A roadmap of exoplanet explorations in Japan. 

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... of both low order modes and high order modes with the same WFS “hardware”: large and small amplitude oscillations of the vibrating membrane are temporally interlaced. The effect of photon noise on the system performance (commonly named “noise propagation”) is reduced. The corresponding gain in Strehl ratio is equivalent to increasing the guide star brightness by more than 1 magnitude. This performance gain is obtained by a better sensing of low order modes, especially tip-tilt. Since coronagraphs are especially sensitive to these modes, this technique is particularly advantageous for HiCIAO. In typical atmospheric conditions, the AO188 system performance is limited by the number of actuators for guide stars brighter than V ≈ 9 (this limit tends to become brighter for high wind speed and fainter for slow turbulence), and is photon noise limited for fainter guide stars. The predicted “bright star” performances, obtained from simulations on a V=5 star, range from 55% to 82% for 0.7” to 0.4” respectively. These numbers exclude non-common path errors and wavefront errors produced by the optics of HiCIAO. See Takami et al. and Hayano et al. (these proceedings) for more details on the LGSAO188 adaptive optics system 14, 15 . The simulations are performed using differential imaging subtraction with a spectral resolution of R=30. The chosen wavelengths are 1625nm and 1680nm intending to match Jupiter-like spectral signature characteristics. However, this wavelength choice is somehow arbitrary and is only intended to reflect multi-spectral differential imaging capabilities for a given spectral resolution. Moreover the wavelengths were chosen to be intermediate ones (H band) keeping in mind that better results can potentially be achieved at longer wavelengths (up to ~2.2μm) and inversely for shorter wavelengths of course. Note that the spatial scale of the images is the same for the two wavelengths. The re-scaling operation is not performed since all the images are generated using the same pupil sampling. Rescaling, registration and rotation can be very efficiently performed using the FFT-based methods described by Marois (2004) 16 , to a much more precise level than actually required. The turbulent phase screens and the 188 actuators AO correction are simulated with a time sampling of 0.5ms. Under very good seeing conditions (0.4” at 500nm) and bright stars (up to V=7) the SR will reach ~80% in H. The top-left graph of Figure 2 shows the SR against the seeing conditions for both natural and laser guide stars (NGS and LGS respectively). The simulation includes several effects: atmospheric dispersion between the spectral channels, static common and differential aberrations, misalignment of the Lyot stop (rotation and translation errors). The common aberrations include terms from the telescope optics (300nm RMS, partially corrected by the AO system), from the AO system and up to the coronagraphic mask (50nm RMS), from relay optics after the coronagraph mask (30nm RMS) and finally differential (uncorrelated) aberrations (10nm RMS). Figure 2 shows typical results of the SDI images with the above simulation parameters (no photon noise). The residual bright speckles (right part of Figure 2) are not atmospheric speckles, but are static speckles due to the differential phase aberrations. Atmospheric speckles are very efficiently subtracted in the SDI process. There are two distinct effects that affect the performance: the coronagraph performance (directly linked to its chromatism and to the AO system correction), and the amplitude of the differential aberrations. In the HiCIAO case, like in all current SDI systems, the performance limit is mainly constrained by the differential aberrations effects. Under these conditions, improving the coronagraph with a more advanced technique does not provide any gain: limitations of the technique due to the coronagraph itself would require Strehl ratios above 95%. Decreasing the spectral channels separation will not provide further gain either, also because of the differential aberrations levels. The latter would require being lower than the natural (differential) chromatism of the light, that is, less than about 4 nm RMS in this configuration. The left part of Figure 2 shows the azimuthally averaged results in this configuration. The 5- σ detection level is computed against rings of width λ /D, and at increasing radial distances, in order to account for the speckle noise. Artificial point sources have been introduced at different position angles (0.21”, 0.55”, 0.88”, 1.23” and 1.57” or 5, 13, 21, 29 and 37 λ /D) and at different contrast levels (from 10 -4 to 10 -6 ). The detection limit lies between 2.5 10 -6 and 5 10 -6 at a distance of around 1”. We are currently working on image processing and calibration procedures which could increase the achievable contrast. In order to introduce the current activities related to the explorations of exoplanets in Japan, we show a roadmap in Figure 3. Our ground-based direct explorations are currently concentrated on the fully-operated and successful Subaru 8.2m telescope. CIAO and HiCIAO will be used for the extensive surveys of young exoplanets. Regarding space-based explorations, the MIR and FIR space telescope, ASTRO-F or AKARI, is developed by JAXA/ISAS 17 and launched successfully in 2006 February. This telescope has a similar size to the Spitzer space telescope, but is optimized to all-sky survey observations. Although its low spatial resolution is not suitable for direct explorations, it will conduct an unbiased survey for faint disk emission. This enables us to make census of exo-zodi and disk evolution with a large sample of stars. As a next project in the OPT/IR community in Japan after Subaru/Akari, SPICA is currently heavily discussed (see Nakagawa et al. 2006) 18 , a cooled large single mirror space-telescope. Because of its high sensitivity and acceptable resolution, the mission will serve for direct exoplanet explorations. Its coronagraph and exoplanet science are discussed by Enya et al. (2006) 19 . A space-telescope dedicated for direct explorations of Earth-like planets, JTPF, is also under discussion in the OPT/IR community in Japan. HiCIAO is a new high-contrast instrument for the Subaru telescope. HiCIAO will be used in conjunction with the new adaptive optics (AO) system (188 actuators and/or its laser guide star - AO188/LGSAO188) at the Infrared Nasmyth platform. It is designed as a flexible camera comprising several modules that can be configured into different modes of operation. The main modules are the AO module with its future extreme AO capability, the warm coronagraph and high contrast optics module, and the cold infrared camera module. HiCIAO will be the first instrument on the 8-m class telescopes which can combine coronagraphic techniques with either simultaneous polarization or spectral differential imaging (SDI/PDI) modes which takes into account the non-common path errors. The basic concept of such differential imaging is to split up the image into two or more images, and then use either different planes of polarization or different spectral bandpass filters to produce a signal that distinguishes faint objects near a bright central object from scattered halo or residual speckles. This enables us to achieve a contrast improvement of at least 10 times or even 100 times better than before. HiCIAO can be utilized for various fields in the high contrast astronomy. Direct imaging of extrasolar planets/brown dwarfs and sub-0.1 arcsec imaging of both disks around Young Stellar Objects and debris disks are two of the most important fields in which HiCIAO plays a vital role. An extensive survey dedicated to these sciences on the Subaru telescope will be a significant contribution to astronomy before the high contrast space telescope era. The application of HiCIAO to other targets might include not only other galactic objects such as late-type stars but also extragalactic objects such as AGNs and quasars, especially with the laser guide stars, where the AO application has been relatively limited so far. A high throughput direct imaging with HiCIAO will be also useful for such objects. The HiCIAO falls into a global high contrast imaging science framework, so that part of its target selection strategy can be tightly linked to these existing or ongoing research programs. We are grateful to the HiCIAO Science working group member, especially, B. Sato, M. Fukagawa, E. Turner, for their contributions in the CDR and PDR. We are grateful to the CIAO and AO development teams, especially, N. Kaifu, N. Ebizuka, S. S. Hayashi, K. Murakawa, Y. Itoh, N. Takato, Y. Hayano, S. Oya, and M. Iye. We also thank the Subaru telescope team and M. Ishii for their telescope/instrument supports. This research is supported by Ministry of Education, Culture, Sports, Science and Technology of Japan, Grant-in-Aid for Scientific Research on Priority Areas, "Development of Extra-Solar Planetary Science". 1. Hodapp, K., et al., “The HiCIAO camera for the Subaru telescope”, these proceedings , 6269-142 (2006) 2. Fukagawa, M., Hayashi, M., Tamura, M., Itoh, Y., Hayashi, S. S., Oasa, Y., Takeuchi, T., Morino, J., Murakawa, K., Oya, S., Yamashita, T., Suto, H., Mayama, S., Naoi, T., Ishii, M., Pyo, T., Nishikawa, T., Takato, N., Usuda, T., Ando, H., Iye, M., Miyama, S. M., Kaifu, N., “Spiral Structure in the Circumstellar Disk around AB Aurigae”, ApJ , 605 , L53 (2004) 3. Fukagawa, M., Tamura, M., Itoh, Y., Kudo, T., Imaeda, Y., Oasa, Y., Hayashi, S. S., Hayashi, M., “Near-Infrared Images of Protoplanetary Disk Surrounding HD 142527”, ApJ , 636 , L153 (2006) 4. Itoh, Y., Hayashi, M., Tamura, M., Tsuji, T., Oasa, Y., Fukagawa, M., Hayashi, Saeko S., Naoi, T., Ishii, M., Mayama, S., Morino, J., Yamashita, T., Pyo, T.-S., Nishikawa, T., Usuda, T., Murakawa, K., Suto, H., Oya, S., Takato, N., Ando, H., Miyama, Shoken M., Kobayashi, N., Kaifu, N., “A ...