We present a near-infrared image of the Herbig Ae star AB Aur obtained with the Coronagraphic Imager with Adaptive Optics mounted on the Subaru Telescope. The image shows a circumstellar emission extending out to a radius of AU, with a double spiral structure detected at AU. The surface brightness r p 580 r p 200-450 decreases as , steeper than the radial profile of the optical emission possibly affected by the scattered Ϫ3.0‫1.0ע‬ r light from the envelope surrounding AB Aur. This result, together with the size of the infrared emission similar to that of the 13 CO ( ) disk, suggests that the spiral structure is indeed associated with the circumstellar J p 1-0 disk but is not part of the extended envelope. We identified four major spiral arms, which are trailing if the brighter southeastern part of the disk is the near side. The weak gravitational instability, maintained for millions of years by continuous mass supply from the envelope, might explain the presence of the spiral structure at the relatively late phase of the pre-main-sequence period.

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We present the detection of a young brown dwarf companion DH Tau B associated with the classical T Tauri star DH Tau. Near-infrared coronagraphic observations with CIAO on the Subaru Telescope have revealed DH Tau B with H = \~15 mag located at 2.3" (330 AU) away from the primary DH Tau A. Comparing its position with a Hubble Space Telescope archive image, we confirmed that DH Tau A and B share the common proper motion, suggesting that they are physically associated with each other. The near-infrared color of DH Tau B is consistent with those of young stellar objects. The near-infrared spectra of DH Tau B show deep water absorption bands, a strong K I absorption line, and a moderate Na I absorption line. We derived its effective temperature and surface gravity of Teff = 2700 -- 2800 K and log g = 4.0--4.5, respectively, by comparing the observed spectra with synthesized spectra of low-mass objects. The location of DH Tau B on the HR diagram gives its mass of 30 -- 50 M_Jupiter.Comment: 10 pages, 14 figures, 1 table, accepted for publication in Ap

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 outÑow source (HM 23). The 10 p 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 classiÐcation 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 identiÐed 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 o Oph core.