伊藤 洋一

We report on the detection of four extrasolar planets orbiting evolved intermediate-mass stars from a precise Doppler survey of G-K giants at Okayama Astrophysical Observatory. All of the host stars are considered to be formerly early F-type or A-type dwarfs when they were on the main sequence. 14 And (K0 III) is a clump giant with a mass of 2.2 M⊙ and has a planet of minimum mass m2 sin i = 4.8 MJ in a nearly circular orbit with a 186 d period. This is one of the innermost planets around evolved intermediate-mass stars, and such planets have only been discovered in clump giants. 81 Cet (G5 III) is a clump giant with 2.4 M⊙ hosting a planet of m2 sin i = 5.3 MJ in a 953 d orbit with an eccentricity of e = 0.21. 6 Lyn (K0 IV) is a less-evolved subgiant with 1.7 M⊙, and has a planet of m2 sin i = 2.4 MJ in a 899 d orbit with e = 0.13. HD 167042 (K1 IV) is also a less-evolved star with 1.5M⊙ hosting a planet of m2 sin i = 1.6 M J in a 418 d orbit with e = 0.10. This planet was independently announced by Johnson et al. (2008, ApJ, 675, 784). All of the host stars have solar or sub-solar metallicity, which supports the lack of a metal-rich tendency in planet-harboring giants in contrast to the case of dwarfs. © 2008. Astronomical Society of Japan.

A high angular resolution near-infrared image of a classical T Tauri star, DO Tau, was obtained with Coronagraphic Imager with Adaptive Optics (CIAO) mounted on the Subaru Telescope. Circumstellar emission was detected at ∼ 1.".7 from the central star in the direction of the redshifted jet. No emission was found toward the opposite direction. We also found one faint point source 3."46 away from the central star. The following observation will reveal whether it is an associated planetary mass object or a background object. © 2008. Astronomical Society of Japan.

We have made a new survey of emission-line stars in the W 5 E H II region to investigate the population of PMS stars near the OB stars by using the Wide Field Grism Spectrograph 2 (WFGS2). A total of 139 Ha emission stars were detected and their g'i'-photometry was performed. Their spatial distribution shows three aggregates, i.e., two aggregates near the bright-rimmed clouds at the edge of the W 5 E H II region (BRC 13 and BRC 14) and one near the exciting O7 V star. The age and mass of each Ha star were estimated from an extinction-corrected color-magnitude diagram and theoretical evolutionary tracks. We found, for the first time in this region, that the young stars near the exciting star are systematically older (4 Myr) than those near the edge of the H II region (1 Myr). This result supports that the formation of stars proceeds sequentially from the center of H II region to the eastern bright rim. We further suggest a possibility that the birth of low-mass stars near the exciting star of the H II region precedes the production of massive OB stars in the pre-existing molecular cloud. © 2008. Astronomical Society of Japan.

We report the detection of an extrasolar planet orbiting f. Tau, one of the giant stars in the Hyades open cluster. This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594.9 ± 5.3 days, a semiamplitude of 95.9 ± 1.8ms-1, and an eccentricity of 0.151 ± 0.023. The minimum mass of the companion is 7.6 ± 0.2 MJ, and the semimajor axis is 1.93 ± 0.03 AU adopting a stellar mass of 2.7 ± 0.1 M⊙. The age of 625 Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 M⊙ for the host star is robustly determined by isochrone fitting, which makes the star the heaviest among planet-harboring stars. Putting together the fact that no planets have been found around about 100 low-mass dwarfs in the cluster, the frequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones. © 2007. The American Astronomical Society. All rights reserved.

We present the status of the development of a coronagraph for the Space Infrared telescope for Cosmology and Astrophysics (SPICA). SPICA is the next generation of infrared space-borne telescope missions following to AKARI, led by Japan. SPICA will carry a telescope that has a 3.5 m diameter monolithic primary mirror and the whole telescope will be cooled to 4.5 K. It is planned to launch SPICA into the sun-earth L2 libration halo orbit using H II-A rocket in the middle of the 2010s and execute infrared observations at wavelengths mainly between 5 and 200 micron. The SPICA mission gives us a unique opportunity for coronagraph observations, because of the large telescope aperture, the simple pupil shape, the capability of infrared observations from space, and the early launch. We have started development of the SPICA coronagraph in which the primary target is direct observation of extra-solar Jovian planets. The main wavelengths of observation, the required contrast and the inner working angle (IWA) of the SPICA coronagraph instrument are set to be 5-27 micron, 10 , and a few λ/D (and as small as possible), respectively, in which λ is the observation wavelength and D is the diameter of the telescope aperture (3.5m). We focused on a coronagraph with a binary shaped pupil mask as the primary candidate for SPICA because of its feasibility. Nano-fabrication technology using electron beam lithography was applied to manufacture a high precision mask and a laboratory experiment with a He-Ne laser (λ=632.8nm) was performed in air without active wavefront control. The raw contrast derived from the average measured in the dark region reached 6.7×10 . On the other hand, a study of Phase Induced Amplitude Apodization (PIAA) was started in an attempt to achieve higher performance, i.e., smaller IWA and higher throughput. A hybrid solution using PIAA and a shaped pupil mask was proposed. A laboratory experiment was performed using a He-Ne laser with active wavefront control via a 32 × 32 channel deformable mirror. A raw contrast of 6.5 × 10 was achieved. Designs of binary shaped pupil mask are presented for the actual SPICA pupil which is obstructed by the telescope's secondary mirror and its support. Subtraction of point spread function (PSF) was also evaluated. -6 -8 -7

We present a near-infrared image of UY Aur, a 0.9″ separated binary system, using the Coronagraphic Imager with Adaptive Optics on the Subaru Telescope. Thanks to adaptive optics, the spatial resolution of our image was ∼0.1″ in the full width at half-maximum of the point-spread function, the highest achieved. By comparison with previous measurements, we estimated that the orbital period is ∼1640 ± 90 yr and the total mass of the binary is ∼ 1.73 ± 0.29 M⊙. The observed H-band magnitude of the secondary varies by as much as 1.3 mag within a decade, while that of the primary is rather stable. This inconstancy may arise from photospheric variability caused by an uneven accretion rate or from the rotation of the secondary. We detected a half-ring-shaped circumbinary disk around the binary with a bright southwestern part but a barely detectable northeastern portion. The brightness ratio is ≳57 ± 5. Its inner radius and inclination are about 520 AU and 42° ± 3°, respectively. The disk is not uniform but has remarkable features, including a clumpy structure along the disk, circumstellar material inside the inner cavity, and an extended armlike structure. The circumstellar material inside the cavity probably corresponds to a clump or material accreting from the disk onto the binary. The armlike structure is part of the disk, created by accretion from the outer region of the disk or encounters with other stellar systems. © 2007. The American Astronomical Society. All rights reserved.

We conducted subarcsecond near-infrared imaging observations of RNO 91 with CIAO (Coronagraphic Imager with Adaptive Optics) mounted on the 8.2m Subaru telescope. We present our JHK band data along with optical images, which when considered together reveal a complex circumstellar structure. We examined the colors of associated nebulae and compared the geometry of the outflow/disk system suggested by our data with that already proposed on the basis of previous studies. Our K-band image shows bright circumstellar nebulosity detected within ∼ 2″ around the central source, while it is less conspicuous at shorter wavelengths. PA and the size of this red color nebulosity agree with those of the previously detected polarization disk. Agreements among these data indicate that this bright nebulosity region, which follows the reddening law, might be attributed to a disklike structure. At J and optical wavelengths, several blue knotlike structures are detected around and beyond the bright circumstellar nebulosity. We suggest that these knotty reflection nebulae may represent disintegrating fragments of an infalling envelope. The three-color composite image has the appearance of arc-shaped nebulosity. We interpret these structures as being roots of a bipolar cavity opening toward the northeast and the southwest. The complex distribution of reflection nebulosity seen around RNO 91 appears to confirm the interpretation that this source is an object dispersing its molecular envelope while transitioning from protostar to T Tauri star. © 2007. Astronomical Society of Japan.

Bright-rimmed clouds (BRCs) found in H II regions are probable sites of triggered star formation due to compression by ionization/shock fronts, and it is hypothesized that star formation proceeds from the exciting star(s) side outward of the H II region ("small-scale sequential star formation"). In order to quantitatively test this hypothesis we undertook B V Ic photometry of four BRC aggregates. The amounts of interstellar extinction and reddening for each star were estimated by using JHKs photometry. We then constructed reddening-corrected V/V - Ic color-magnitude diagrams, where the age of each star was derived. All of the stars turned out to be a few tenths to a few Myr old. Although the scatters were large and the numbers of the sample stars are small, we found a clear trend that the stars inside or in the immediate vicinity of the bright rim are younger than those outside it in all four aggregates, confirming the hypothesis in question. © 2007. Astronomical Society of Japan.

We searched for small main-belt asteroids in the Cosmic Evolution Survey (COSMOS) field using the Subaru Prime Focus Camera (Suprime-Cam). The ecliptic latitude of the region is about -10°. Using a method we developed to detect moving objects from two images, we detected 202 moving objects with a limiting magnitude of i′ = 23.8 mag in 1.7 deg2. Most of them are sub-km asteroid candidates. The number density is 122 deg-2, less than half of that in the ecliptic. The slope of the cumulative size distribution is 1.30 ± 0.05 for asteroids with a diameter of 0.5-2 km, consistent with that of sub-km asteroids in the ecliptic derived from a previous survey. © 2007. Astronomical Society of Japan.

We present the results of a coronagraphic imaging search for extrasolar planets around the young main-sequence stars ε Eri and Vega. Concentrating the stellar light into the core of the point-spread function by the adaptive optic system and blocking the core by the occulting mask in the coronagraph, we have achieved the highest sensitivity for point sources in close vicinity of the both central stars. Nonetheless, we had no secure detection of a point source around the stars. The observations give the upper limits on the masses of the planets to (4-6)MJ and (5-10)MJ at a few arcseconds from ε Eri and Vega, respectively. Difruse structures are also not detected around both stars. © 2006. The American Astronomical Society. All rights reserved.

We present the results of deep near-infrared observations searching for very low mass young stellar objects (YSOs) in the massive star-forming region of S106 taken with the Subaru Telescope. The survey, whose limiting magnitude exceeds 20 mag in the JHK′ bands, is sensitive enough to provide unprecedented details in the two nebular lobes. In addition, it reveals a census of the stellar population down to objects below the deuterium-burning limit, a fiducial boundary between brown dwarfs and planetary-mass objects. Based on color-color diagrams, nearly 600 embedded YSO candidates with near-infrared excesses have been identified in an area of ∼5′ × 5′ that are not uniformly distributed but centrally concentrated. Combining the reddening-corrected luminosity of the YSO candidates with the theoretical evolutionary models, we suggest that there exists a substantial substellar population. If we divide the cluster into three subgroups based on stellar and cloud properties, both their luminosity functions and mass functions across the stellar-substellar boundary demonstrate local variation on a subparsec scale. Nevertheless, all the mass functions have distinct characteristics in common: they do not have any turnover and do not decline down to the completeness limits. Furthermore, the mass function for S106 appears to be more abundant in young substellar objects compared to those obtained for other young clusters, such as Trapezium and IC 348. This implies that the substellar mass function for young clusters may have a variation. © 2006. The American Astronomical Society. All rights reserved.

High angular resolution near-infrared (JHK) adaptive optics images of T Tau were obtained with the infrared camera Coronagraphic Imager with Adaptive Optics (CIAO) mounted on the 8.2 m Subaru Telescope in 2002 and 2004. The images resolve a complex circumstellar structure around a multiple system. We resolved T Tau Sa and Sb as well as T Tau N and S. The estimated orbit of T Tau Sb indicates that it is probably bound to T Tau Sa. The K band flux of T Tau S decreased by ∼ 1.7 Jy in 2002 November compared with that in 2001 mainly because T Tau Sa became fainter. The arc-like ridge detected in our near-infrared images is consistent with what is seen at visible wavelengths, supporting the interpretation in previous studies that the arc is part of the cavity wall seen relatively pole-on. Halo emission is detected out to ∼ 2″ from T Tau N. This may be light scattered off the common envelope surrounding the T Tauri multiple system. © 2006. Astronomical Society of Japan.

We report the results of a prototype photometric search for transiting extrasolar planets using Subaru Suprime-Cam. Out of about 100000 stars monitored around the Galactic plane (l = 90°, b = 0°), we find that 7700 show photometric precision better than 1% for 60 s exposures, which is required to detect extrasolar planets by the transit method. Thus, Suprime-Cam has the photometric stability and accuracy necessary for a transiting planet survey. During this observing run, we detected three objects (around 18.5 mag for i′-band) that exhibit a single full transit-like light curve with a fractional depth of &lt 5%. While a spectroscopic follow-up remains to be done using future telescopes in the 20-30 m class, the estimated parameters for the three systems are consistent with the planetary size companions around main-sequence stars. We also found two eclipsing binary candidates and eleven variable stars exhibiting W UMa-like light curves. © 2006. Astronomical Society of Japan.

Using the Wide Field Grism Spectrograph 2 (WFGS2), we carried out slit-less spectroscopy, g′r′i′ photometry, and slit spectroscopy on the L 1014 dense core. We detected three Hα emission-line stars. We interpret one as being a weak-line T Tauri star (WTTS) and the others as classical T Tauri stars (CTTS). Since their g′ - i′ colors and/or classified spectral types are consistent with those of T Tauri stars and two of them show less extinction than the cloud, these three stars are likely to be T Tauri stars associated with L 1014. Adopting an age range for T Tauri stars, 1-10Myr, the color-magnitude diagram suggests a distance of ∼ 400-900pc, rather than the previously assumed distance, 200 pc. This could strongly affect the mass estimate of L 1014-IRS, which is thought to be either a very young protostar or proto-brown dwarf. © 2006. Astronomical Society of Japan.

We consider a combination of two techniques, namely the nulling interferometer and halo suppression by modified pupil such as shaped pupils and apodized square apertures, which achieves very high dynamic range, in order to detect faint extra-solar planets around nearby stars. The effects of the nulling interferometer and the modified pupil are independent and in combination hence multiplied. We show that one can achieve higher dynamic range in the case of a resolved star than with either method alone. By numerical simulations, we show that the combination method can achieve dynamic range levels of 10-10 at 3λ/D. Used alone, the two-telescope interferometer would give the nulling depth of 10-3 whilst the halo suppression by a shaped aperture would do 10-7, with a point-spread-function core radius less than 3λ/D for a shaped aperture. The introduction of the modified pupil has the same effect whether it is made at the entrance apertures of the interferometer or at a re-imaged common pupil plane after the nulling interferometry. From another point of view, a nulling interferometer works as pre-optics in front of any single telescope methods, which reduces the intensity of a resolved source transmitting some uniform wavefront residuals. © ESO 2005.

We discovered a unique morphology in a disk around the Herbig Ae star HD 142527 by near-infrared ( H and K bands) adaptive optics imaging observations. The almost face-on disk consists of two bright arcs facing one another along the east-west direction (banana-split structure) and one spiral arm extending to the north from the western arc. The eastern arc is located at similar to 100 - 400 AU in radius from the star, and the western one is detected at similar to 150 - 490 AU. The stellar position is displaced from the center of the disk by about 20 AU to the north, and also from the center of the inner hole. The two arcs show an asymmetry in their size and brightness; the larger western arc is brighter than the east one by about 2 mag. The morphology of the disk, consisting of a banana-split structure and a spiral arm, most likely suggests the presence of an unseen eccentric binary and a recent stellar encounter.

We have started a corongraphic search for brown dwarfs and planets around young nearby stars within 20 pc of the Sun, using the adaptive optics coronagraph, CIAO, on Subaru. The dynamic range we have achieved is ΔK = 13 at 2.5″ from the central star. For a typical target with K = 7 at 10 pc, the limiting absolute magnitude is MK = 20. We apply two kinematical age criteria to select M and K dwarfs statistically younger than 350Myr. The first criterion is a small velocity deviation from the velocity of LSR. The second is a (U, V, W) velocity vector similar to a particular young moving group. The combination of the age and magnitude limits implies that the mass limit for giant planet detection is about 2MJ. We show a sample image of a target field at 3 pc of the Sun with faint companion candidates, to be followed up for the common proper motion test. We give a brief description of our procedures for data acquisition, reduction, and analysis. ©2005 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

We are currently conducting three kinds of IR surveys of star forming regions (SFRs) in order to seek for very low-mass young stellar populations. First is a deep JHKs-bands (simultaneous) survey with the SIRIUS camera on the IRSF 1.4m or the UH 2.2m telescopes. Second is a very deep JHKs survey with the CISCO IR camera on the Subaru 8.2m telescope. Third is a high resolution companion search around nearby YSOs with the CIAO adaptive optics coronagraph IR camera on the Subaru. In this contribution, we describe our SIRIUS camera and present preliminary results of the ongoing surveys with this new instrument.

We present high angular resolution spectra taken along the jets from L1551 IRS 5 and DG Tau obtained with the Subaru Telescope. The position-velocity diagrams of the [Fe II] λ 1.644 μm emission line revealed remarkably similar characteristics for the two sources, showing two distinct velocity components separated from each other in both velocity and space with the entire emission range blueshifted with respect to the stellar velocity. The high velocity component (HVC) has a velocity of -200 - -300 km s-1 with a narrow line width, while the low velocity component (LVC) is around -100 km s-1 exhibiting a broad line width. The HVC is located farther away from the origin and is more extended than the LVC. Our results suggest that the HVC is a well-collimated jet originating from the region close to the star, while the LVC is a widely-opened wind accelerated in the region near the inner edge of the accretion disk.

We present a new [Fe II] λ1.644 μm emission-line image of the L1551 IRS 5 jets and a velocity-resolved spectrum obtained with the Subaru Telescope. In the [Fe II] line image showing two separate jetlike features, a strong and spatially wide [Fe II] emission feature was found elongated along the northern jet at its base. The echelle spectrum taken along this feature shows two prominent velocity components together with underlying pedestal and wing emissions. The entire emission range is blueshifted with respect to the systemic velocity, which shows that the emission originates from an outflow. The high-velocity component (HVC) at VLSR = -300 km s-1, which has a corresponding Hα emission, consistently shows a narrow line width of ∼40 km s-1 (deconvolved FWHM value). The low-velocity component (LVC) at VLSR = -100 km s-1, on the other hand, has no corresponding Hα feature and is located spatially closer to the IRS 5 VLA sources (IRS 5 VLA) than is the HVC. The LVC shows broad line widths of 130-160 km s-1 (FWHM) near IRS 5 VLA, while its line width decreases with increasing distance from it. We interpret from these characteristics that the HVC is a spatially narrow, well-collimated ionized stellar jet and that the LVC is a widely opened, partially ionized disk wind that is being collimated as it travels away from the origin. The inclination-corrected velocity of the HVC, 440 km s-1, may imply that the accreting protostar driving the jet is rotating at almost its breakup speed. The LVC has an inclination-corrected wind velocity of ∼200 km s -1, suggesting that it is accelerated at the region where the accretion disk is interacting with the stellar magnetic field. The pedestal and red wing features may represent the LVC gas entrained and accelerated by the HVC. It is possible that the two apparently parallel jetlike features seen more than 6″ away from the IRS 5 VLA sources are the brightened edges of the partially ionized wind.

High-resolution near-infrared imaging of a Herbig Ae/Be star LkHα 198 has been carried out using the coronagraphic camera CIAO (Coronagraphic Imager with Adaptive Optics) on the Subaru 8.2 m Telescope. Obtained images resolve the infrared companion (IRC) and an associated reflection nebula. The IRC exhibits near-infrared colors suggestive of a class I-like source. Faint nebulae associated with outflow activity and a bright flattened envelope are also found around the primary. Our observations provide morphological evidence of the IRC being an outflow source as well as the primary star.

The highest angular resolution near-infrared images of the GG Tau A binary system were obtained with the Coronagraphic Imager with Adaptive Optics (CIAO) mounted on the Subaru Telescope. The images show that its circumbinary disk is smooth and does not have 0″1 scale structures. Asymmetries in the surface brightness as well as a gap are also resolved. Combined with previous measurements of the stellar positions, our data indicate that the central binary is unlikely to have a large eccentricity. The fifth point source toward this double binary system is probably a background star, based on astrometric measurements.

We carried out near-infrared spectroscopic observations of 23 very low-luminosity young stellar object (YSO) candidates and 5 of their companions in Heiles Cloud 2, one of the densest parts of the Taurus molecular cloud. Twelve objects were confirmed to be YSOs by the Bγ feature. The effective temperatures of the YSOs and of the companions are estimated from the 2.26-μm feature, the 2.21-μm feature, and the H2O band strengths. Detailed comparisons of our photometric and spectroscopic observations with evolutionary tracks on the HR diagram suggest some objects to be very low-mass YSOs.

High-resolution near-infrared (J H K L′ M′) images of a massive star-forming region, S 255-2, were obtained with the Subaru Telescope and an infrared camera, CIAO. These images clearly resolve two sets of bipolar nebulae illuminated by two independent massive YSOs. A number of cluster member YSOs have been detected, including 3 new sources. Seven early-B type stars have been identified within 10pc of S 255-2 three are associated with extended optical H II regions. The variation in the appearance of these early-B type stars suggests an evolutionary sequence of massive stars in the S 255 region.

This paper describes the first light and subsequent test observations with the 8.2 in aperture Subaru Teleseope constructed at the summit of Mauna Kea. Following the engineering first light, which started 1998 December, the astronomical first light and test observations were carried out in 1999 January with 4 testing instruments under seeing conditions of 0.″2-0.″5 for near-infrared and 0.″3-0.″6 for optical wave-lengths. The actively supported primary mirror was shown to achieve an overall imaging performance of 0.″1 (FWHM) or better in the absence of any atmospheric disturbance. The pointing accuracy of the telescope is about 1″ rms, and a closed-loop tracking accuracy of ≲ 0.″07 rms has been achieved. Infrared images of the Orion Nebula covering 5′ × 5′, obtained with J, K′, and H2 υ = 1-0 S(1) filters, have revealed much finer and fainter details of the BN/KL region, the bright bar, and other conspicuous features compared with previous observations. K′ band photometry of 516 point sources yielded a luminosity function with a peak at K′ ∼ 12 mag with a long tail in K′ ∼ 13 mag down to K′ ∼ 17 mag, suggesting a fairly large number of young brown dwarfs existing in the Trapezium cluster. Several new features around the Orion BN/KL region are also reported.

We present high spatial resolution images of the gravitational-lens system PG 1115+080 taken with the near-infrared camera (CISCO) on the Subaru telescope. The FWHM of the combined image is 0.″32 in the K′-band, yielding spatial resolution of 0.″14 after a deconvolution procedure. This is a first detection of an extended emission adjacent to the A1/A2 components, indicating the presence of a fairly bright, emission region with a characteristic angular radius of ∼ 5 mas (40 pc). The near-infrared image of the Einstein ring was extracted in both the J and K′ bands. The J - K′ color is found to be significantly redder than that of a synthetic model galaxy with an age of 3 Gyr, the age of the universe at the quasar redshift.

We report on the results of optical and near-infrared photometry of the cluster of galaxies A 851 (= CL 0939+4713) carried out as one of the first-light observations of the 8.2 m Subaru telescope on Mauna Kea. Images as sharp as 0.″3 FWHM in K′ and 0.″45 in R were obtained in these observations. The 3 σ limiting magnitudes for points sources in 0.″3 seeing with a 0.″'6 software aperture were shown to go down to R = 28.1, J = 25.1. and K′ = 24.0, respectively, for about 1 hour integration. Subaru photometry of the galaxies in this cluster has shown in its color-magnitude diagrams a well-defined sequence of the early-type galaxy population that is consistent with the track predicted for a single-burst passive-evolution model of galaxies at z = 0.4. We attempted a morphological classification of galaxies in the Subaru R-band image using the Cin method, and found that discrimination between ellipticals and spirals can be achieved fairly consistently with the types assigned by MORPHs on the HST F702W image down to R &lt 23. A weak lensing analysis made on the Subaru R-band image yielded a reconstructed surface mass-density distribution that shows a significant maximum corresponding to the peak of the smoothed luminosity distribution of cluster galaxies. We found no significant excess of faint and/or small galaxies in the putative cluster area around the z = 2 quasar and no difference in the colors for galaxies in this area from those in the main cluster either. Two extremely red objects (ERO) with R - K &gt 6 are newly identified in the field. The colors of the reddest, disk-shaped galaxy R1 are found compatible with those of an unreddened E/S0 galaxy at z = 1.6.

High-resolution near-infrared J-band imaging with the Subaru Telescope has demonstrated, for the first time from the ground, two independent jets emanating from the protostar L 1551 IRS 5. Successive near-infrared spectroscopy has revealed that the jet emission is dominated by [Fe II] lines in the J- and H-bands. While the visual-extinction estimated from the [Fe II] line ratios reaches more than 20 mag in the close vicinity of IRS 5, it decreases rapidly at ∼1″ from IRS 5 and remains constant around 7 mag at larger distances. The electron density in the jets is estimated to be 106 cm-3 near IRS 5 and is 104 to 105 cm-3 in their outer section. The twisted appearance of the jets, with bright knots especially prominent in the northern jet, is similar to the R-band jets observed with the Hubble Space Telescope. These results suggest that the twisted structure and bright emission knots are intrinsic to the jets and are not due to a spatial variation of the extinction.

We present results of imaging polarimetry of the H2 υ = 1-0 S(1) line towards the outflow lohes of DR21. The polarization vectors are produced by dichroic absorption in a medium of aligned grains and therefore delineate the direction of the magnetic field, projected on to the plane of the sky, aligning those grains. It is argued that the polarization is dominated by grains within the DR21 cloud. The magnetic field strength, estimated from the dispersion of position angles of the polarization vectors, is ∼0.07 mG, which is not sufficient to play a significant role in either halting the outflow or confining its lateral expansion. The polarimetry of the H2 υ = 1-0 S(1) line suggests that the magnetic field is parallel to the outflow axis for the eastern outflow lobe, whereas it appears twisted perpendicular to this axis for the western lobe. This peculiar morphology for the magnetic field may imply the presence of either a two-component magnetic field along the line of sight or a complex, wound-up magnetic field.

We have carried out a near-infrared search for companions around 23 very low luminosity young stellar objects (YSOs) in the Taurus molecular cloud. After sophisticated photometry and analysis, five extremely low luminosity YSO (ELL-YSO) candidate companions were identified by both their near-infrared colors and proximity to the primary (separation less than 6″). They show infrared excess, as do ordinary YSOs. Their absolute J-band magnitudes range from 9 to 12 mag. The masses of these companions are estimated from their J-band luminosities, which use recent evolutionary tracks for very low mass objects. It is found that all are young brown dwarf candidates. The Ks-band magnitude difference between the ELL-YSO candidate companions and their primaries ranges from 2 to 6 mag, significantly larger than observed in T Tauri binaries. It is suggested that the companions are formed by the fragmentation of a disk around the primary. The binary frequency is 22-9-10% for systems with a period ranging between 107-108 days. This frequency is consistent with that of T Tauri stars, but it is significantly higher than that of low-mass main-sequence stars.

Infrared imaging observations have detected a dozen faint young stellar objects (YSOs) in the Taurus and Chamaeleon molecular clouds whose near- infrared colors are similar to those of classical T Tauri stars (TTS). They are around four magnitudes fainter than low-luminosity YSOs in Taurus detected in earlier surveys and as much as eight magnitudes fainter than typical TTS. The extreme faintness of the objects and their lower luminosity relative to previously identified brown dwarfs in the Pleiades indicate that these faint YSOs are very young brown dwarfs on the order of 1 million years old.

CLAO is a stellar Coronagraph Imager with Adaptive Optics for use on the Subaru 8.2-m telescope, whose aim is to obtain diffraction limited images of faint objects in close vicinity of bright objects at infrared wavelengths. The instrument is a near-infrared camera with a sophisticated coronagraph capability optimized for use between 1 and 5 micron and is used in conjunction with the adaptive optics system. A number of occulting masks and pupil masks with various sizes and shapes are selectable, all cooled down to below 80 K. Care is taken to minimize the diffraction and scattering effects both in the telescope and in the instrument. Great care is also taken to design the tension-strap supported cryostat which minimizes the flexure within the cryostat. The camera will utilize an SBRC 1024 X 1024 InSb detector array. Facilities are also provided for polarimetry and grism spectroscopy. CLAO will be a very powerful instrument to study circumstellar disks around both young stellar objects and main-sequence stars, companion brown dwarfs and other kinds of companions, extra-solar planets, jets and outflows from both young stars and evolved stars, circumnuclear regions around AGNs, and host galaxies of QSOs.

An infrared camera optimized for stellar coronagraphic observations is now under construction for use on the Subaru 8 m telescope. This instrument, called CIAO (Coronagraphic Imager with Adaptive Optics), aims to observe faint infrared objects in the very close vicinity of a bright source, such as brown dwarfs in binary systems and protoplanetary disks. We have conducted extensive computer simulations in order to optimize the transmittance patterns of the stellar coronagraph components and to evaluate the performance of the camera for various astronomical objects. The simulations took into account atmospheric turbulence and telescope aberration, where the wavefront is expanded in terms of Zernike polynomials. We examined the effect of the truncation of the Zernike expansion to the properties of the resultant PSFs, and found that the higher terms of the Zernike expansion omitted by former studies should be included rather in the simulation for coronagraphic observations than in the simulation for direct imaging observations. The results of the simulations show that a few minute integrations will be required for the detection of a young brown dwarf in a binary system in the Pleiades, while it will take a few nights for a protoplanetary disk in the Taurus molecular cloud.

We have identified the exciting source of the Herbig-Haro objects HH 135 and 136 by near-infrared polarimetric imaging of the associated extensive reflection nebula. The source is situated at the middle of the two Herbig-Haro objects, coincident with a luminous IRAS source, and it is responsible for the excitation of both objects, although it has been suggested that there are two independent exciting sources. The source appears to be a young stellar object (presumably a Herbig Ae/Be star) surrounded by a thick circumstellar disc and it is one of the most luminous (L ≃ 104 L⊙) exciting sources of Herbig-Haro objects. ©1997 RAS.

We have mapped CO J=1-0 outflows from two low-mass protostar candidates in Taurus, IRAS 04365+2535 (TMC1-A) and IRAS 04368+2557 (L1527), using the Nobeyama Millimeter Array. The outflow structure is compared with deep near-infrared images obtained by IR array detectors. The molecular outflow from IRAS 04365+2535 (TMC1-A) shows a clear bipolar structure whose blueshifted lobe extends to the north-west and redshifted one to the south-east. The extension of the blueshifted lobe is consistent with that of the cometary infrared reflection nebula, suggesting the presence of a large (∼2500 AU) tilted dust disk or elongated envelope which obscures the other side of the nebula, i.e., the redshifted lobe. The outflow associated with IRAS 04368+2557 (L1527) appears to be almost perfectly in the plane of the sky with blueshifted and redshifted lobes significantly overlapped with each other. Both lobes show clear bow-tie shaped, shell-like structure whose geometrical center is coincident with a millimeter continuum point-like source. Deep near-infrared images have revealed that a faint bipolar infrared nebulosity is associated with the "edge-on" molecular outflow. The central source, totally invisible even at near-infrared wavelengths, might be heavily obscured by a disk or an elongated envelope whose size is comparable to the IR nebula (∼10 000 AU). These two IRAS sources show large difference in their near-infrared appearance even though they have similar physical properties including bolometric luminosity, association of CO outflows, and submillimeter/millimeter flux. This difference might be due to the inclination effect: the central part of IRAS 04368+2557 is more heavily obscured (Aν∼70 mag) along the line of sight than that of IRAS 04365+2535 by the envelope of a disk-like structure with almost edge-on geometry. The difference of the appearance of CO outflow between these two sources is consistent with this interpretation. Although IRAS 04368+2557 is regarded as a Class O source, our observations suggest that this source could be similar to IRAS 04365+2535, a Class 1 source, in nature, except for the geometry of the circumstellar envelope. © 1996 American Astronomical Society.