Mihoko Konishi

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.

DISCOVERY OF AN INNER DISK COMPONENT AROUND HD 141569 A

We report the discovery of a scattering component around the HD 141569 A circumstellar debris system, interior to the previously known inner ring. The discovered inner disk component, obtained in broadband optical light with Hubble Space Telescope/Space Telescope Imaging Spectrograph coronagraphy, was imaged with an inner working angle of 0 25 arcseconds, and can be traced from 0 4 seconds (approximately 46 atomic units) to 1.0 arcseconds (approximately 116 atomic units) after deprojection using inclination = 55 degrees. The inner disk component is seen to forward scatter in a manner similar to the previously known rings, has a pericenter offset of approximately 6 atomic units, and break points where the slope of the surface brightness changes. It also has a spiral arm trailing in the same sense as other spiral arms and arcs seen at larger stellocentric distances. The inner disk spatially overlaps with the previously reported warm gas disk seen in thermal emission. We detect no point sources within 2 arcseconds (approximately 232 atomic units), in particular in the gap between the inner disk component and the inner ring. Our upper limit of 9 plus or minus 3 mass Jupiter (M (sub J)) is augmented by a new dynamical limit on single planetary mass bodies in the gap between the inner disk component and the inner ring of 1 mass Jupiter, which is broadly consistent with previous estimates.

The infrared Doppler (IRD) instrument for the Subaru telescope: instrument description and commissioning results

The Infrared Doppler (IRD) instrument is a fiber-fed high-resolution NIR spectrometer for the Subaru telescope covering the Y,J,H-bands simultaneously with a maximum spectral resolution of 70,000. The main purpose of IRD is a search for Earth-mass planets around nearby M-dwarfs by precise radial velocity measurements, as well as a spectroscopic characterization of exoplanet atmospheres. We report the current status of the instrument, which is undergoing commissioning at the Subaru Telescope, and the first light observation successfully done in August 2017. The general description of the instrument will be given including spectrometer optics, fiber injection system, cryogenic system, scrambler, and laser frequency comb. A large strategic survey mainly focused on late-type M-dwarfs is planned to start from 2019.

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.

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.

Performance tests of Subaru/IRD for very precise and stable infrared radial velocity observations

The InfraRed Doppler (IRD) instrument is a high-dispersion spectrograph that is available on Subaru Telescope to explore extrasolar planets via infrared radial velocity (RV) observations. The Subaru/IRD is especially useful in the search of a low-mass planet around cool M-type dwarfs for which infrared RV observations are essential. We report our early performance tests for IRD. IRD’s two H2RG detectors have been evaluated with our detector readout technique, ensuring that their readout noise is made sufficiently smaller than the stellar photon noise expected in our planned survey. We have also tested the instrumental stability of RV measurements from the laboratory data obtained with the IRD’s calibration systems including a laser frequency comb (LFC). Among our tested three types of velocity stability, the stability of comb spectra obtained with a multi-mode fiber (MMF) relative to that with another MMF is measured to be ∼1 m s−1. We also infer from these tests that stellar RV measurements with an MMF can be calibrated with a short-term stability of 2 m s−1 or better by the simultaneously-observed reference spectra of LFC. Furthermore, we report preliminary on-sky RV measurements calibrated with a Thorium-Argon hollow-cathode lamp for RV-stable stars (τ Ceti and Barnards star) and a planet-host (51 Pegasi). These preliminary RV measurements help the further performance test of IRD that will be performed by the on-sky observations with LFC.

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.