Naoto Kobayashi

H- and K-Band Spectra of Brown Dwarf Candidates in the Core of the ρ Ophiuchi Molecular Cloud Complex

We present low-resolution H- and K-band spectra of 14 brown dwarf candidates located in the core of the ρ Ophiuchi molecular cloud complex. Half of the spectra exhibit broad water absorption bands characteristic of M stars, while half of the spectra are featureless. We derive spectral types and effective temperatures for the seven candidates whose spectra exhibit water absorption bands by comparing the strength of these bands with those of standard M dwarf stars. We estimate their masses by first combining the spectral types with published near-infrared photometry to estimate bolometric luminosities and then by comparing their positions in a Hertzsprung-Russell diagram with theoretical evolutionary tracks and isochrones. The masses of four of the candidates lie below the substellar limit when the errors in their effective temperatures and bolometric luminosities are considered, while the remaining three candidates have masses on the stellar-substellar border. We find our mass estimates to be consistent with those determined using near-infrared photometry alone. The derived ages of three candidates are considerably older than the previously determined age of the embedded cluster. If these objects are members of the cluster, then they may belong to an older and previously unrecognized population of the ρ Ophiuchi molecular cloud complex.

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.

Evaluation of large pixel CMOS image sensors for the Tomo-e Gozen wide field camera

Tomo-e Gozen (Tomo-e) is a wide field optical camera for the Kiso 1.05 m f/3.1 Schmidt telescope operated by the University of Tokyo. Tomo-e is equipped with 84 chips of front-illuminated CMOS image sensors with a microlens array. The field of view is about 20 square degrees and maximum frame rate is 2 fps. The CMOS sensor has 2160x1200 pixels and a size of pixel is 19 microns, which is larger than those of other CMOS sensors. We have evaluated performances of the CMOS sensors installed in Tomo-e. The readout noise is 2.0 e- in 2 fps operations when an internal amplifier gain is set to 16. The dark current is 0.5 e-/sec/pix at room temperature, 290K, which is lower than a typical sky background flux in Tomo-e observations, 50 e-/sec/pix. The efficiency of the camera system peaks at approximately 0.7 in 500 nm.

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.

Type-Ia SNR at z=3.5 seen in the three lines of sight toward the gravitationally lensed QSO B1422+231

Using Subaru 8.2m Telescope and IRCS Echelle spectrograph, we obtained spatially-resolved near-infrared spectra (R=10,000) of images A and B (AB=0.5 arcsec) of the gravitationally lensed quasar B1422+231 (z=3.628), which consists of four known lensed images. We detected Mg II and Fe II absorption lines of a gas cloud at z=3.54 intervening these lines of sight, which show a large variance of column densities and velocities between the lines of sight A and B with a projected separation of only 8.4 h70−1 pc at the redshift. This is the smallest spatial structure of the high-z gas clouds ever detected after Rauch et al. found a 20 pc scale structure for the same z=3.54 absorption system using optical spectra of images A and C. Considering the physical origin of the differences of α-element absorption lines among three images A, B and C, we conclude that the z=3.54 system is an expanding shell as suggested by Rauch. From the three-dimensional structure of the absorbing gas cloud, which can be analyzed by the i...