Four nearby L dwarfs

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000\AA in the L0 dwarf 2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I 4227\AA and Na I 5890/5896\AA are extremely strong, with the latter having an equivalent width of 240\AA in the L4 dwarf. By spectral type L5, the D lines extend over $\sim1000$\AA and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband colour. The KI resonance doublet at 7665/7699\AA increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.Comment: to appear in AJ, January 2000; 27 pages, including 3 tables and 7 figures embedded in the tex

Primary Meningeal Rhabdomyosarcoma

Primary meningeal rhabdomyosarcoma is a rare primary brain malignancy, with scant case reports. While most reports of primary intracranial rhabdomyosarcoma occur in pediatric patients, a handful of cases in adult patients have been reported in the medical literature. We report the case of a 44-year-old male who developed primary meningeal rhabdomyosarcoma. After developing episodes of right lower extremity weakness, word finding difficulty, and headaches, a brain magnetic resonance imaging (MRI) demonstrated a vertex lesion with radiographic appearance of a meningeal-derived tumor. Subtotal surgical resection was performed due to sagittal sinus invasion and initial pathology was interpreted as an anaplastic meningioma. Re-review of pathology demonstrated rhabdomyosarcoma negative for alveolar translocation t(2;13). Staging studies revealed no evidence of disseminated disease. He was treated with stereotactic radiotherapy with concurrent temozolamide to be followed by vincristine, actinomycin-D, and cyclophosphamide (VAC) systemic therapy

Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition

We present the results of a Hubble Space Telescope NICMOS imaging survey of 22 T-type field brown dwarfs. Five are resolved as binary systems with angular separations of 0"05-0"35, and companionship is established on the basis of component F110W-F170M colors (indicative of CH4 absorption) and low probabilities of background contamination. Prior ground-based observations show 2MASS 1553+1532AB to be a common proper motion binary. The properties of these systems - low multiplicity fraction (11[+7][-3]% resolved, as corrected for sample selection baises), close projected separations (a = 1.8-5.0 AU) and near-unity mass ratios - are consistent with previous results for field brown dwarf binaries. Three of the binaries have components that span the poorly-understood transition between L dwarfs and T dwarfs. Spectral decomposition analysis of one of these, SDSS 1021-0304AB, reveals a peculiar flux reversal between its components, as its T5 secondary is ~30% brighter at 1.05 and 1.27 micron than its T1 primary. This system, 2MASS 0518-2828AB and SDSS 1534+1615AB all demonstrate that the J-band brightening observed between late-type L to mid-type T dwarfs is an intrinsic feature of this spectral transition, albeit less pronounced than previously surmised. We also find that the resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L and T dwarfs, an anomaly that can be explained by a relatively rapid evolution of brown dwarfs through the L/T transition, perhaps driven by dynamic (nonequilibrium) depletion of photospheric condensates.Comment: ~40 pages, 17 figures, accepted for publication to ApJ. Note that emulateapj style file cuts off part of Table

Crawford, Davis & the Right of Confrontation: Where Do We Go from Here?

Afternoon Panel Discussio

Physical tests for Random Numbers in Simulations

We propose three physical tests to measure correlations in random numbers used in Monte Carlo simulations. The first test uses autocorrelation times of certain physical quantities when the Ising model is simulated with the Wolff algorithm. The second test is based on random walks, and the third on blocks of n successive numbers. We apply the tests to show that recent errors in high precision simulations using generalized feedback shift register algorithms are due to short range correlations in random number sequences. We also determine the length of these correlations.Comment: 16 pages, Post Script file, HU-TFT-94-

Discovery of a Bright Field Methane (T-type) Brown Dwarf by 2MASS

We report the discovery of a bright (J = 13.83$\pm$0.03) methane brown dwarf, or T dwarf, by the Two Micron All Sky Survey. This object, 2MASSI J0559191-140448, is the first brown dwarf identified by the newly commissioned CorMASS instrument mounted on the Palomar 60-inch Telescope. Near-infrared spectra from 0.9 - 2.35 \micron show characteristic CH$_4$ bands at 1.1, 1.3, 1.6, and 2.2 \micron, which are significantly shallower than those seen in other T dwarfs discovered to date. Coupled with the detection of an FeH band at 0.9896 \micron and two sets of K I doublets at J-band, we propose that 2MASS J0559-14 is a warm T dwarf, close to the transition between L and T spectral classes. The brightness of this object makes it a good candidate for detailed investigation over a broad wavelength regime and at higher resolution.Comment: 21 pages, 3 figures, 2 tables, accepted to AJ for publication August 200

A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages

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 (Dec < 0 deg) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus. We use a homogeneously observed sample of roughly one hundred and fifty 6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-Resolution Imaging Spectrometer at the W. M. Keck Observatory to examine the strength of the 6708-A Li I line as a function of spectral type and further corroborate the trends noted by Kirkpatrick et al. (2000). We use our low-gravity spectra to investigate the strength of the Li I line 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 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 in Kirkpatrick et al. (2006) that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.Comment: 73 pages with 22 figures; to appear in ApJ (Dec 20, 2008, v689n2 issue