Photometric Variability at the L/T Dwarf Boundary
We have monitored the photometric variability of nine field L and T brown dwarfs for 10 nights during the course of 1 month. Observations were obtained in the K_s band with the Palomar 60 inch (1.5 m) telescope Near-Infrared Camera. Results of statistical analyses indicate that at least three of the nine targets show significant evidence for variability, and three more are possibly variable. Fractional deviations from the median flux vary from 5% to 25%. Two of the variable targets, 2MASS 0030-14 (L7) and SDSS 0151+12 (T1), have marginally significant peaks in their periodograms. The phased light curves show evidence for periodic behavior on timescales of 1.5 and 3.0 hr, respectively. No significant correlations between variability amplitude and spectral type or J-K_s color are found. While it is clear that variability exists in objects near the L/T dwarf boundary, we find no evidence that variability near the L/T boundary is more likely than it is for early L dwarfs.

title: The 2MASS Wide-Field T Dwarf Search. IV. Hunting Out T Dwarfs with Methane Imaging
abstract: We present first results from a major program of methane filter photometry for low-mass stars and brown dwarfs. The definition of a new methane filter photometric system is described. A recipe is provided for the differential calibration of methane imaging data using existing Two Micron All Sky Survey (2MASS) photometry. We show that these filters are effective in discriminating T dwarfs from other types of stars, and we demonstrate this with Anglo-Australian Telescope observations using the IRIS2 imager. Methane imaging data and proper motions are presented for 10 T dwarfs identified as part of the 2MASS Wide-Field T Dwarf Search, seven of them initially identified as T dwarfs using methane imaging. We also present near-infrared moderate-resolution spectra for five T dwarfs newly discovered by this technique. Spectral types obtained from these spectra are compared to those derived from both our methane filter observations and spectral types derived by other observers. Finally, we suggest a range of future programs to which these filters are clearly well suited: the winnowing of T dwarf and Y dwarf candidate objects coming from the next generation of near-infrared sky surveys, the robust detection of candidate planetary-mass brown dwarfs in clusters, the detection of T dwarf companions to known L and T dwarfs via deep methane imaging, and the search for rotationally modulated time-variable surface features on cool brown dwarfs.

title: A SAMPLE OF VERY YOUNG FIELD L DWARFS AND IMPLICATIONS FOR THE BROWN DWARF LITHIUM TEST AT EARLY AGES
abstract: Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6% ± 1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (decl: < 0°) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, β Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few x 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well-determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

title: Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with the Hubble Space Telescope Wide Field P
abstract: We present the discovery of two T dwarf binaries, 2MASS 1225-2739AB and 2MASS 1534-2952AB, identified in a sample of 10 T dwarfs imaged with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2. Companionship is established by the uniquely red F814W-F1042M colors of the binary components, caused by heavily pressure-broadened K I absorption centered at 7665 and 7699 A. The separations of the two binary systems are 0."282 ± 0."005 and 0."065 ± 0."007, implying projected separations of 3.17 ± 0.14 and 1.0 ± 0.3 AU, respectively. These close separations are similar to those found in previous brown dwarf binary searches and permit orbital mapping over the coming decade. 2MASS 1225-2739AB has a substantially fainter secondary, with ΔM_(F814W) = 1.59 ± 0.04 and ΔM_(F1042M) = 1.05 ± 0.03; this system is likely composed of a T6 primary and T8 secondary with mass ratio 0.7-0.8. The observed binary fraction of our HST sample, 20^(+17%)_(-1), is consistent with results obtained for late-type M and L field dwarfs and implies a bias-corrected binary fraction of 9^(+15%)_(-4) for α ≳ 1 AU and q ≳ 0.4, significantly lower than the binary fractions of F-G and early-type M dwarf stars. Neither of the T binaries have separations α ≳ 10 AU, consistent with results from other brown dwarf binary searches. Using the statistical models of Weinberg, Shapiro, & Wasserman, we conclude that tidal disruption by passing stars or giant molecular clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages ≾ 1-10 Myr) or a formation mechanism that precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M^2_(total), a possible clue to the physical mechanism that restricts wide substellar systems.

title: NIMBUS: the Near-infrared Multi-Band Ultraprecise Spectroimager for SOFIA
abstract: We present a new and innovative near-infrared multi-band ultraprecise spectroimager (NIMBUS) for SOFIA. This design is capable of characterizing a large sample of extrasolar planet atmospheres by measuring elemental and molecular abundances during primary transit and occultation. This wide-field spectroimager would also provide new insights into Trans-Neptunian Objects (TNO), Solar System occultations, brown dwarf atmospheres, carbon chemistry in globular clusters, chemical gradients in nearby galaxies, and galaxy photometric redshifts. NIMBUS would be the premier ultraprecise spectroimager by taking advantage of the SOFIA observatory and state of the art infrared technologies. This optical design splits the beam into eight separate spectral bandpasses, centered around key molecular bands from 1 to 4μm. Each spectral channel has a wide field of view for simultaneous observations of a reference star that can decorrelate time-variable atmospheric and optical assembly effects, allowing the instrument to achieve ultraprecise calibration for imaging and photometry for a wide variety of astrophysical sources. NIMBUS produces the same data products as a low-resolution integral field spectrograph over a large spectral bandpass, but this design obviates many of the problems that preclude high-precision measurements with traditional slit and integral field spectrographs. This instrument concept is currently not funded for development.

title: L Dwarfs and the Substellar Mass Function
abstract: Analysis of initial observations sky surveys has shown that the resulting photometric catalogs, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between ≈2000 ± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10-3.5 and 10. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the solar neighborhood population and constrain Ψ(M) by comparing the predicted L dwarf surface densities and temperature distributions against observations from the Deep Near-Infrared Survey (DENIS) and 2 Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can be represented by a power-law mass function, Ψ(M) ∝ M-α, with 1 M/M☉ > 0.01 brown dwarfs is 0.10 systems pc-3. In that case, brown dwarfs are twice as common as main-sequence stars but contribute no more than ~15% of the total mass of the disk.

title: Large-Scale Structure Shocks at Low and High Redshifts
abstract: Cosmological simulations show that, at the present time, a substantial fraction of the gas in the intergalactic medium (IGM) has been shock heated to T 105 K. Here we develop an analytic model to describe the fraction of shocked, moderately overdense gas in the IGM. The model is an extension of the Press & Schechter description for the mass function of halos: we assume that large-scale structure shocks occur at a fixed overdensity during nonlinear collapse. This in turn allows us to compute the fraction of gas at a given redshift that has been shock heated to a specified temperature. We show that if strong shocks occur at turnaround, our model provides a reasonable description of the temperature distribution seen in cosmological simulations at z ~ 0, although it does overestimate the importance of weak shocks. We then apply our model to shocks at high redshifts. We show that before reionization, the thermal energy of the IGM is dominated by large-scale structure shocks (rather than virialized objects). These shocks can have a variety of effects, including stripping 10% of the gas from dark matter minihalos, accelerating cosmic rays, and creating a diffuse radiation background from inverse Compton and cooling radiation. This radiation background develops before the first stars form and could have measurable effects on molecular hydrogen formation and the spin temperature of the 21 cm transition of neutral hydrogen. Finally, we show that shock heating will also be directly detectable by redshifted 21 cm measurements of the neutral IGM in the young universe.