Yumiko Oasa

A Deep Near-Infrared Survey of the Chamaeleon I Dark Cloud Core

We have carried out a deep near-infrared imaging survey to search for low-mass young stellar objects (YSOs) in the densest star-forming core of the Chamaeleon I dark cloud. Our observations cover an area of 30 arcmin2, including an early B9 star (HD 97300) and an outflow source (HM 23). The 10 σ limiting magnitudes are 18.1, 17.0, and 16.2 mag at J, H, and K, respectively, which is sensitive enough to provide a census of the embedded stellar population down to substellar objects in the cloud. Source classification is performed based on the near-infrared (NIR) color-color diagram. Many of the YSO candidates with NIR excesses are more than 7 mag fainter than typical T Tauri stars in the same cloud. Some of them are even fainter than the known brown dwarfs in the Pleiades. The luminosities of newly identified YSO candidates and the recent evolutionary models for very low mass objects suggest that they appear to be substellar, if their typical age is assumed to be similar to that of classic T Tauri stars or, namely, 1 Myr with an upper limit of 10 Myr. Therefore it is highly likely that young brown dwarfs form in this molecular cloud core. The J-band luminosity function of the YSO candidates does not appear to turn over down to the completeness limit. In the Chamaeleon I dark cloud core, stars form in a clustered mode characterized by both a high star formation efficiency and high stellar density such as in the ρ Oph core.

Near-Infrared Imaging of the Circumstellar Disk around Herbig Ae Star HD 150193A*

We present 01 resolution near-infrared imaging of Herbig Ae star HD 150193 (~6 Myr) and its T Tauri companion with the stellar coronagraphic camera (Coronagraphic Imager with Adaptive Optics) on the Subaru 8.2 m telescope. The images obtained reveal a circumstellar disk around the primary star extending from the edge of the coronagraphic mask at 50 AU to about 190 AU (13). No circumstellar structure was detected around the companion, which was separated by 165 AU (110) from the primary. For the circumprimary disk, the lower limit of the ratio between the disk and the total flux is estimated to be 1.3 × 10-2. Asymmetries are seen in the surface brightness of this disk. Azimuthal asymmetry of the radial profile suggests that the companion has an effect on the distortion of the circumprimary disk. HD 150193 provides insight into the disk evolutionary process in binary systems.

Interferometric Observations of the T Tauri Stars in the MBM 12 Cloud

We have carried out a millimeter interferometric continuum survey toward seven young stellar objects (YSOs) in the MBM 12 cloud. Thermal emissions associated with two YSOs were detected above the 3 σ level at 2.1 mm, and one also showed a 1.3 mm thermal emission. Another object was marginally detected at 2.1 mm. Spectral energy distributions of the YSOs are well fitted by a simple power-law disk model. Masses of the circumstellar disks are estimated to be of the order of 0.05 M☉. The circumstellar disks in the MBM 12 cloud have properties in common with the disks in nearby star-forming regions, in terms of disk parameters, such as disk mass, as well as an infrared excess.

Near infrared coronagraph images of IRC +10216 - Faint structures at 1-5 from the central star

We present J, H and K band coronagraph images of the circumstellar envelope around IRC +10216 (CW Leo) obtained with a near infrared camera, CIAO and the 8.2 m Subaru telescope. A circular occulting mask of 2 �� in diameter was used to block out the light from the bright central object. The images show 2 collimated radial structures to the NNW and WNW, 2 fan-like structures to the S and NE, respectively, and 3 arc-like structures at a radius of 4 to 5 arcsec from the stellar center. We compare this intermediate size-scale structure to that seen on larger and smaller scales and find evidence for a deviation from spherically symmetric outflow beginning ∼150 years ago. Previous near infrared speckle imaging has revealed a complex clumpy structure on a scale of less than 200 mas, and it is likely that at least some of the radial features seen in our images could be due to shadowing by dust clumps close to the star.

Deep Near-Infrared Surveys and Young Brown Dwarf Populations in Star-Forming Regions

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.