The Cornell High-order Adaptive Optics Survey for brown dwarfs (CHAOS), currently about 90% complete, uses the Palomar Hale Telescope's adaptive optics system to survey 80 bright stars out to 13 parsecs. Using the telescope's AO Science camera, brown dwarf companions 4–200 AU can be identified using coronagraphic imaging and spectroscopic modes. We will be using monte carlo simulations to create population models consistent with the CHAOS data set.

Currently, we have observed 70 systems out of a target sample of 80. Five candidate companions await follow-up observations. As of now, no systems in the target sample have shown strong evidence of having brown dwarf companions. These early results support previous speculations of a “brown dwarf desert” at orbital separations out to 200 AU.

While the target sources revealed no evidence of brown dwarf companions, accompanying observations of calibration stars provided evidence of an early methane dwarf candidate around the binary system HD150451AB. Forty seven parsecs from earth, the candidate indicated a projected orbital separation of 280 AU.

We present the first results obtained at CFHT with the TRIDENT infrared camera, dedicated to the detection of faint companions close to bright nearby stars. Its main feature is the acquisition of three simultaneous images in three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6μm, that enables a precise subtraction of the primary star PSF while keeping the companion signal. Thirty-five stars have been observed in two observing missions, with no detection so far. It is shown that a faint companion with a ΔH of 10 magnitudes would be detected at 0.5″ from the primary.

We present results of the first large (> 100 stars) infrared coronographic search for substellar companions to nearby stars. The search consisted of two surveys of stars chosen for their youth and proximity to Earth: 1.) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30MJ, orbiting between about 75 and 300 AU, 2.) a 102 star survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, orbiting between about 75 and 300 AU.

This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions (of any mass) to G, K and M stars orbiting between 75 and 300 AU is measured to be 1 ± 1 %. The frequency of massive (> 30 MJ) brown dwarf companions is found to be 0.6 ± 0.6 %. The frequency of giant planets with masses larger than 10 MJ, between 75 and 300 AU, is measured here for the first time to be no more than about 3 %.

Many widely separated companions to main-sequence stars have been found using 2MASS. These companions include both stars and brown dwarfs. I discuss a number of systems of particular interest. Present indications are that a few percent of G dwarfs have brown dwarf companions.

We have started a program to search for Brown Dwarf (BD) companions to nearby stars (5 ≤ d ≤ 10 pc) on long period orbits (up to 2000 AU) using wide field optical/infrared imaging. Our goals in surveying a volume-limited sample of stars are: 1) to determine the binary fraction of widely separated BDs and help constrain the BD formation processes, 2) to search for very cold BDs, 3) to provide a better determination of the age and luminosity of a BD sample. Observations are currently conducted at three observatories, CFHT (3.6m), CTIO (4m) and Observatoire du mont Mégantic (1.6m) and should yield a sample of ~100 star systems to a detection limit of J ≈ 20.

The infrared search for substellar companions to nearby white dwarfs has been going on for a little more than a decade. The most recent phase has been a wide field proper motion search carried out primarily at Steward Observatory, where we are complete down to J = 18. Earlier phases included near field searches at the IRTF and Keck Observatory. In the last year we have discovered ten previously unrecognized faint proper motion companions. Of the recent discoveries, most are white dwarfs and a few M dwarfs. GD165B, discovered in 1988 as part of our program, is still the only known companion to a white dwarf with spectral type later than M.

Near-infrared coronagraphic images of GG Tau and GK Tau are presented as preliminary results of the search for young brown dwarfs around T Tauri stars.

Brown dwarfs occupy the important region in the mass range between stars and planets. Their existence, ambiguous until only recently, and their properties give insight into stellar and planetary formation. We present statistical results of an infrared, coronagraphic survey of young, nearby stars that includes probable companions to three young G-type stars, G1 503.2 (G2V), HD 102982 (G3V), and G1 577 (G5V). The companion to G1 577 is a possible binary brown dwarf, according to evolutionary models. A dynamical determination of the components' masses will be achievable in the near future and will provide an excellent test of the predictive ability of the evolutionary models.

I use simulated adaptive optics data to demonstrate a noise removal technique exploiting prior knowledge of the angular support of the object. Using both detector-noise limited and speckle-noise limited data, I show that this method can result in significant increases in the sensitivity of speckle interferometry to faint companions under demanding observing conditions.

We present the results of two imaging surveys for companions to low-luminosity dwarfs, one with Keck to a limiting magnitude of K = 20, and the other with HST to a limiting magnitude of I = 23. No companions were found at separations of 0″.5 to 20″.0, but seven companions were found within 0″.5. Based on sub-stellar evolutionary models, we estimate the companion detection efficiency as a function of mass ratio. This modeling, coupled with the data, place strong upper limits on the mass ratios of unobserved companions. We conclude that no companions exist in the optimal sensitivity regions of both surveys for q > 0.85 (HST), and q > 0.4 (Keck).

Microlensing is sensitive to binary, brown dwarf (BD), and planetary companions to normal stars in the Galactic bulge with separations between about 1–10 AU. The accurate, densely-sampled photometry of microlensing events needed to detect planetary companions has been achieved by several follow-up collaborations. Detailed analysis of microlensing events toward the bulge demonstrates that less than 45% of M-dwarfs in the bulge have MJup companions between 1 and 5 AU. Detection of binary and BD companions using microlensing is considerably easier; however, the interpretation is hampered by their non-perturbative influence on the parent lightcurve. I demonstrate that ~ 25% of BD companions with separations 1 – 10AU should be detectable with survey-quality data (~ 1 day sampling and ~ 5% photometry). Survey data is more amenable to generic, brute-force analysis methods and less prone to selection biases. An analysis of the ~ 1500 microlensing events detected by OGLE-III in the next three years should test whether the BD desert exists at separations 1 – 10AU from M-dwarfs in the Galactic bulge.

We obtained optical and infrared spectra of Cha Hα 5/cc 1, a faint possibly sub-stellar companion candidate next to the M6-type brown dwarf candidate Cha Hα 5 in Cha I, using FORS1 and ISAAC at the VLT. The VRIJHK colors of Cha Hα 5/cc 1 are consistent with either an L-type companion or a K-type background giant. Our spectra show that the companion candidate actually is a background star.

We present results based on a 6 year survey of a solar vicinity M-dwarf sample. We have obtained their multiplicity rate and orbital elements for the binaries. Such results could give us keys for a better understanding of stellar formation.

Dwarf cepheids DY Her, AD CMi, and BE Lyn show light time effects, which may be caused by orbital motion due to a low mass unseen companion. Their mass is probably between 0.015 to 0.07 M⊙, so they may be brown dwarfs.

The photospheric spectra of all L and T dwarfs contain strong molecular bands and alkali absorption features, and those of L and early T dwarfs are also affected by dust. Although work on atmospheric models is addressing the treatment of dust, and is increasing the completeness of molecular opacity linelists, the effective temperatures of L and T dwarfs cannot be derived confidently from the modelled spectral energy distributions. However, because the radii of brown dwarfs older than 0.1 Gyr vary little with mass, measurements of intrinsic luminosity (i.e. total integrated flux and parallax) accurately determine effective temperature. Using this method we find a well constrained relationship between the effective temperatures of L and T dwarfs and spectral type, but with temperature nearly constant from L7 to T4. More work is required to determine the uniqueness of this relationship and to constrain the masses and ages of brown dwarfs.

Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.

A sophisticated approach to condensate opacity is required to properly model the atmospheres of L and T dwarfs. Here we review different models for the treatment of condensates in brown dwarf atmospheres. We conclude that models which include both particle sedimentation and upwards transport of condensate (both gas and particles) provide the best fit for the L dwarf colors. While a globally uniform cloud model fits the L dwarf data, it turns to the blue in J - K too slowly to fit the T dwarfs. Models which include local clearings in the global cloud deck, similar to Jupiter's prominent five-micron hot spots, better reproduce the available photometric data and also account for the observed resurgence of FeH absorption in early type T dwarfs.

Carbon monoxide and ammonia have been detected in the spectrum of G1 229B at abundances that differ substantially from those obtained from chemical equilibrium. Vertical mixing in the atmosphere is a mechanism that can drive slowly reacting species out of chemical equilibrium. We explore the effects of vertical mixing as a function of mixing efficiency and effective temperature on the chemical abundances in the atmospheres of brown dwarfs and on their spectra. The models compare favorably with the observational evidence and indicate that vertical mixing plays an important role in brown dwarf atmospheres.

A continuous L dwarf classification sequence requires the combined use of far optical (visible) and near infrared spectral indices. However, the visible and near infrared indices currently in use assign subtypes that differ by up to three subclasses due to differences in cloud opacity for objects with the same effective temperature. Therefore, it may be impossible to combine visible and near infrared spectral indices to create one L dwarf classification system, and two classification variables may be necessary.

We present the preliminary results of a programme aimed at defining a low-resolution near-infrared spectral classification scheme for faint M, L, and T-dwarfs. The method is based on the global shape of R˜100 complete near-infrared spectra from 0.8 to 2.4μm as obtained through a high-throughput prism-based optical element, the Amici device, mounted inside the NICS instrument at the TNG 3.5m telescope (Baffa et al. 2001; Oliva 2000). The aim of our project is to provide an efficient classification scheme based on very low-resolution near infrared spectroscopy, which can be carried on at a 4m-class telescope. The results for the L-type dwarfs have already been presented in Testi et al. (2001), sample spectra for the M and T-dwarfs range are shown in Figure 1. A preliminary application of the method to the classification of young embedded brown-dwarf candidates has been successfully attempted by Testi et al. (2002) and Natta et al. (2002). The method is shown to be accurate and competitive: the high system throughput coupled with the possibility of obtaining in a “single shot” the complete spectrum of the objects make the NICS/TNG system more efficient than existing large telescopes.