Takashi Onaka

Cryogenic optical testing of sandwich-type silicon carbide mirrors

The experimental cryogenic performance of 160-mm-diameter silicon carbide (SiC) mirrors, one of which, a 700-mm-diameter mirror, is to be used as a primary mirror of the Japanese Infrared Astronomical Satellite ASTRO-F, is described. The mirrors are made from a sandwich-type SiC material that comprises a light porous core and a dense chemical-vapor-deposited coat of SiC. Three mirrors were manufactured consecutively, and changes in their surface contours related to temperature were measured with an interferometer when the mirrors were placed in a liquid-helium cryostat. Owing to significant improvements in manufacturing, the third SiC mirror showed only slight deformation as the temperature decreased from 300 to 6 K, which indicates high thermal strain homogeneity for a well-controlled sandwich-type SiC mirror.

Laboratory performance evaluation of the mid-infrared camera and spectrograph MIMIZUKU for the TAO 6.5-m telescope

The Mid-Infrared Multi-field Imager for gaZing at the UnKnown Universe (MIMIZUKU) is a mid-infrared camera and spectrograph developed as a first-generation instrument on the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope. MIMIZUKU covers a wide wavelength range from 2 to 38 μm and has a unique optical device called Field Stacker which realizes accurate calibration of variable atmospheric transmittance with a few percent accuracy. By utilizing these capabilities, MIMIZUKU realizes mid-infrared long-term monitoring, which has not been challenged well. MIMIZUKU has three optical channels, called NIR, MIR-S, and MIR-L, to realize the wide wavelength coverage. The MIR-S channel, which covers 6.8–26 μm, has been completed by now. We are planning to perform engineering observations with this channel at the Subaru telescope before the completion of the TAO 6.5-m telescope. In this paper, we report the results of the laboratory tests to evaluate the optical and detector performances of the MIR-S channel. As a result, we confirmed a pixel scale of 0.12 arcsec/pix and a vignetting- free field of view of 2./0 1./8. The instrument throughputs for imaging modes are measured to be 20–30%. Those for N - and Q -band spectroscopy modes are 17 and 5%, respectively. As for the detector performance, we derived the quantum efficiency to be 40–50% in the mid-infrared wavelength region and measured the readout noise to be 3000–6000 electrons, which are larger than the spec value. It was found that this large readout noise degrades the sensitivity of MIMIZUKU by a factor of two.

DEVELOPMENT OF NEW STITCHING INTERFEROMETRY FOR THE SPICA TELESCOPE

The telescope to be onboard SPICA (Space Infrared Telescope for Cosmology and Astrophysics) has an aperture diameter of 2.5 m and its imaging performance is to be diffraction-limited at a wavelength of 20 μm at the operating temperature of <8 K. Because manufacturing precise autocollimating flat mirrors (ACFs) with sizes comparable to the SPICA telescope is not technically feasible, we plan to use sub-aperture stitching interferometry through ACFs for optical testing of the telescope. We have verified the applicability of the sub-aperture stitching technique to the SPICA telescope by performing stitching experiments in a vacuum at a room temperature, using the 800-mm telescope and a 300-mm ACF. We have also developed a new method to reduce uncertainties possibly caused by cryogenic and gravitational deformations of ACFs.

AKARI NIR spectroscopy of interstellar ices

The Infrared Camera (IRC) onboard AKARI has a near-infrared (2–5 μm) spectroscopic capability with high sensitivity that allows us to study the major ice components in various objects. In particular, H2O and CO2 ice absorption features have been detected towards nearby galaxies, including several young stellar objects (YSOs) in the Large Magellanic Cloud (LMC), as well as a number of HII region-PDR complexes for the first time by IRC spectroscopy. While observations in the LMC show a high ratio (∼ 0.34) of the CO2 to H2O ice column densities, the ratios in Galactic HII-region-PDR complexes are in the range of 0.1–0.2, being compatible with those found in Galactic massive YSOs in previous studies. The good correlation supports concurrent formation of the two ice species on the grain surface and the higher ratio in the low-metallicity LMC suggests possible environmental effects in the formation process.

Near-Infrared Spectroscopy of Diffuse Galactic Radiation with AKARI/IRC

Takashi Onakaa∗, Tamami I. Moria, Itsuki Sakona, Ryou Ohsawaa, Tomohiko Nakamura,a Ho-Gyu Leea†, Ingrid M. Kochb, Takashi Shimonishic, Hidehiro Kanedad , Yoko Okadae, and Masahiro Tanaka f aDepartment of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan bDepartment of Physics and Astronomy, University of Rochester, NY 14627-0171, U. S. A. cDepartment of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan dGraduate School of Science, Nagoya University, Nagoya 464-8602, Japan eI. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany f Center for Computational Sciences, Tsukuba University, Ibaraki 305-8577, Japan E-mail: onaka@astron.s.u-tokyo.ac.jp

Evolution of Planetary Nebulae and PAH features

Near- to mid-infrared spectra of 17 Galactic Planetary Nebulae (PNe), which show prominent C-rich dust features, were obtained with the AKARI/IRC and Spitzer/IRS. Relative intensities of Polycyclic Aromatic Hydrocarbon (PAH) features were measured. The evolutionary stage of PNe is estimated by the effective temperature of the central star. The evolution of the PAH emission is investigated in terms of the evolution of the central stars. As PNe evolve, the ionization fraction of PAHs decreases, the 3:4–3:5mm aliphatic features become stronger and the peak position of the 6:2mm feature moves toward shorter wavelengths. Possible mechanisms for the evolution of the PAH emission are discussed.

AKARI IRC INFRARED 2.5-5 ㎛ SPECTROSCOPY OF NEARBY LUMINOUS INFRARED GALAXIES

We present the result of systematic AKARI IRC infrared 2.5‐5 „m spectroscopy of >100 nearby luminous infrared galaxies, to investigate the energetic roles of starbursts and optically-elusive buried AGNs. Based on (1) the equivalent widths of the 3.3 „m PAH emission features, (2) the optical depths of absorption features, and (3) continuum slopes, we can disentangle emission from starbursts and AGNs. We find that the energetic importance of buried AGNs increases with increasing galaxy infrared luminosities, suggesting that the AGN-starburst connections (and thereby possible AGN feedback to host galaxies) are luminosity dependent.

AKARI IRC SURVEY OF THE LARGE MAGELLANIC CLOUD: AN OVERVIEW OF THE SURVEY AND A BRIEF DESCRIPTION OF THE POINT SOURCE CATALOG

We observed an area of 10 deg² of the Large Magellanic Cloud using the Infrared Camera (IRC) onboard AKARI. The observations were carried out using five imaging filters (3, 7, 11, 15, and 24 ㎛) and the prism disperser (2 - 5 ㎛, λ/Δλ~20) equipped in the IRC. This paper presents an outline of the survey project and also describes very briefly the newly compiled near- to midinfrared point source catalog. The 10 ¾ limiting magnitudes are 17.9, 13.8, 12.4, 9.9, and 8.6 mag at 3.2, 7, 11, 15 and 24 ㎛, respectively. The photometric accuracy is estimated to be about 0.1 mag at 3.2 ㎛ and 0.06 - 0.07 mag in the other bands. The position accuracy is 0.300 at 3.2, 7 and 11 ㎛ and 1.000 at 15 and 24 ㎛. The sensitivities at 3.2, 7, and 24 ㎛ are roughly comparable to those of the Spitzer SAGE LMC point source catalog, while the AKARI catalog provides the data at 11 and 15 ㎛, covering the near- to mid-infrared spectral range continuously.