Astrometric monitoring of the nearby early-L dwarf DE0823$-$49 has revealed a low-mass companion in a 248-day orbit that was announced in an earlier work. Here, we present new astrometric and spectroscopic observations that allow us to characterise t
he system in detail. The optical spectrum shows LiI-absorption indicative of a young age and/or substellar mass for the primary component. The near-infrared spectrum is best reproduced by a binary system of brown dwarfs with spectral types of L1.5 $+$ L5.5 and effective temperatures of $2150pm100$ K and $1670pm140$ K. To conform with the photocentric orbit size measured with astrometry and the current understanding of substellar evolution, the system must have an age in the 80--500 Myr range. Evolutionary models predict component masses in the ranges of $M_1simeq0.028-0.063,M_odot$ and $M_2simeq0.018-0.045,M_odot$ with a mass ratio of $qsimeq0.64-0.74$. Multi-epoch radial velocity measurements unambiguously establish the three-dimensional orbit of the system and allow us to investigate its kinematic properties. DE0823$-$49 emerges as a rare example of a nearby brown dwarf binary with orbit, component properties, and age that are characterised well. It is a juvenile resident of the solar neighbourhood, but does not appear to belong to a known young association or moving group.
The aim of the project is to search for lithium in absorption at 6707.8 Angstroms to constrain the nature and the mass of the brightest low-metallicity L-type dwarfs (refered to as L subdwarfs) identified in large-scale surveys. We obtained low- to
intermediate-resolution (R~2500-9000) optical (~560-770 nm) spectra of two mid-L subdwarfs, SDSSJ125637.13-022452.4 (SDSS1256; sdL3.5) and 2MASSJ162620.14+392519.5 (2MASS1626; sdL4) with spectrographs on the European Southern Observatory Very Large Telescope and the Gran Telescopio de Canarias. We report the presence of a feature at the nominal position of the lithium absorption doublet at 6707.8 Angstroms in the spectrum of SDSS1256, with an equivalent width of 66+/-27 Angstroms at 2.4 sigma, which we identify as arising from a CaH molecular transition based on atmosphere models. We do not see any feature at the position of the lithium feature in the spectrum of 2MASS1626. The existence of overlapping molecular absorption sets a confusion detection limit of [Li/H]=-3 for equivalently-typed L subdwarfs. We provided improved radial velocity measurements of -126+/-10 km/s and -239+/-12 km/s for SDSS1256 and 2MASS1626, respectively, as well as revised Galactic orbits. We implemented adjusting factors for the CaH molecule in combination with the NextGen atmosphere models to fit the optical spectrum of SDSS1256 in the 6200-7300 Angstroms range. We also estimate the expected Li abundance from interstellar accretion ([Li/H]=-5), place limits on circumstellar accretion (10^9 g/yr), and discuss the prospects of Li searches in cooler L and T subdwarfs.
We fit a sample of 49 R=6000 NIR (0.9 - 2.5 micron) T dwarf spectra obtained with Magellans FIRE spectrograph with two different atmospheric model sets to compare the derived physical parameters such as Teff, log g, cloud opacity, and rotational velo
city between the models, as well as their reliability. Many of our T dwarfs have distance measurements, which allows us to calculate their radii during the fitting, which can be compared to evolutionary models to determine age, mass and potentially the presence of unseen companions. We present our spectral sample and model fits, and comment on the measured fundamental properties of these T dwarfs. Our analysis allows us to identify global deviations between models and observed spectra, and hence provides important feedback for the next generation of substellar atmospheric models.
Luhman (2013) has reported the discovery of a brown dwarf binary system only 2.01+/-0.15 pc from the Sun. The binary is well-resolved with a projected separation of 1.5, and spectroscopic observations have identified the components as late-L and earl
y-T dwarfs. The system exhibits several remarkable traits, including a flux reversal, where the T dwarf is brighter over 0.9-1.3 micron but fainter at other wavelengths; and significant (~10%) short-period (~4.9 hr) photometric variability with a complex light curve. These observations suggest spatial variations in condensate cloud structure, which is known to evolve substantially across the L dwarf/T dwarf transition. Here we report preliminary results from a multi-site monitoring campaign aimed at probing the spectral and temporal properties of this source. Focusing on our spectroscopic observations, we report the first detections of NIR spectral variability, present detailed analysis of K I lines that confirm differences in condensate opacity between the components; and preliminary determinations of radial and rotational velocities based on high-resolution NIR spectroscopy.
