In this paper, I briefly review the theoretical and observational work done since 1958 on the gaseous objects with mass below the H-burning limit. Special attention is paid to the theory that I developed during the period 1958–1962 for the hydrogen-rich gaseous objects with mass below the minimum main sequence mass of ~0.08 M⊙. The three main predictions of this theory (Kumar 1962a; Kumar 1962b; Kumar 1963a; Kumar 1963b) are discussed. Fragmentation processes for interstellar clouds are discussed, and it is pointed out that the minimum mass of a gaseous fragment may be as low as 0.001 M⊙. Observational results obtained since 1995 on the luminous and dark objects of very low mass are briefly reviewed. Comments are made on the basic nature of the stars and planets, and it is pointed out that the processes of star formation are fundamentally different from those of planet formation. Arguments are presented to show that some of the very-low-mass dark companions, discovered since 1995, were most probably formed by the star formation processes and not by the planet formation processes.

We discuss a scenario in which brown dwarfs are formed like stars, except that their full collapse phases are interrupted through dynamical interactions in small multiple systems, leading to the ejection of the lightest member. This disintegration is a stochastic process, often resulting in the expulsion of newborn low mass stars, but when it occurs early enough the ejected stellar embryo will be a substellar object. This process may be so common at early ages that a large fraction of the ubiquitous brown dwarfs could have formed in this manner. Detailed gas dynamical simulations are required in order to better understand the details of the decay of small newborn multiple systems. We discuss the observational consequences of the ejection hypothesis, noting especially the importance of binaries with brown dwarf components as an observational test. Finally, we note that brown dwarfs that have recently been ejected may be so disturbed, by infall from the collapsing core and also by heavy accretion from perturbed circumstellar disks, that traditional spectral and luminosity criteria may fail to identify their substellar nature.

Searches for very low mass objects in young star clusters have uncovered evidence for free-floating objects with inferred masses possibly as low as 5 to 15 Jupiter masses (MJup), similar to the masses of several extrasolar planets. We show here that the process which forms single and multiple protostars, namely collapse and fragmentation of molecular clouds, might be able to produce self-gravitating objects with initial masses less than ˜ 1MJup. Models are calculated with a three dimensional, finite differences code which solves the equations of hydrodynamics, gravitation, and radiative transfer in the Eddington and diffusion approximations. Magnetic pressure is added to the gas pressure, magnetic tension is approximated by gravity dilution once collapse is well underway, and ambipolar diffusion is treated approximately as well. Initially oblate clouds fragment into multiple protostar systems containing a small number (of order four) of fragments. If such fragments can be ejected from an unstable quadruple protostar system, prior to gaining significantly more mass, protostellar collapse might then be able to explain the formation of free-floating objects with masses below 13MJup. These objects might then be best termed “sub-brown dwarfs”.

We present results from the most complex hydrodynamical star formation calculation performed to date. It follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form dozens of stars and brown dwarfs. It resolves all fragmentation down to the opacity limit, binary stars with separations as small as 1 AU, and circumstellar disks with radii down to ≈ 10 AU. In this proceedings, we examine the formation mechanism of the brown dwarfs and compare the initial mass function and the properties of the brown dwarfs with observations.

The orbital elements of extreme mass ratio binaries will be modified by interactions with surrounding circumstellar disks. For brown dwarf companions to Solar-type stars the resulting orbital migration is sufficient to drive short period systems to merger, creating a brown dwarf desert at small separations. We highlight the similarities and the differences between the migration of brown dwarfs and massive extrasolar planets, and discuss how observations can test a migration model for the brown dwarf desert.

Numerical simulations are used to explore how gravitational interactions within young multiple star systems may determine the binary properties of brown dwarfs. We compare different scenarios for cluster formation and decay and find that brown dwarf binaries, although possible, generally have a low frequency. We also discuss the frequencies of brown dwarf companions to normal stars expected from these models.

We report on high-resolution hydrodynamic simulations of small stellar clusters that explore the effects of substructure in star-forming regions. These calculations, when applied to the substellar regime, provide clear evidence for dynamical interactions to play a key role as a formation mechanism for brown dwarfs. Several observational consequences of this process are discussed.

We analyse evolutionary tracks at young ages for low mass stars with masses m ≤ 1.4 M⊙ and brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages ≲ 1 Myr.

Most of the star forming regions within 200 pc from the Sun are located South of the celestial equator. Brown dwarfs or strong candidates have been discovered in all of them, and have been often studied at multiple wavelengths. This paper summarizes some highlights of the research on young brown dwarfs in these regions.

We present very deep JHK imaging of the Trapezium Cluster obtained with Gemini South/Flamingos and Gemini North/Hōkūpa'a. These images probe the IMF down to ˜ 2 MJup in a total area of ˜ 8 arcmin2. Several very faint new planetary mass candidates are detected and sources previously detected with UKIRT are verified. Photometry of 124 point sources in this field produces a Luminosity Function which drops to zero at K=18.75. Allowing for modest extinction this corresponds to a possible turn-down in the IMF near 5 Mjup. A minority of PMOs exhibit large K band excesses attributed to hot dust, confirming their extreme youth. Some of the faintest sources are associated with short trails of light of uncertain nature. These may provide a clue to the origin of PMOs, perhaps marking evacuated paths cleared by rapidly moving objects.

We have employed an advanced suite of Monte Carlo calculations to model the infrared luminosity function (KLF) of the Trapezium cluster and to derive its Sub-stellar Initial Mass Function down to the deuterium burning limit. Independent of the details, we find that sub-stellar objects can account for no more than ~ 22% of the total number of likely cluster members. This cluster's substellar IMF steadily declines in a power-law fashion over most of the brown dwarf regime, however the formation of a secondary peak in the cluster's substellar KLF suggests the existence of a corresponding secondary peak in the substellar IMF between 10 and 30 times the mass of Jupiter based upon the seemingly degenerate behavior of the theoretical substellar mass-luminosity relation.

Red spectra of a sample of low-mass stars and brown dwarfs in the Orion Trapezium cluster are discussed. They show late-type spectral characteristics confirming cluster membership and some show evidence of circumstellar activity.

We have performed a photometric search for brown dwarf candidates in Taurus using the CFH12K wide-field mosaic camera. We are in the process of obtaining follow-up spectroscopy of likely brown dwarf candidates. We summarize the current state of the photometric analysis and the optical spectroscopy obtained to date. We also discuss the evidence for accretion in these systems.

We have obtained low resolution infrared spectra of 21 brown dwarf candidates, selected from a survey performed at the Canada-France-Hawaii Telescope. The spectra were obtained at the Telescopio Nazionale Galileo (La Palma), using the NICS spectrometer in the Amici configuration, which allows to observe the range 0.85–2.45 μm in a single shot. The bands shown by most of the spectra confirm that we are dealing with low effective temperature objects. Preliminary results of spectral classification are presented.

Preliminary results from a deep R, I and Z band survey of ~ 6 square degrees of the Taurus Dark Cloud region are presented. 186 potential brown dwarfs have been unearthed, with seven having follow-up spectroscopic data. The spectra reveal three mid/late-type M dwarfs, of which two show weak Hα emission. If these objects are members of the TDC region, theoretical models suggest masses in the range 10 – 20 MJ.