Hideyo Kawakita

Cryogenic performance of high-efficiency germanium immersion grating

Immersion gratings will play important roles for infrared astronomy in the next generation. We have been developing immersion gratings with a variety of kinds of materials and have succeeded in fabricating a high-efficiency germanium (Ge) immersion grating with both a reflection coating on the grating surface and an AR coating on the entrance surface. The grating will be installed in a K-, L-, and M-bands (2-5μm) high-resolution (R=80,000) spectrograph, VINROUGE, which is a prototype for the TMT MIR instrument. In this paper, we report the preliminary results on the evaluation of the Ge immersion grating. We confirmed that the peak absolute diffraction efficiency was in the range of 70-80%, which was as expected from the design, at both room and cryogenic temperatures.

Optical and Near-infrared High-resolution Spectroscopic Observations of Nova V2659 Cyg: Structure of Nova Ejecta and Origin of Two-distinct Velocity Systems

Two distinct absorption-line systems distinguished by radial velocities have often been observed in the optical high-resolution spectra of classical novae during their early decline phase. The origin of these absorption-line systems is under debates. We present optical high-resolution spectroscopic observations spectra of nova V2659 Cyg and discuss about the temporal evolution of those absorption-line systems observed in this nova during its early decline phase. The observed temporal evolution of absorption-line profiles with relatively higher velocities (the high-velocity component) can be explained qualitatively by the clumpy ejecta and movement of the ionization fronts in the ejecta with time. Conversely, the low-velocity component may originate in the cool region compressed by the shock caused by collision between the fast nova wind and the slow expanding, equatorially focused dense ejecta. We also present high-resolution spectra of V2659 Cyg during its nebular phase in optical and near-infrared wavelength regions. Emission lines detected during the nebular phase also showed two velocity components, suggesting that the velocity structure of the ejecta during the nebular phase is similar to that during the early decline phase. The double-horned profiles of emission lines with lowu3000velocities imply a ring-like distribution of materials with lower velocities. The observations during both the early-decline phase and the nebular phase support the multiple ejection of ejecta at a nova explosion, with different velocities.

A Review of the Evolution of Classical Nova V2676 Oph: Formation of Molecules and Dust Grains

Nova V2676 Oph is the first classical nova in which C2 has been detected during its early phase near the visual-brightness maximum in addition to CN. The presence of C2 and CN in the nova indicates that its atmosphere was enriched in carbon with C/O > 1. Furthermore, molecule formation in the early phase is likely to be associated with the dust formation that started ∼90 days after the discovery. We have obtained isotopic ratios of carbon and nitrogen for this nova, and they are consistent with model predictions. Based on the lightcurves and optical spectra of the nova, the inferred mass of the white-dwarf component of V2676 Oph is relatively small (∼0.6M⊙). The absence of strong [Ne II] emission at 12.8 μm and the relatively small ejected mass from V2676 Oph support this hypothesis. However, the mass of the white-dwarf component should be higher (>∼1.0M⊙) according to the observed isotopic ratios and theoretical predictions based on thermonuclear runaways.

Reflective optical system made entirely of ultra low thermal expansion ceramics: a possibility of genuine athermal cryogenic IR instrument

Reflective optical system free from chromatic aberration is essential for astronomical instruments, which usually require wider wavelength coverage. However, it cannot always be the optimum choice compared with refractive optical system in terms of cost-effectiveness because mirrors require high surface accuracy and also because non-co-axial systems force tough alignment work. This dilemma could be overcome by a monolithic reflective optical system made entirely of cordierite CO-720, a ceramic material by Kyocera, which is the first material that offers both high-precision 3D-shaping and surface polishing for optical quality. This material also possesses a very low coefficient of thermal expansion (CTE) enabling a genuine athermal system useful for various astronomical applications. This athermality could make a significant breakthrough especially for cryogenic infrared instruments since optical systems made of cordierite are expected to keep as-built performance throughout the cooling process, providing extremely high wavefront accuracy that has never been possible at cryogenic temperature with conventional optical systems made of glasses or metals. In this paper, we report the first cryogenic optical testing of a small cordierite-made imaging optical system that was simply assembled with mechanical accuracy at room temperature. We confirmed that the diffraction-limited optical performance is kept even down to ~80K as built in the room temperature.