exhibits an empty region of sources at l = -04-10 and Vlsr = 20-150 km s-1, which is interpreted as an undersampling effect of sources in the IRAS catalog at the Galactic central disk. The rotation rate, velocity dispersion, tilt angle of the rotation axis, and velocity shift at l = 0° are derived by fitting the velocities of the sources with a straight line as a function of Galactic longitude. These quantities are compatible with those obtained from previous observations of bulge stars with |b| > 3°. The average radial velocity of subsamples of stars tends to increase with distance, suggesting the presence streaming motion of stars in a barlike bulge. We conclude that most of the IRAS sources in the sample belong to the bulge.