Miki Ishii

76 T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density

We report the discovery of 76 new T dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). Near-infrared broad- and narrow-band photometry and spectroscopy are presented for the new objects, along with Wide-field Infrared Survey Explorer (WISE) and warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are presented from a new two epoch LAS proper motion catalogue. We use these motions to identify two new benchmark systems: LHS 6176AB, a T8p+M4 pair and HD 118865AB, a T5.5+F8 pair. Using age constraints from the primaries and evolutionary models to constrain the radii, we have estimated their physical properties from their bolometric luminosity. We compare the colours and properties of known benchmark T dwarfs to the latest model atmospheres and draw two principal conclusions. First, it appears that the H - [4.5] and J - W2 colours are more sensitive to metallicity than has previously been recognized, such that differences in metallicity may dominate over differences in T-eff when considering relative properties of cool objects using these colours. Secondly, the previously noted apparent dominance of young objects in the late-T dwarf sample is no longer apparent when using the new model grids and the expanded sample of late-T dwarfs and benchmarks. This is supported by the apparently similar distribution of late-T dwarfs and earlier type T dwarfs on reduced proper motion diagrams that we present. Finally, we present updated space densities for the late-T dwarfs, and compare our values to simulation predictions and those from WISE.

Near-Infrared Polarimetric Study of Monoceros R2 IRS

We present H- and Ks-band imaging polarimetry for the star-forming region Mon R2 IRS, and spectropolarimetry from 1.2 to 4.2 μm for the infrared sources IRS 2 and IRS 3. The nebular complex in Mon R2 IRS is revealed to be composed of four domains dominated by the infrared sources IRS 1, IRS 2, IRS 3, and IRS 6. IRS 2 is the predominant illuminating source in the region and dominates the emission of the IR ring and most of the western part of the nebulosity. IRS 1 is responsible for the enhanced unpolarized intensity in the southeast part of the IR ring and inside the IR ring. IRS 3 illuminates the bright eastern nebula, and together with IRS 2 illuminates the eastern region inside the IR ring. IRS 6 illuminates a small nebula to the south of the IR ring. An arc structure of polarized intensity is observed abutting the northwest IR ring, consistent with the outflow cavity structure inferred from millimeter-wave observations. IRS 3 is associated with a disklike condensation oriented southeast-northwest, perpendicular to the elongated eastern nebula. The magnetic field in the core region exhibits an hourglass structure oriented north-south.

A circumstellar disk associated with a massive protostellar object

The formation process for stars with masses several times that of the Sun is still unclear. The two main theories are mergers of several low-mass young stellar objects, which requires a high stellar density, or mass accretion from circumstellar disks in the same way as low-mass stars are formed, accompanied by outflows during the process of gravitational infall. Although a number of disks have been discovered around low- and intermediate-mass young stellar objects, the presence of disks around massive young stellar objects is still uncertain and the mass of the disk system detected around one such object, M17, is disputed. Here we report near-infrared imaging polarimetry that reveals an outflow/disk system around the Becklin–Neugebauer protostellar object, which has a mass of at least seven solar masses (M[circdot]). This strongly supports the theory that stars with masses of at least 7M[circdot] form in the same way as lower mass stars.

Unveiling Deeply Embedded Sources by Near-Infrared Polarimetric Imaging

Near-infrared polarimetric images are presented for six molecular outflow sources: IRAS 20050+2720, IRAS 20126+4104, IRAS 20188+3928, S233, AFGL 5180, and AFGL 6366S. All the regions are found to exhibit reflection nebulae and to be associated with massive and clustered star formation. By inspecting polarimetric patterns in the nebulae, we have identified six deeply embedded sources (DESs) which illuminate circumstellar nebulosity but are not detectable in wavelengths shorter than 2 μm. While the DES in IRAS 20050 coincides with an infrared source in a previous, longer wavelength observation and the one in IRAS 20126 with a hot molecular core, the nature of the other newly discovered DESs is not known. From the compilation of the observations of DESs over a large wavelength range, we suspect that the DESs possess characteristics similar to hot molecular cores and are likely to be in the pre-ultracompact H II region phase of massive star formation.

K-Band Spectroscopy of Luminous Young Stellar Objects

We present spectroscopy from 2.0 to 2.33 mum of 32 luminous young stellar objects (YSOs), which are presumed to be precursors of Herbig Ae/Be stars. From these stars, Br gamma, H(2), CO, He I, and Fe II were found in emission with detection rates of 97%, 34%, 22%, 9%, and 3%, respectively. We compare the spectral features with those of Herbig Ae/Be stars in the literature to investigate the spectral behavior of intermediate- to high-mass YSOs and to search for their relations to the spectral energy distributions (SEDs). H(2) emission is detected only in Class I SEDs with particularly large spectral indices. The detection of H(2) emission is related to the degree of the dispersal of circumstellar envelopes, where H(2) molecules are probably excited by shocks from outflows. On the other hand, Br gamma emission, which is generally thought to occur in stellar winds close to the stars, does not depend on the SEDs. This indicates that stellar wind from luminous YSOs does not change much from the embedded phase to the optically visible phase. CO emission is also independent of the SEDs, but the detection rate is much lower than that of Br gamma emission. Probably, more specific physical conditions regarding circumstellar disks and stellar radiation are necessary for CO emission to take place.

A Survey of Dust Features in the 3 Micron Spectra of YSO Candidates

We present the results of spectrophotometry from 1.3 to 4.2 μm of 45 IRAS sources in the list of Campbell, Persson, & Matthews that have been presumed to be luminous young stellar objects because of their infrared colors and large IRAS fluxes. About 70% of the objects show 3.1 μm H2O ice absorption or 3.3 μm unidentified infrared emission, or both. Among those with 3.1 μm H2O ice absorption, three objects seem to have a distinct absorption near 3.4 μm, and their absorption profiles are different than the optical depth profiles of young stellar objects in the literature. In the HKL two-color diagram, objects with ice absorption occupy the same region as previously known young stellar objects with the ice feature, and those with unidentified infrared emission overlap with Herbig Ae/Be stars. We interpret the distribution in the HKL two-color diagram as indicating the evolutionary sequence of intermediate- to high-mass young stars.

Near-Infrared Polarimetric Study of S140 IRS

We present H- and Ks-band imaging polarimetry for the star-forming region S140 IRS and spectropolarimetry from 1.26 to 4.18 μm for the infrared source IRS 1. The polarimetric images reveal two reflection nebulae illuminated by the deeply embedded infrared sources IRS 1 and IRS 3. The nebula associated with IRS 1 does not simply show a symmetric structure, as expected in the well-known pole-on outflow model, but displays three reflection lobes. Two bipolar reflection lobes are apparently extended in the northeast-southwest direction and lie nearly in the plane of the sky. Another pole-on lobe, corresponding to the blueshifted pole-on molecular outflow, is projected to the southeast of IRS 1. The polarimetric images also resolve an asymmetric density distribution around IRS 1; to the west there is a dense disk structure elongated in the east-west direction; whereas to the east there is a cavity. On the basis of these features, as well as previous radio continuum and CO molecular line observations, we propose that S140 IRS 1 may drive a quadrupolar outflow.

The Structure of Pre-transitional Protoplanetary Disks. II. Azimuthal Asymmetries, Different Radial Distributions of Large and Small Dust Grains in PDS 70

The formation scenario of a gapped disk, i.e., transitional disk, and its asymmetry is still under debate. Proposed scenarios such as disk-planet interaction, photoevaporation, grain growth, anticyclonic vortex, eccentricity, and their combinations would result in different radial distributions of the gas and the small (sub-µm size) and large (millimeter size) dust grains as well as asymmetric structures in a disk. Optical/near-infrared (NIR) imaging observations and (sub-)millimeter interferometry can trace small and large dust grains, respectively; therefore multi-wavelength observations could help elucidate the origin of complicated structures of a disk. Here we report SMA observations of the dust continuum at 1.3 mm and 12 CO J = 2 → 1 line emission of the pre-transitional protoplanetary disk around the solar-mass star PDS 70. PDS 70, a weak-lined T Tauri star, exhibits a gap in the scattered light from its disk with a radius of ∼65 AU at NIR wavelengths. However, we found a larger gap in the disk with a radius of ∼80 AU at 1.3 mm. Emission from all three disk components (the gas and the small and large dust grains) in images exhibits a deficit in brightness in the central region of the disk, in particular, the dust-disk in small and large dust grains has asymmetric brightness. The contrast ratio of the flux density in the dust continuum between the peak position to the opposite side of the disk reaches 1.4. We suggest the asymmetries and different gap-radii of the disk around PDS 70 are potentially formed by several (unseen) accreting planets inducing dust filtration. Subject headings: planetary systems — protoplanetary disks — stars: individual (PDS 70) — stars: pre-main sequence — submillimeter: general — polarization

A Survey of Near Infrared Nebulosities around Luminous Young Stellar Objects: J, H, and K Imaging

We present J, H, and K images of 45 IRAS sources in the recent list by Campbell, Persson, & Matthews, which have been presumed to be luminous young stellar objects (YSOs) because of their infrared colors and their large IRAS fluxes. These objects have been classified into four groups according to two dust features in the 3 mum band the H(2)O ice absorption and the unidentified infrared band (UIB) emission. Our near-infrared images reveal that most of them (42/45) have nebulosity in at least one of the three bands. By examining the nebulosities around these YSOs, we find that (1) nebulae around the objects with the ice absorption can be explained by the scattered light of the central stars, (2) nebulae around the objects with the UIB emission cannot be explained by the scattered light alone additional emission, most likely from very small grains, is necessary to explain the nebular colors and the surface brightness, and (3) the objects with neither the ice absorption nor the UIB emission tend to have faint or no nebulosity. We interpret the variation of nebular brightness and the color as evolutionary phases of circumstellar matter of intermediate- to high-mass YSOs.

3.4 MICRON FEATURE ON THE SHOULDER OF ICE-BAND ABSORPTIONS IN THREE LUMINOUS YOUNG STELLAR OBJECTS: IRAS 18511+0146, IRAS 21413+5442, AND IRAS 04579+4703

An absorption feature at 3.4 μm has been detected in the long-wavelength wing of the 3.1 μm H2O ice feature in three young stellar objects: IRAS 18511+0146, IRAS 21413+5442, and IRAS 04579+4703. The profile of the 3.4 μm absorption in IRAS 18511+0146 has distinct subfeatures at 3.38, 3.41, and 3.48 μm and is not similar to those found in molecular clouds but is similar to those in the diffuse interstellar medium, such as the lines of sight toward the Galactic center and Cyg OB2 12. Spectropolarimetry of the 3.4 μm feature in IRAS 18511+0146 shows no excess polarization accompanying the feature, which is consistent with the observation toward the Galactic center source IRS 7 by Adamson et al.The 3.4 μm absorption in IRAS 18511+0146 probably occurs in the diffuse interstellar medium intervening in the line of sight to the molecular cloud where the H2O ice absorption occurs. Furthermore, the 3.4 μm absorption carrier seems to reside in a population of nonpolarizing grains in the diffuse interstellar medium, physically separate from other polarizing grains.