Hidehiko Nakaya

Hyper Suprime-Cam

Hyper Suprime-Cam (HSC) is an 870 Mega pixel prime focus camera for the 8.2 m Subaru telescope. The wide field corrector delivers sharp image of 0.25 arc-sec FWHM in r-band over the entire 1.5 degree (in diameter) field of view. The collimation of the camera with respect to the optical axis of the primary mirror is realized by hexapod actuators whose mechanical accuracy is few microns. As a result, we expect to have seeing limited image most of the time. Expected median seeing is 0.67 arc-sec FWHM in i-band. The sensor is a p-ch fully depleted CCD of 200 micron thickness (2048 x 4096 15 μm square pixel) and we employ 116 of them to pave the 50 cm focal plane. Minimum interval between exposures is roughly 30 seconds including reading out arrays, transferring data to the control computer and saving them to the hard drive. HSC uniquely features the combination of large primary mirror, wide field of view, sharp image and high sensitivity especially in red. This enables accurate shape measurement of faint galaxies which is critical for planned weak lensing survey to probe the nature of dark energy. The system is being assembled now and will see the first light in August 2012.

Variable stars in the Magellanic Clouds: results from OGLE and SIRIUS

We have performed a cross-identification between Optical Gravitational Lensing Experiment II (OGLE-II) data and single-epoch Simultaneous three-colour Infrared Imager for Unbiased Surveys (SIRIUS) near-infrared (NIR) JHK survey data in the Large and Small Magellanic Clouds (LMC and SMC, respectively). After eliminating obvious spurious variables, variables with too few good data and variables that seem to have periods longer than the available baseline of the OGLE-II data, we determined the pulsation periods for 8852 and 2927 variables in the LMC and SMC, respectively. Based on these homogeneous data, we studied the pulsation properties and metallicity effects on period–K magnitude (PK) relations by comparing the variable stars in the LMC and SMC. The sample analysed here is much larger than the previous studies, and we found the following new features in the PK diagram. (1) Variable red giants in the SMC form parallel sequences on the PK plane, just like those found by Wood in the LMC. (2) Both sequences A and B of Wood have discontinuities, and they occur at the K-band luminosity of the tip of the red giant branch. (3) The sequence B of Wood separates into three independent sequences B± and C′. (4) A comparison between the theoretical pulsation models and observational data suggests that the variable red giants on sequences C and newly discovered C′ are pulsating in the fundamental and first overtone modes, respectively. (5) The theory cannot explain the pulsation mode of sequences A± and B±, and they are unlikely to be the sequences for the first and second overtone pulsators, as was previously suggested. (6) The zero-points of PK relations of Cepheids in the metal deficient SMC are fainter than those of the LMC by ≈0.1 mag but those of SMC Miras are brighter than those of the LMC by ≈0.13 mag (adopting the distance modulus offset between the LMC and SMC to be 0.49 mag and assuming the slopes of the PK relations are the same in the two galaxies), which are probably due to metallicity effects.

Early Phase Obserbations of Extermely Luminous Type Ia Supernova 2009dc

We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is ?m 15(B) = 0.65 ? 0.03, which is one of the slowest among SNe Ia. The peak V-band absolute magnitude is estimated to be MV = ?19.90 ? 0.15?mag if no host extinction is assumed. It reaches MV = ?20.19 ? 0.19?mag if we assume the host extinction of AV = 0.29?mag. SN 2009dc belongs to the most luminous class of SNe Ia, like SNe 2003fg and 2006gz. Our JHKs -band photometry shows that this SN is also one of the most luminous SNe Ia in near-infrared wavelengths. We estimate the ejected 56Ni mass of 1.2 ? 0.3 M ? for the no host extinction case (and of 1.6 ? 0.4 M ? for the host extinction of AV = 0.29?mag). The C II ?6580 absorption line remains visible until a week after the maximum brightness, in contrast to its early disappearance in SN 2006gz. The line velocity of Si II ?6355 is about 8000?km?s?1 around the maximum, being considerably slower than that of SN 2006gz. The velocity of the C II line is similar to or slightly less than that of the Si II line around the maximum. The presence of the carbon line suggests that the thick unburned C+O layer remains after the explosion. Spectropolarimetric observations by Tanaka et?al. indicate that the explosion is nearly spherical. These observational facts suggest that SN 2009dc is a super-Chandrasekhar mass SN Ia.

Current performance and on-going improvements of the 8.2 m Subaru Telescope

An overview of the current status of the 8.2m Subaru Telescope constructed and operated at Mauna Kea, Hawaii, by the National Astronomical Observatory of Japan is presented. The basic design concept and the verified performance of the telescope system are described. Also given are the status of the instrument package offered to the astronomical community, the status of operation, and some of the future plans. The status of the telescope reported in a number of SPIE papers as of the summer of 2002 are incorporated with some updates included as of 2004 February. However, readers are encouraged to check the most updated status of the telescope through the home page, http://subarutelescope.org/index.html, and/or the direct contact with the observatory staff.

Near-Infrared Imaging Survey of Bok Globules: Density Structure

On the basis of near-infrared imaging observations, we derived the visual extinction (AV) distribution toward 10 Bok globules through measurements of both the color excess (EH-K) and the stellar density at J, H, and Ks (star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our 10 globules and four globules in the literature, we investigated the stability of globules on the basis of ξmax, which characterizes the Bonnor-Ebert sphere, as well as the stability of the equilibrium state against gravitational collapse. We found that more than half the starless globules are located near the critical state (ξmax = 6.5 ± 2). Thus, we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. The remaining starless globules show clearly unstable states (ξmax > 10). Since unstable equilibrium states are not long maintained, we expect that these globules are on the way to gravitational collapse or that they are stabilized by nonthermal support. It was also found that all the star-forming globules show unstable solutions of ξmax > 10, which is consistent with the fact that they have started gravitational collapse. We investigated the evolution of a collapsing gas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere. We found that the column density profiles of the collapsing sphere mimic those of the static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gas sphere resembles marginally unstable Bonnor-Ebert spheres for a long time. We found that the frequency distribution of ξmax for the observed starless globules is consistent with that from model calculations of the collapsing sphere. In addition to the near-infrared observations, we carried out radio molecular line observations (C18O and N2H + ) toward the same 10 globules. We confirmed that most of the globules are dominated by thermal support. The line width of each globule was used to estimate the cloud temperature including the contribution from turbulence, with which we estimated the distance to the globules from the Bonnor-Ebert model fitting.

A Distinct Structure inside the Galactic Bar

We present the result of a near-infrared (JHKs) survey along the Galactic plane, -105 ≤ l ≤ 105 and b = +1°, with the IRSF 1.4 m telescope and the SIRIUS camera. Ks versus H - Ks color-magnitude diagrams reveal a well-defined population of red clump stars whose apparent magnitude peak changes continuously along the Galactic plane, from Ks = 13.4 at l = -10° to Ks = 12.2 at l = 10° after dereddening. This variation can be explained by the barlike structure found in previous studies, but we find an additional inner structure at l 4°, where the longitude-apparent magnitude relation is distinct from the outer bar and where the apparent magnitude peak changes by only ≈0.1 mag over the central 8°. The exact nature of this inner structure is as yet uncertain.

HyperSuprime: project overview

HyperSuprime is a next generation wide field camera proposed for the 8.3 m Subaru Telescope. The targeted field of view is larger than 1.5 deg in diameter, which will give us roughly 10 times increase of the survey speed compared with the existing prime focus camera (Suprime-Cam). An overview of the current status of the feasibility study is given.

Variable stars in the Magellanic Clouds – II. The data and infrared properties

The data of 8852 and 2927 variable stars detected by the OGLE survey in the Large and Small Magellanic Clouds are presented. They are cross-identified with the SIRIUS JHK survey data, and their infrared properties are discussed. Variable red giants are well separated on the period-(J - K) plane, suggesting that it could be a good tool to distinguish their pulsation mode and type.

TRISPEC: A Simultaneous Optical and Near-Infrared Imager, Spectrograph, and Polarimeter

We present the design, construction, and performance of TRISPEC (Triple Range Imager and Spectrograph), a simultaneous optical and near-infrared imager, spectrograph, and polarimeter. This instrument splits the incoming light from a telescope into the three beams—one optical channel (0.45-0.90 μm) and two infrared channels (0.90-1.85 and 1.85-2.5 μm)—by means of two dichroic mirrors. It is capable of simultaneous three-band imaging or spectroscopy, with or without polarimetry. Low-resolution grisms (R ~ 70-360) cover a wide wavelength range of 0.46-2.5 μm, simultaneously. The instrument employs three arrays—one 512 × 512 SITe CCD and two 256 × 256 SBRC InSb arrays—to cover the three channels. It has been commissioned in the imaging, spectroscopic, and polarimetric modes on the United Kingdom Infrared Telescope, the University of Hawaii 2.2 m telescope, and the Okayama Astrophysical Observatory 1.88 m telescope since 1999 June.

The period–luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations (JHK s ) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period-luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster. The period-luminosity relation can be reproduced using the pulsation equation (P√ρ = Q) assuming that all the stars have the same mass. Cluster RR Lyr variables were found to lie on an extrapolation of the period-luminosity relation. These results provide important constraints on the parameters of the variable stars. Using Two Micron All-Sky Survey (2MASS) data, we show that the type II Cepheids in the Large Magellanic Cloud (LMC) fit our period-luminosity relation within the expected scatter at the shorter periods. However, at long periods (P > 40 d, i.e. in the RV Tau star range) the LMC field variables are brighter by about one magnitude than those of similar periods in galactic globular clusters. The long-period cluster stars also differ from both these LMC stars and galactic field RV Tau stars in a colour-colour diagram. The reasons for these differences are discussed.