56 research outputs found
Faint warm debris disks around nearby bright stars explored by AKARI and IRSF
Context: Debris disks are important observational clues for understanding
planetary-system formation process. In particular, faint warm debris disks may
be related to late planet formation near 1 AU. A systematic search of faint
warm debris disks is necessary to reveal terrestrial planet formation. Aims:
Faint warm debris disks show excess emission that peaks at mid-IR wavelengths.
Thus we explore debris disks using the AKARI mid-IR all-sky point source
catalog (PSC), a product of the second generation unbiased IR all-sky survey.
Methods : We investigate IR excess emission for 678 isolated main-sequence
stars for which there are 18 micron detections in the AKARI mid-IR all-sky
catalog by comparing their fluxes with the predicted fluxes of the photospheres
based on optical to near-IR fluxes and model spectra. The near-IR fluxes are
first taken from the 2MASS PSC. However, 286 stars with Ks<4.5 in our sample
have large flux errors in the 2MASS photometry due to saturation. Thus we have
measured accurate J, H, and Ks band fluxes, applying neutral density (ND)
filters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF,
the \phi 1.4 m near-IR telescope in South Africa, and improved the flux
accuracy from 14% to 1.8% on average. Results: We identified 53 debris-disk
candidates including eight new detections from our sample of 678 main-sequence
stars. The detection rate of debris disks for this work is ~8%, which is
comparable with those in previous works by Spitzer and Herschel. Conclusion:
The importance of this study is the detection of faint warm debris disks around
nearby field stars. At least nine objects have a large amount of dust for their
ages, which cannot be explained by the conventional steady-state collisional
cascade model.Comment: 17 pages, 21 figures, accepted for publication in Astronomy and
Astrophysic
Molecular-beam epitaxial growth of a far-infrared transparent electrode for extrinsic Germanium photoconductors
We have evaluated the optical and electrical properties of a far-infrared
(IR) transparent electrode for extrinsic germanium (Ge) photoconductors at 4 K,
which was fabricated by molecular beam epitaxy (MBE). As a far-IR transparent
electrode, an aluminum (Al)-doped Ge layer is formed at well-optimized doping
concentration and layer thickness in terms of the three requirements: high
far-IR transmittance, low resistivity, and excellent ohmic contact. The
Al-doped Ge layer has the far-IR transmittance of >95 % within the wavelength
range of 40--200 microns, while low resistivity (~5 ohm-cm) and ohmic contact
are ensured at 4 K. We demonstrate the applicability of the MBE technology in
fabricating the far-IR transparent electrode satisfying the above requirements.Comment: 18 pages, 7 figures, accepted for publication in the PAS
Demonstration of intrahepatic accumulated microbubble on ultrasound represents the grade of hepatic fibrosis
OBJECTIVES: To examine the feasibility of perflubutane-based ultrasound for grading hepatic fibrosis. METHODS: This prospective study included 202 subjects; main study (controls:33, F0–1:35, F2:26, F3:23, cirrhosis:29) and subsequent study (controls:16, F0–1:7, F2:20, F3:7, cirrhosis:6). Diagnostic abilities for assessing fibrosis grade were compared between contrast findings and FIB4 (age × AST/[platelet count × ALT(0.5)]). RESULTS: High-power emission produced an intrahepatic band-like structure, and the three-layer appearance was less frequent and monolayer appearance was more frequent in cirrhosis than controls/chronic hepatitis (P < 0.0001). Intensity difference at 15-min phase showed most significant correlation with fibrosis grade (ρ = 0.79, P < 0.0001), and the best areas under the receiver operating characteristic curves are 0.88 for marked fibrosis, 0.95 for advanced fibrosis and 0.97 for cirrhosis, which were significantly higher than those of FIB4, 0.85 for marked fibrosis, 0.89 for advanced fibrosis and 0.90 for cirrhosis. Sensitivity, specificity and efficiency of the intensity difference were 88%, 72% and 81% for marked fibrosis, 85%, 91% and 89% for advanced fibrosis and 97%, 90% and 91% for cirrhosis, respectively. The subsequent study validated the main study results; significant correlation between the intensity difference and the fibrosis grade (ρ = 0.73–0.77, P < 0.0001). CONCLUSIONS: Perflubutane-based ultrasound accurately predicts the grade of hepatic fibrosis. KEY POINTS: • The behaviour of intrahepatic microbubbles depends on the severity of hepatic fibrosis. • Layer enhancement pattern simply represents the degree of chronic liver disease. • Parenchymal intensity change due to high-power emission predicts the hepatic fibrosis grade
AKARI infrared imaging of reflection nebulae IC4954 and IC4955
We present the observations of the reflection nebulae IC4954 and IC4955
region with the Infrared Camera (IRC) and the Far-Infrared Surveyor (FIS) on
board the infrared astronomical satellite AKARI during its performance
verification phase. We obtained 7 band images from 7 to 160um with higher
spatial resolution and higher sensitivities than previous observations. The
mid-infrared color of the S9W (9um) and L18W (18um) bands shows a systematic
variation around the exciting sources. The spatial variation in the
mid-infrared color suggests that the star-formation in IC4954/4955 is
progressing from south-west to north-east. The FIS data also clearly resolve
two nebulae for the first time in the far-infrared. The FIS 4-band data from
65um to 160um allow us to correctly estimate the total infrared luminosity from
the region, which is about one sixth of the energy emitted from the existing
stellar sources. Five candidates for young stellar objects have been detected
as point sources for the first time in the 11um image. They are located in the
red S9W to L18W color regions, suggesting that current star-formation has been
triggered by previous star-formation activities. A wide area map of the size of
about 1 x 1 (deg^2) around the IC4954/4955 region was created from the AKARI
mid-infrared all-sky survey data. Together with the HI 21cm data, it suggests a
large hollow structure of a degree scale, on whose edge the IC4954/4955 region
has been created, indicating star formation over three generations in largely
different spatial scales.Comment: 23 pages, 7 figures, accepted for publication in PASJ AKARI special
issu
A 0.3-V operating, Vth-variation-tolerant SRAM under DVS environment for memory-rich SoC in 90-nm technology era and beyond
元・大学院自然科学研究科 現・神戸大学大学院自然科学研究科We propose a voltage control scheme for 6T SRAM cells that makes a minimum operation voltage down to 0.3 V under DVS environment. A supply voltage to the memory cells and wordline drivers, bitline voltage, and body bias voltage of load pMOSFETs are controlled according to read and write operations, which secures operation margins even at a low operation voltage. A self-aligned timing control with a dummy wordline and its feedback is also introduced to guarantee stable operation in a wide range of the supply voltage. A measurement result of a 64-kb SRAM in a 90-nm process technology shows that a power reduction of 30 can be achieved at 100 MHz. In a 65-nm 64-Mb SRAM, a 74 power saving is expected at 1/6 of the maximum operating frequency. The performance penalty by the proposed scheme is less than 1, and area overhead is 5.6. Copyright © 2006 The Institute of Electronics, Information and Communication Engineers
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 um, 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 arcmin, and the pixel pitch is matches the diffraction
limit of the telescope. Derived point spread functions (PSFs) from observations
of asteroids are similar to 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 its
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.Comment: 23 pages, 10 figures, and 2 tables. Accepted for publication in the
AKARI special issue of the Publications of the Astronomical Society of Japa
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