Kodai Yamamoto

Direct imaging search for extrasolar planets in the pleiades

We carried out an imaging survey for extrasolar planets around stars in the Pleiades (125 Myr, 135 pc) in the H and KS bands using HiCIAO combined with adaptive optics, AO188, on the Subaru telescope. We found 13 companion candidates fainter than 14.5 mag in the H band around 9 stars. Five of these 13 were confirmed to be background stars by measurement of their proper motion. One was not found in the second epoch observation, and thus was not a background or companion object. One had multi-epoch images, but the precision of its proper motion was not sufficient to conclude whether it was a background object. Four other candidates are waiting for second-epoch observations to determine their proper motion. Finally, the remaining two were confirmed to be 60 MJ brown dwarf companions orbiting around HD 23514 (G0) and HII 1348 (K5), respectively, as had been reported in previous studies. In our observations, the average detection limit for a point source was 20.3 mag in the H band beyond 1.′′ 5 from the central star. On the basis of this detection limit, we calculated the detection efficiency to be 90% for a planet with 6 to 12 Jovian masses and a semi-major axis of 50-1000 AU. For this reason we extrapolated the distribution of the planet mass and the semi-major axis derived from radial velocity observations, and adopted the planet evolution model Baraffe et al. (2003, A&A, 402, 701). Since there was no detection of a planet, we estimated the frequency of such planets to be less than 17.9% (2 σ) around one star of the Pleiades cluster.

Indications of M-Dwarf Deficits in the Halo and Thick Disk of the Galaxy

We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20 +/- 13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52 +/- 13%) or the scale height must be decreased (approximately 600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities.

Real-time point-diffraction interferometer and its analytical formulation.

We propose a novel wavefront sensor and study its performance with an analytical formulation. The sensor has a polarizing point-diffraction beam splitter. Using transmitted and reflected beams, we can build a real-time point-diffraction interferometer with high precision and efficiency. Our analytical studies reveal that wavefront errors might be measured incorrectly and that less precise estimates of wavefronts appear as the pinhole radius Rpin is increased. An investigation of propagating uncertainties shows that the wavefront measurement can be calibrated by estimating the pinhole effects and the polarizing properties with a precision of a few percent. Based on these studies, Rpin should be smaller than half of the Airy disk for better performance.

Development of new optical adjustment system for FITE (Far-Infrared Interferometric Telescope Experiment)

We have developed a balloon-borne, astronomical far-infrared interferometer (FITE). Because the interferometer is a Fizeau-type two beam interferometer consisting of two off-axis parabolic mirrors, it is important to establish a method by which the two beams can be adjusted simultaneously. A conventional Hartmann test was originally employed in our previous system, but it enabled the adjustment of only one beam at one time, thus quite inefficient. We developed a new optical adjustment system that can simultaneously measure and evaluate two beams by using a Shack - Hartmann wave front sensor. In the first stage, the field of view (FOV) of the wave front sensor was adapted to the full beam size of 40 cm (the beam diameter), and the mirror surface accuracy as well as the mirror alignment were measured and adjusted for each beam. After the adjustment of both beams, they are focused at the input aperture hole of the far-infrared sensor system by expanding the FOV of the wave front sensor so that it included both beams. With this new method, we can make real-time measurements and analyses of converging beams, and can also realize fast switching between the single beam mode and double beam mode. We demonstrated this new adjustment method by performing laboratory measurements, and designed and assembled the new optical adjustment system for FITE.

Far-Infrared Interferometric Telescope Experiment: Optical Adjustment System

We developed a balloon-borne, astronomical far-infrared interferometer telescope experiment (FITE), which is a Fizeau-type two-beam interferometer consisting of two off-axis parabolic mirrors. Therefore, it is important to establish a method by which the two beams can be simultaneously adjusted. A conventional Hartmann test was originally employed; however, it has two disadvantages in that we cannot simultaneously measure two beams, and it is time-consuming. We developed a new optical adjustment system that can simultaneously measure and evaluate two beams using a Shack-Hartmann wavefront sensor. In the first stage, the field-of-view (FOV) of a wavefront sensor is adapted to a full beam size of 40 cm (the beam diameter), and the mirror surface accuracy and mirror alignment are measured and adjusted for one beam. After the adjustment of both beams, both focuses coincide at the input aperture hole of the far-infrared sensor system by expanding the FOV of the wavefront sensor so that it includes both beams. Using this new method, we can realize the real-time measurement and analysis of converging beams, and can also achieve fast switching between the single- and double-beam modes. We demonstrated this new adjustment method by performing laboratory measurements; we have also designed a new optical adjustment system for FITE. To manufacture this system, we required a large and precise spherical mirror as the reference mirror with R = 300 mm and D = 300 mm to simultaneously calibrate the two beams using the wavefront sensor. We also manufactured an F/1 lens for the collimation of the two beams, a Keplarian beam expander and a beam-switching system. We will soon assemble this optical adjustment system and apply it to the optical adjustment of the FITE interferometer system.