We present new 5.2-14.5 mu m low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0-4.1 mu m spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (similar to 0.6-2.5 mu m), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing et al. We compare these spectra to those generated from the model atmospheres of Saumon & Marley. The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381-0158521. The broad wavelength coverage allows us to constrain almost independently the four parameters used to describe these photospheres in our models: effective temperature (T(eff)), surface gravity, grain sedimentation efficiency (f(sed)), and vertical gas transport efficiency (K(zz)). The CH(4) bands centered at 2.2, 3.3, and 7.65 mu m and the CO band at 2.3 mu m are sensitive to K(zz), and indicates that chemical mixing is important in all L and T dwarf atmospheres. The sample of L3.5 to T5.5 dwarfs spans the range 1800 K greater than or similar to T(eff) greater than or similar to 1000 K, with an L-T transition (spectral types L7 to T4) that lies between 1400 and 1100 K for dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K warmer or cooler, respectively, when using infrared spectral types. When using optical spectral types, the bluer dwarfs have more typical T(eff) values as they tend to have earlier optical spectral types. In this model analysis, f(sed) increases rapidly between types T0 and T4, indicating that increased sedimentation can explain the rapid disappearance of clouds at this stage of brown dwarf evolution. There is a suggestion that the transition to dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.

We present Hubble Space Telescope observations of six binary transneptunian systems: 2000 QL(251), 2003 TJ(58), 2001 XR(254), 1999 OJ(4), (134860) 2000 OJ(67), and 2004 PB(108). The Mutual orbits of these systems are found to have periods ranging from 22 to 137 days, semmajor axes ranging from 2360 to 10500 km, and eccentricities ranging from 0.09 to 0.55. These orbital parameters enable estimation of system masses ringing from 0.2 to 9.7 x 10(18) kg. For reasonable assumptions of bulk density (0.5 to 2,0 gcm (3)), the masses can be combined with visible photometry to constrain sizes and albedos. The resulting albedos are consistent with an emerging picture of the dynamically "Cold" Classical sub-population having relatively high albedos, compared with comparably-sized objects on more dynamically excited orbits. (C) 2008 Elsevier Inc. All rights reserved.

We report resolved photometry of the primary and secondary components of 23 transneptunian binaries obtained with the Hubble Space Telescope. V-I colors of the components range from 0.7 to 1.5 with a median uncertainty of 0.06 magnitudes. The colors of the primaries and secondaries are correlated with a Spearman rank correlation probability of 99.99991%, 5 sigma for a normal distribution. Fits to the primary vs. secondary colors are identical to within measurement uncertainties. The color range of binaries as a group is indistinguishable from that of the larger population of apparently single transneptunian objects. Whatever mechanism produced the colors of apparently single TNOs acted equally on binary systems. The most likely explanation is that the colors of transneptunian objects and binaries alike are primordial and indicative of their origin in a locally homogeneous, globally heterogeneous protoplanetary disk. Published by Elsevier Inc.

We fit theoretical model atmospheres to the spectral energy distribution of 21 L and T dwarfs recently observed with the Spitzer Space Telescope to identify and isolate four key physical parameters used in the model characterization of their atmospheres. The wide range of wavelengths observed ( ∼0.6 to 14.5 μm ) lets us constrain almost independently the four model parameters used to describe these photospheres: effective temperature (Teff), grain sedimentation (fsed), vertical gas transport efficiency (Kzz), and gravity. We find that the ratio of the mid‐infrared to near‐infrared flux is a good indicator of Teff, while the slope in the near‐infrared is strongly dependent on fsed. The CH4 bands found at 2, 3 and 8 μm are sensitive to the timescale for vertical mixing, and gravity will influence the flux at 2 μm.

We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon-Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 +/- 0.006 days and a semimajor axis of 1628 +/- 29 km, implying a system mass of (9.49 +/- 0.52) x 10(17) kg. The eccentricity of the orbit is 0.526 +/- 0.015. Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76(-16)(+14) and 42(-9)(+8) km, respectively. These numbers give an average bulk density of only 0.44(-0.17)(+0.44) g cm(-3), consistent with very low bulk densities recently reported for two other small transneptunian binaries. (C) 2008 Elsevier Inc. All rights reserved.

Context. L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Indeed, a number of ultra-cool dwarfs have been reported to vary. Additional time-resolved spectral observations of brown dwarfs offer the opportunity for further constraining and characterising atmospheric variability. Aims. It has been proposed that growth of heterogeneities in the global cloud deck may account for the L-to T-type transition when brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs to test this hypothesis. Methods. We obtained spectroscopic time series using the near-infrared spectrographs ISAAC on VLT-ANTU, over 0.99-1.13 mu m, and SpeX on the Infrared Telescope Facility for two of our targets in the J, H, and K bands. We searched for statistically variable lines and for a correlation between those. Results. High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence of large-amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, depending on the targets and wavelengths, on the time scale of a few hours. In contrast to the rest of the sample, the T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations. Conclusions. Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud-cover disruption should be smaller than 5-8% of the disk area for four of our targets, using simplistic heterogeneous atmospheric models. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud-breaking hypothesis.