Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey

We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7, and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early- to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1 types, and a similar spread in H-K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects.Comment: accepted by AJ, 18 pages, 10 figures, 5 tables, emulateapj layou

Sub-Pixel Response Measurement of Near-Infrared Sensors

Wide-field survey instruments are used to efficiently observe large regions of the sky. To achieve the necessary field of view, and to provide a higher signal-to-noise ratio for faint sources, many modern instruments are undersampled. However, precision photometry with undersampled imagers requires a detailed understanding of the sensitivity variations on a scale much smaller than a pixel. To address this, a near-infrared spot projection system has been developed to precisely characterize near-infrared focal plane arrays and to study the effect of sub-pixel non uniformity on precision photometry. Measurements of large format near-infrared detectors demonstrate the power of this system for understanding sub-pixel response.Comment: 9 pages, 13 figures, submitted to PAS

Ground-Based Coronagraphy with High Order Adaptive Optics

We summarize the theory of coronagraphic optics, and identify a dimensionless fine-tuning parameter, F, which we use to describe the Lyot stop size in the natural units of the coronagraphic optical train and the observing wavelength. We then present simulations of coronagraphs matched to adaptive optics (AO) systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under various atmospheric conditions, and identify useful parameter ranges for AO coronagraphy on these telescopes. Our simulations employ a tapered, high-pass filter in spatial frequency space to mimic the action of adaptive wavefront correction. We test the validity of this representation of AO correction by comparing our simulations with recent K-band data from the 241-channel Palomar Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs

Physical and Spectral Characteristics of the T8 and Later-Type Dwarfs

We use new and published near-IR spectra, with synthetic spectra, to derive physical properties of three of the latest-type T dwarfs. A new R~1700 spectrum of the T7.5 dwarf HD 3651B, with existing data, allows a detailed comparison to the well-studied and very similar dwarf, Gl 570D. We find that HD 3651B has both higher gravity and metallicity than Gl 570D, with Teff=820-830K, log g= 5.4-5.5, [m/H]= +0.2 and Kzz=10^4cm^2/s. Its age is 8-12 Gyr and its implied mass is 60-70 M_Jup. We perform a similar analyis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that the two dwarfs have the same Teff as the reference dwarf, and similar gravities, but lower metallicities. The parameters are Teff=725-775K and [m/H]= -0.3; log g=5.3-5.45 for 2MASS J09393548-2448279 and log g=5.0-5.3 for 2MASS J11145133- 261823. The age and mass are ~10Gyr and 60M_Jup for 2MASS J09393548-2448279, and ~5 Gyr and 40M_Jup for 2MASS J11145133-261823. A serious limitation is the incompleteness of the line lists of CH4 and NH3 at lambda <1.7um. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y- and J-bands may be useful as indicators of the next cooler spectral type, and not features in the H- and K-bands as previously thought. However large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value, and inclusion of laboratory NH3 opacities predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to low temperatures around 400K, when water clouds condense in the atmosphere [abridged].Comment: 34 pages including 10 figures and two tables; accepted for publication in the Astrophysical Journa

Hubble space telescope advanced camera for surveys coronagraphic observations of the dust surrounding HD 100546

We present HST ACS coronagraphic observations of HD 100546, a B9.5 star, 103 pc away from the Sun, taken in the F435W, F606W, and F814W bands. Scattered light is detected up to 14" from the star. The observations are consistent with the presence of an extended flattened nebula with the same inclination as the inner disk. The well-known "spiral arms" are clearly observed and trail the rotating disk material. Weaker arms never before reported are also seen. The interarm space becomes brighter, but the structures become more neutral in color at longer wavelengths, which is not consistent with models that assume that they are due to the effects of a warped disk. Along the major disk axis, the colors of the scattered light relative to the star are Δ(F435W - F606W) ≈ 0.0-0.2 mag and Δ(F435W - F814W) ≈ 0.5-1 mag. To explain these colors, we explore the role of asymmetric scattering, reddening, and large minimum sizes on ISM-like grains. We conclude that each of these hypotheses by itself cannot explain the colors. The disk colors are similar to those derived for Kuiper Belt objects, suggesting that the same processes responsible for their colors may be at work here. We argue that we are observing only the geometrically thick, optically thin envelope of the disk, while the optically thick disk responsible for the far-IR emission is undetected. The observed spiral arms are then structures on this envelope. The colors indicate that the extended nebulosity is not a remnant of the infalling envelope but reprocessed disk material

Hubble and Spitzer Space Telescope Observations of the Debris Disk around the Nearby K Dwarf HD 92945

[ABRIDGED] We present the first resolved images of the debris disk around the nearby K dwarf HD 92945. Our F606W (V) and F814W (I) HST/ACS coronagraphic images reveal an inclined, axisymmetric disk consisting of an inner ring 2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface brightness declines slowly with increasing radius 3".0-5".1 (65-110 AU) from the star. A precipitous drop in the surface brightness beyond 110 AU suggests that the outer disk is truncated at that distance. The radial surface-density profile is peaked at both the inner ring and the outer edge of the disk. The dust in the outer disk scatters neutrally but isotropically, and it has a low V-band albedo of 0.1. We also present new Spitzer MIPS photometry and IRS spectra of HD 92945. These data reveal no infrared excess from the disk shortward of 30 micron and constrain the width of the 70 micron source to < 180 AU. Assuming the dust comprises compact grains of astronomical silicate with a surface-density profile described by our scattered-light model of the disk, we successfully model the 24-350 micron emission with a minimum grain size of a_min = 4.5 micron and a size distribution proportional to a^-3.7 throughout the disk, but with a maximum grain size of 900 micron in the inner ring and 50 micron in the outer disk. Our observations indicate a total dust mass of ~0.001 M_earth. However, they provide contradictory evidence of the dust's physical characteristics: its neutral V-I color and lack of 24 micron emission imply grains larger than a few microns, but its isotropic scattering and low albedo suggest a large population of submicron-sized grains. The dynamical causes of the disk's morphology are unclear, but recent models of dust creation and transport in the presence of migrating planets indicate an advanced state of planet formation around HD 92945.Comment: 29 pages, 10 figures; to be published in The Astronomical Journa