We present near-infrared long slit and multi-slit spectra of low mass brown dwarf candidates in the Orion Nebula Cluster. The long slit data were observed in the H-& K-bands using NIRI on the Gemini North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo Australian Telescope at H-band. We develop a spectral typing scheme based on optically calibrated, near infrared spectra of young sources in the Taurus and IC 348 star forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on H-R diagrams overlaid with theoretical premain-sequence isochrones. The majority of our objects lie close to or above the 1 Myr isochrone, leading to an average cluster age that is <1 Myr. We find 38 sources lie at or below the hydrogen burning limit (0.075 M ⊙ ). 10 sources potentially have masses below the deuterium burning limit (0.012 M ⊙ ). We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions. The lowest χ 2 values are produced by an age distribution centred at 1 Myr, with a mass function that declines at sub-stellar masses according to an M α power law in the range α=0.3 to 0.6. We find that truncating the mass function at 0.012 M ⊙ produces luminosity functions that are starved of the faintest magnitudes, even when using bimodal age populations that contain 10 Myr old sources. The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC.

The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright Royal Astronomical SocietyWe have used blue near-infrared colours to select a group of 12 spectroscopically confirmed United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) T dwarfs later than T4. From amongst these, we identify the first two kinematic halo T-dwarf candidates. Blue near-infrared colours have been attributed to collisionally induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming V-rad = 0. From this, ULAS J0926+0835 is found to have U = 62 km s(-1) and V = -140 km s(-1), and ULAS J1319+1209 is found to have U = 192 km s(-1) and V = -92 km s(-1). These values are consistent with potential halo membership. However, these are not the bluest objects in our selection. The bluest is ULAS J1233+1219, with J - K = -1.16 +/- 0.07, and surprisingly this object is found to have young disc-like U and V. Our sample also contains Hip 73786B, companion to the metal-poor K5 dwarf Hip 73786. Hip 73786 is a metal-poor star, with [Fe/H] = -0.3 +/- 0.1 and is located at a distance of 19 +/- 0.7 pc. U, V, W space velocity components are calculated for Hip 73786A and B, finding that U = -48 +/- 7 km s(-1), V = -75 +/- 4 km s(-1) and W = -44 +/- 8 km s(-1). From the properties of the primary, Hip 73786B is found to be at least 1.6-Gyr old. As a metal-poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks

The definitive version is available at www.blackwell-synergy.com Copyright Blackwell Publishing DOI : 10.1111/j.1745-3933.2006.00244.

International audienceWe report the discovery of 21 hitherto unknown bright southern ultracool dwarfs with spectral types in the range M7 to L5.5, together with new observations of a further three late M dwarfs previously confirmed. Three more objects are already identified in the literature as high proper motion stars; we derive their spectral types for the first time. All objects were selected from the Two Micron All-Sky Survey (2MASS) all-sky and SuperCOSMOS point source data bases on the basis of their optical/near-infrared colours, J-band magnitudes and proper motions. Low-resolution (R ~ 1000) JH spectroscopy with the European Southern Observatory (ESO)/New Technology Telescope (NTT) Son of Isaac (SOFI) spectrograph has confirmed the ultracool nature of 24 targets, out of a total of 25 candidates observed. Spectral types are derived by direct comparison with template objects and compared to results from H2O and FeH indices. We also report the discovery of one binary, as revealed by SOFI acquisition imaging; spectra were taken for both components. The spectral types of the two components are L2 and L4 and the distance ~19 pc. Spectroscopic distances and transverse velocities are derived for the sample. Two ~L5 objects lie only at ~0 pc distant. Such nearby objects are excellent targets for further study to derive their parallaxes and to search for fainter, later companions with adaptive optics and/or methane imaging. Based on observations made at the European Southern Observatory, Chile (New Technology Telescope/SOFI programs 076C.0382, 076D.0872 and 077C.0117) and on data from the Two Micron All-Sky Survey project (University of Massachusetts and IPAC/Caltech, USA). E-mail: trk@star.herts.ac.u

The optical-infrared afterglow of the LAT-detected long duration burst, GRB 090902B, has been observed by several instruments. The earliest detection by ROTSE-IIIa occurred 80 minutes after detection by the GBM instrument onboard the Fermi Gamma-Ray Space Telescope, revealing a bright afterglow and a decay slope suggestive of a reverse shock origin. Subsequent optical-IR observations followed the light curve for 6.5 days. The temporal and spectral behavior at optical-infrared frequencies is consistent with synchrotron fireball model predictions; the cooling break lies between optical and XRT frequencies ∼ 1.9 days after the burst. The inferred electron energy index is p = 1.8 ± 0.2, which would however imply an X-ray decay slope flatter than observed. The XRT and LAT data have similar spectral indices and the observed steeper value of the LAT temporal index is marginally consistent with the predicted temporal decay in the radiative regime of the forward shock model. Absence of a jet break during the first 6 days implies a collimation-corrected γ-ray energy E γ > 2.2 × 10 52 ergs, one of the highest ever seen in a long-duration GRBs. More events combining GeV photon emission with multi-wavelength observations will be required to constrain the nature of the central engine powering these energetic explosions and to explore the correlations between energetic quanta and afterglow emission.