We present preliminary results from a deep near-infrared survey of a ~ 1 square degree area in the young open cluster Alpha Persei using the wide-field Omega-Prime camera on the Calar Alto 3.5m telescope, yielding a list of new low-mass cluster members, including brown dwarf candidates.

We have compiled the largest magnitude limited sample of candidate substellar Pleiads to date. We fit King profiles to their spatial distribution to determine the Pleiades brown dwarf core radius to be Subsequently we have used our improved spatial model to place stringent limits on the shape of the cluster mass function across and below the stellar/substellar regime. We find this to be a power law with index α = 0.41±0.08 (0.3M⊙ ≥M≥ 0.035M⊙). Extrapolation of this mass function to M= 0.012M⊙ indicates that brown dwarfs contribute only ~ 2% to the total mass of the cluster hence we conclude that brown dwarfs do not contribute significantly to disk dark matter.

We present the preliminary findings of a deep imaging survey covering 17 square degrees in the Pleiades. We have identified in excess of 300 candidate cluster members in the i, i-z diagram. The survey is complete to ~ i=22, with candidates detected as faint as i=23.

We present preliminary results from a deep near-infrared J-band and I-band photometric survey of the Pleiades for freely-floating superplanets and brown dwarfs (BD) near the deuterium burning limit (DBL). With limiting magnitudes of J=20.5 and I=23.5, we have selected candidate Pleiads on the basis of evolutionary tracks, color-magnitudes, and I-J color lower limits (non-detections at deep I-band). Likelihoods of membership will be ultimately determined by a combination of image profile analysis, spectral energy distribution, proper motion, and low-resolution measurements of near-infrared water and/or methane absorption slopes. If confirmed, our faintest candidates are predicted to have made the transition from L to T spectral types with temperatures down to 820 K, and masses approaching 10 MJup.

Studies of brown dwarfs as a distinct galactic population have been largely pioneered by dedicated, vast-area surveys at deep optical and near-infrared wavebands. The Two Micron All Sky Survey has, because of its full-sky coverage and depth, been the most successful at providing new targets for further study. The author briefly reviews some of the ways in which 2MASS data have been used by brown dwarf researchers. One of the by-products – namely the 2MASS Team's follow-up spectroscopy of brown dwarf candidates – is potentially valuable as a scientific resource itself and is also being released to the public.

We present new M and L-dwarfs confirmed through follow-up of 2MASS color-selected objects with the CorMASS near-infrared spectrograph (R ˜ 300) on the Palomar 60-inch telescope as part of a continuing follow-up survey. Most of the objects are bright (Ks < 13).

We present initial results of an all-sky search for cool, low-mass stars and brown dwarfs within 20 pc. We have used the near-infrared photometry provided by the 2-Micron All Sky Survey (2MASS) Second Incremental Data Release to create a target list of 1793 candidates with 6 < J < 17 and J – K > 1. Follow-up observations and existing data have enabled over 85% of the sample to be classified. Spectral types and distances have been derived for over 250 nearby dwarfs, including 39 newly discovered L dwarfs. In this poster, we discuss our selection methods and present preliminary results.

The initial results of the Deep Lens Survey (http://dls.bell-labs.com) to identify possible brown dwarfs and extremely metal poor red halo subdwarfs near the hydrogen burning limit are presented. Individual deep CCD high galactic latitude survey fields appear to offer a low probability of discovering field BD's, but taken collectively offer an opportunity to begin addressing questions regarding the scale height and distribution of these objects. In all likelihood, the very depth of such surveys will greatly increase our knowledge of the coolest extreme halo objects, which currently are known in far fewer numbers than T dwarfs. Ultimately, the large volume surveyed by the Large Synoptic Survey Telescope will identify vast numbers of such objects, providing a more complete picture of their spatial distribution.

We present a 1.4 square degree survey of the Praesepe open cluster complete to I = 21.3 and Z = 20.2. This corresponds to masses down to 0.06M⊙ and 0.075M⊙ for an assumed cluster age of 1.0 or 0.5 Gyrs respectively. Over fifty new Praesepe candidate members have been discovered with masses less than 0.25M⊙. We discuss plans to establish proper motions of identified Praesepe candidates, and to measure NIR photometry of our candidates.

We present new infrared photometry of Pleiades and Praesepe sources, and define the cluster sequences in colour magnitude and 2-colour diagrams. We identify non-members and unresolved binaries, and discuss the clusters binary fractions.

In spite of all the attempts to find them, no one has yet detected any brown dwarf in a globular cluster. Although powerful instruments such as the VLT and Advanced Camera could further push the frontiers of this search, globular clusters will probably hold tight to their secrets for a while longer. Nonetheless, the search for very low mass stars in globular clusters has taught us a lot about their original mass distribution (IMF) and its evolution in time. I shall review the results of an investigation carried out over what is presently the largest, most homogeneous sample, and discuss the reasons suggesting that: 1. dynamical evolution (internal and external) has reshaped the cluster mass function over time, but the imprint of the IMF is still visible; 2. the IMF appears to vary very little from cluster to cluster; 3. the most likely functional form of the IMF is that of a power law that rises to a peak at ˜ 0.3 M⊙ and tapers off at smaller masses.

Up to now, most planet search projects have concentrated on G and K stars. In order to considerably widen the view, we have stated a survey for planets of old, nearby brown dwarfs and very low-mass stars. Using UVES, we have observed 26 brown dwarfs and very low-mass stars. As it turned out these objects are very inactive and thus highly suitable for such a project. For 19 objects, we can exclude a planet with the mass of 3 MJ, and a period of 100 days or less with a probability of more than 60%. For these objects, we can also exclude Pegasi-planets with a high probability. For another 4 objects, we can exclude at least a brown dwarf companion. One object is a double line spectroscopic binary, and one object shows significant radial-velocity variations that can not be caused by a normal stellar-spot. This object either has a planetary-mass companion, or the variations are caused by surface structures that are quite different from normal star-spots.

We have studied twelve very young (1–5 Myr) bona fide and candidate brown dwarfs in the Cha I star forming region in terms of their kinematic properties, the occurrence of multiple systems among them as well as their rotational characteristics. Based on high-resolution spectra taken with UVES at the VLT (8.2 m), radial and rotational velocities have been measured. A kinematic study of the sample showed that their radial velocity dispersion is relatively small suggesting that they are not ejected during their formation as proposed in recent formation scenarios. By means of time-resolved UVES spectra, a radial velocity survey for close companions to the targets was conducted. The radial velocities of the targets turned out to be rather constant setting upper limits for the mass M2 sin i of possible companions to 0.1 MJup – 2 MJup. These findings hint at a rather low (≤ 10%) multiplicity fraction of the studied brown dwarfs. Furthermore, a photometric monitoring campaign of the targets yielded the determination of rotational periods for three brown dwarf candidates in the range of 2.2 to 3.4 days. These are the first rotational periods for very young brown dwarfs and among the first for brown dwarfs at all.

Brown dwarf binary systems offer the unique opportunity to measure some of the underlying physical characteristics, like, e.g., the mass. In addition, the individual components of brown dwarf binaries are expected to be coeval and have the same chemical composition. This provides crucial constraints on any (evolutionary) model by greatly reducing the number of free parameters. Finally, brown dwarf binary statistics should shed light on the origin of brown dwarfs and possible formation mechanisms and environments. We summarize on-going surveys for brown dwarf binaries, and present first results of follow-up studies.

We review the results of HST/WFPC2 observations of a sample of 50 very-low-mass objects in the solar neighborhood (30 from our own program and 20 from the archive). Using high spatial resolution HST/WFPC2 images, we observed these objects in two filters (F814W and F675W) in order to identify close companions, measure their colours, and obtain first epoch data of the newly discovered binaries. In addition to 3 previously known binaries we discovered 3 new multiple systems. Second epoch data are required to confirm that they are real common proper motion pairs. Binary properties like multiplicity, distribution of binary separations and brightness ratios hold clues on the origin and evolution of free-floating brown dwarf binaries.

Use of the highly sensitive Hōkūpa'a/Gemini curvature wave-front sensor has allowed for the first time direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT=M8.0–L0.5. A survey of 39 such objects detected 9 VLM binaries (7 of which were discovered for the first time to be binaries). Most of these systems (55%) are tight (separation < 5 AU) and have similar masses (ΔKs < 0.8 mag; 0.85 < q < 1.0). However, 2 systems (LHS 2397a, and 2M2331016-040618) have large ΔKs > 2.38 mag and consist of a VLM star orbited by a much cooler L6.5–L8.5 brown dwarf companion. Based on our initial flux limited (Ks < 12 mag) survey of 39 M8.0–L0.5 stars (mainly from the sample of Gizis et al. 2000) we find a binary fraction in the range 19±7% for M8.0–L0.5 binaries with separations > 2.6 AU. This is slightly less than the 32 ± 9% measured for more massive (M0–M4) stars over the same separation range (Fischer & Marcy 1992). It appears M8.0–L0.5 binaries (as well as L and T dwarf binaries) have a much smaller semi-major axis distribution peak (~ 4 AU) compared to more massive M and G stars which have a broad peak at larger ~ 30 AU separations. We also find no VLM binaries (Mtot < 0.18M⊙) with separations > 20 AU. We find that a velocity “kick” of ~ 3 km/s can reproduce the observed cut-off in the semi-major axis distribution at ˜ 20 AU. This kick may have been from the VLM system being ejected from its formation mini-cluster.

We have used the adaptive optics system Hōkūpa'a at Gemini North to search for companions from a flux-limited (Ks > 12) survey of 30 nearby M6.0–M7.5 dwarfs. Our observations, which are sensitive to companions with separations > 0.1″ (~ 2.8 AU), detect 3 new binary systems. This implies an overall binary fraction of 9±4% for M6.0–M7.5 binaries. This binary frequency is somewhat less than the 19±7% measured for late M stars and ~ 20% for L stars, but is still statistically consistent. However, the result is significantly lower than the binary fractions observed amongst solar mass main sequence stars (~60%) and early M stars (~35%).

Using the adaptive optics system, Hōkūpa'a, at Gemini-North, we have directly imaged a companion around the UKIRT faint standard M8 star, LHS 2397a (FS 129) at a separation of 2.96 AU. Near-Infrared photometry obtained on the companion has shown it to be an L7.5 brown dwarf and confirmed the spectral type of the primary to be an M8. We also derive a substellar mass of the companion of 0.068M⊙, although masses in the range (0.061 – 0.069) are possible, and the primary mass as 0.090M⊙ (0.089 – 0.094). Reanalysis of archival imaging from HST has confirmed the secondary as a common proper motion object. This binary represents the first clear example of a brown dwarf companion within 4 AU of a low mass star, and should be the first L7.5 to have a dynamical mass. As part of a larger survey of M8-M9 stars, this object may indicate that there is no “brown dwarf desert” around low mass primaries.

We present the results of a search for low-mass companions around a sample of young, solar-analog stars using the Hōkūpa'a adaptive optics instrument mounted on the Gemini North 8 m telescope. Out of 31 stars observed, one binary brown dwarf system was found as a companion to the star HD 130948 (HIP 72567), as confirmed by proper motion and near-IR spectra. Orbital motion between the two brown dwarfs was measured, but our 14 month time baseline is inadequate to accurately measure the system's dynamical mass. Upcoming spectroscopic observations of the brown dwarfs will measure their lithium absorption lines to provide a more accurate age estimate of the system. The eventual dynamical mass determination coupled with the age determination will provide a valuable check of brown dwarf evolutionary models.

We present near-infrared spectroscopy of low-mass companions in the HD 130948 system (Goto et al. 2002a). Adaptive optics on the Subaru Telescope allowed for spectroscopy of the individual components of the 0″.13 binary system. Based on a direct comparison with a series of template spectra, we determined the spectral types of HD 130948B and C to be L4 ± 1. We find they are most likely a binary brown dwarf system.