We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive, extended discs around Sun-like stars. We argue that such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs typically fragment within a few thousand years to produce mainly brown dwarfs (including planetary-mass brown dwarfs) and low-mass hydrogen-burning stars. Subsequently most of the brown dwarfs are ejected by mutual interactions. We analyse the properties of these objects that form by disc fragmentation, and compare them with observations.
Our first aim is to calculate the minimum mass for Primary Fragmentation in a variety of potential star-formation scenarios, i.e. (i) hierarchical fragmentation of a 3-D medium; (ii) one-shot, 2-D fragmentation of a shock-compressed layer; (iii) fragmentation of a circumstellar disc. Our second aim is to evaluate the role of H2 dissociation in facilitating Secondary Fragmentation and thereby producing close, low-mass binaries. Results: (i)For contemporary, local star formation, the minimum mass for Primary Fragmentation is in the range 0.001-0.004Msun, irrespective of the scenario considered. …
We investigate the formation of brown dwarfs and very low-mass stars through the gravitational fragmentation of infalling gas into stellar clusters. The gravitational potential of a forming stellar cluster provides the focus that attracts gas from the surrounding molecular cloud. Structures present in the gas grow, forming filaments flowing into the cluster centre. These filaments attain high gas densities due to the combination of the cluster potential and local self-gravity. The resultant Jeans masses are low, allowing the formation of very low-mass fragments. The tidal shear and high veloci…
We argue that gravitational instability of typical protostellar disks is not a viable mechanism for the fragmentation into multiple systems -- binary stars, brown dwarf companions, or gas giant planets -- except at periods above roughly 20,000 years. Our conclusion is based on a comparison between prior numerical work on disk self-gravity by Gammie (2001) with our own analytical models for the dynamical and thermal state of protostellar disks. For this purpose we first develop a simple theory for the initial conditions of low-mass star formation, accounting for the effect of turbulence on the …
The origin of brown dwarfs (BDs) is an important component of the theory of star formation, because BDs are approximately as numerous as solar mass stars. It has been suggested that BDs originate from the gravitational fragmentation of protostellar disks, a very different mechanism from the formation of hydrogen burning stars. We propose that BDs are instead formed by the process of turbulent fragmentation, like more massive stars. In numerical simulations of turbulence and star formation we find that gravitationally unstable density peaks of BD mass are commonly formed by the turbulent flow. …
We review five mechanisms for forming brown dwarfs: (i) turbulent fragmentation of molecular clouds, producing very low-mass prestellar cores by shock compression; (ii) collapse and fragmentation of more massive prestellar cores; (iii) disc fragmentation; (iv) premature ejection of protostellar embryos from their natal cores; and (v) photo-erosion of pre-existing cores overrun by HII regions. These mechanisms are not mutually exclusive. Their relative importance probably depends on environment, and should be judged by their ability to reproduce the brown-dwarf IMF, the distribution and kinemat…
We suggest that low-mass hydrogen-burning stars like the Sun should sometimes form with massive extended discs; and we show, by means of radiation hydrodynamic simulations, that the outer parts of such discs (R>100 AU) are likely to fragment on a dynamical timescale (10^3 to $10^4 yr), forming low-mass companions: principally brown dwarfs (BDs), but also very low-mass hydrogen-burning stars and planetary-mass objects. A few of the BDs formed in this way remain attached to the primary star, orbiting at large radii. The majority are released into the field, by interactions amongst themselves; in…
We present results from the first hydrodynamical star formation calculation to demonstrate that brown dwarfs are a natural and frequent product of the collapse and fragmentation of a turbulent molecular cloud. The brown dwarfs form via the fragmentation of dense molecular gas in unstable multiple systems and are ejected from the dense gas before they have been able to accrete to stellar masses. Thus, they can be viewed as `failed stars'. Approximately three quarters of the brown dwarfs form in gravitationally-unstable circumstellar discs while the remainder form in collapsing filaments of mole…
We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Most of the the PM stars and BDs are ejected by mutual interactions. We analyse the statistic…
We present results from the largest numerical simulation of star formation to resolve the fragmentation process down to the opacity limit. The simulation follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form a stellar cluster and, simultaneously, the formation of circumstellar discs and binary stars. This large range of scales enables us to predict a wide variety of stellar properties for comparison with observations. The calculation clearly demonstrates that star formation is a highly-dynamic and chaotic process. Star-disc encounters form binaries and trunc…
Hundreds of brown dwarfs (BDs) have been discovered in the last few years in stellar clusters and among field stars. BDs are almost as numerous as hydrogen burning stars and so a theory of star formation should also explain their origin. The ``mystery'' of the origin of BDs is that their mass is two orders of magnitude smaller than the average Jeans' mass in star--forming clouds, and yet they are so common. In this work we investigate the possibility that gravitationally unstable protostellar cores of BD mass are formed directly by the process of turbulent fragmentation. Supersonic turbulence …
We review and evaluate four mechanisms for forming brown dwarves: (i) dynamical ejection of a stellar embryo from its placental prestellar core; (ii) opacity-limited fragmentation of a shock-compressed layer; (iii) gravitational instabilities in discs, triggered by impulsive interactions with other discs or naked stars; and (iv) photo-erosion of pre-existing cores. All these mechanisms can produce free-floating brown dwarves, but only (ii) and (iii) are likely to produce brown dwarves in multiple systems, and (i) has difficulty delivering brown dwarves with discs.
The formation of brown dwarfs via encounters between proto-stars has been confirmed with high-resolution numerical simulations with a restricted treatment of the thermal conditions. The new results indicate that young brown dwarfs (BDs) formed this way are disk-like and often reside in multiple systems. The newly-formed proto-BDs disks are up to 18 AU in size and spin rapidly making small-scale bipolar outflows, fragmentation and the possible formation of planetary companions likely as have recently been observed for BDs. The object masses range from 2 to 73 Jupiter masses, distributed in a ma…
The observational properties of brown dwarfs pose challenges to the theory of star formation. Because their mass is much smaller than the typical Jeans mass of interstellar clouds, brown dwarfs are most likely formed through secondary fragmentation processes, rather than through the direct collapse of a molecular cloud core. In order to prevent substantial post-formation mass accretion, young brown dwarfs must leave the high density formation regions in which they form. We propose here that brown dwarfs are formed in the circumbinary disks. Through post-formation dynamical interaction with the…
We present the results of an ensemble of simulations of the collapse and fragmentation of dense star-forming cores. We show that even with very low levels of turbulence the outcome is usually a binary, or higher-order multiple, system. We take as the initial conditions for these simulations a typical low-mass core, based on the average properties of a large sample of observed cores. All the simulated cores start with a mass of $M = 5.4 M_{\odot}$, a flattened central density profile, a ratio of thermal to gravitational energy $α_{\rm therm} = 0.45$ and a ratio of turbulent to gravitational ene…
The observational properties of brown dwarfs pose challenges to the theory of star formation. Because their mass is much smaller than the typical Jeans mass of interstellar clouds, brown dwarfs are most likely formed through secondary fragmentation processes, rather than through the direct collapse of a molecular cloud core. In order to prevent substantial post-formation mass accretion, young brown dwarfs must leave the high density formation regions in which they form. We propose here that brown dwarfs are formed in the optically thin outer regions of circumbinary disks. Through post-formatio…
In this review, we examine the successes and weaknesses of modern low-mass star and brown dwarf theory.(1) We first focus on the mechanical (equation of state) and thermal (atmosphere) properties and on the evolution. We then examine the current shortcomings of the theory and we discuss recent observational analysis which have suggested discrepancies between models and observations.(2) We then examine the stellar and brown dwarf IMF and suggest that a power-law above the average thermal Jeans mass (about 1 Msol) rolling over a lognormal form below this limit adequately reproduces the observati…
We review four mechanisms for forming brown dwarfs: (i) turbulent fragmentation (producing very low-mass prestellar cores); (ii) gravitational instabilities in discs; (iii) dynamical ejection of stellar embryos from their placental cores; and (iv) photo-erosion of pre-existing cores in HII regions. We argue (a) that these are simply the mechanisms of low-mass star formation, and (b) that they are not mutually exclusive. If, as seems possible, all four mechanisms operate in nature, their relative importance may eventually be constrained by their ability to reproduce the binary statistics of bro…
Brown dwarfs, which occupy a fuzzy gap in mass between stars and planets, appear to be common both in the solar neighborhood and in star-forming regions. Their origin is a topic of significant current interest and debate. Some astronomers have suggested that brown dwarfs are stellar embryos, ejected from newborn multiple star systems. Others argue that they form in the same manner as stars, through fragmentation and collapse of interstellar clouds. Observations of young brown dwarfs may help distinguish among different scenarios for their formation. The recent discovery that many of them harbo…
We review the current state of observational work on the formation of brown dwarfs, focusing on their initial mass function, velocity and spatial distributions at birth, multiplicity, accretion, and circumstellar disks. The available measurements of these various properties are consistent with a common formation mechanism for brown dwarfs and stars. In particular, the existence of widely separated binary brown dwarfs and a probable isolated proto-brown dwarf indicate that some substellar objects are able to form in the same manner as stars through unperturbed cloud fragmentation. Additional me…
In this paper, we use high-resolution smoothed particle hydrodynamics (SPH) simulations to investigate the response of a marginally stable self-gravitating protostellar disc to a close parabolic encounter with a companion discless star. Our main aim is to test whether close brown dwarfs or massive planets can form out of the fragmentation of such discs. We follow the thermal evolution of the disc by including the effects of heating due to compression and shocks and a simple prescription for cooling and find results that contrast with previous isothermal simulations. In the present case we find…
Abridged/ We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of SPH + Nbody simulations of the fragmentation of small molecular clouds, that fully resolve the opacity limit for fragmentation. We have produced a statistically significant number of stable multiple systems, with components separations in the range 1-1000 AU. At the end of the hydrodynamical evolution (0.5 Myr) we find that ~60% of stars and brown dwarfs are members of multiples systems, with about a third of these being low mass, weakly bound outliers in …
The issue of fragmentation in self-gravitating gaseous accretion discs has implications both for the formation of stars in discs in the nuclei of active galaxies, and for the formation of gaseous planets or brown dwarfs in circumstellar discs. It is now well established that fragmentation occurs if the disc is cooled on a timescale smaller than the local dynamical timescale, while for longer cooling times the disc reaches a quasi-steady state in thermal equilibrium, with the cooling rate balanced by the heating due to gravitational stresses. We investigate here how the fragmentation boundary d…
We present high resolution optical spectroscopy of three candidate members of the Taurus-Auriga star forming region. Based on the spectral type, the strength, profile and width of the H(alpha) line, the detected lithium at 6708 angstrom, the location of these objects in a H-R diagram and the comparison with similar objects belonging to young stellar associations, we determine that they are bona fide members of the SFR, with about ~3 Myr, have masses at or below the substellar limit and, at least in one case, there is active accretion from a circum(sub)stellar disk. This result suggests that hi…
Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the yet cooler Y-type spectral class (Teff <~ 500K?) has not been seen. Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5 Mj) are rare. Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic o…
A new model for the formation of Jovian planets is proposed. We consider planets forming at large distances from a protostar (>100 AU) through direct fragmentation of a gas cloud, by the same formation mechanism as wide stellar and brown dwarf binaries. We model the gravitational evolution of a system of these distant planets and a second population formed in a disk closer to the star. We compute the typical closest approach of these planets to the star (i.e., smallest pericenter) over the course of their evolution. When the planets reach a pericenter within a gaseous disk surroundig the star,…
Sun-like stars have stellar, brown dwarf and planetary companions. To help constrain their formation and migration scenarios, we analyse the close companions (orbital period < 5 years) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost two orders of magnitude from 1 M_Sun stellar companions to the brown dwarf desert and then rises by more than an order of magni…
A substellar-mass object in orbit at about 300 astronomical units (AU) from the young low-mass star G196-3 was detected by direct imaging. Optical and infrared photometry and low- and intermediate-resolution spectroscopy of the faint companion, hereafter referred to as G196-3B, confirms its cool atmosphere and allows its mass to be estimated at 25^{+15}_{-10} Jupiter masses. The separation between both objects and their mass ratio suggest the fragmentation of a collapsing cloud as the most likely origin for G196-3B, but alternatively it could have originated from a proto-planetary disc which h…
Context: The principal mechanism by which brown dwarfs form, and its relation to the formation of higher-mass (i.e. hydrogen-burning) stars, is poorly understood. Aims: We advocate a new model for the formation of brown dwarfs. Methods: In this model, brown dwarfs are initially binary companions, formed by gravitational fragmentation of the outer parts (R > 100 AU) of protostellar discs around low-mass hydrogen-burning stars. Most of these binaries are then gently disrupted by passing stars to create a largely single population of brown dwarfs and low-mass hydrogen-burning stars. Results: This…
Self-gravitating protostellar discs are unstable to fragmentation if the gas can cool on a time scale that is short compared to the orbital period. We use a combination of hydrodynamic simulations and N-body orbit integrations to study the long term evolution of a fragmenting disc with an initial mass ratio to the star of M_disc/M_star = 0.1. For a disc which is initially unstable across a range of radii, a combination of collapse and subsequent accretion yields substellar objects with a spectrum of masses extending (for a Solar mass star) up to ~0.01 M_sun. Subsequent gravitational evolution …
We perform SPH simulations of the collapse and fragmentation of low-mass cores having different initial levels of turbulence (alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation which captures the transport of cooling radiation against opacity due to both dust and gas (including the effects of dust sublimation, molecules, and H^- ions). We also perform comparison simulations using a standard barotropic equation of state. We find that -- when compared with the barotropic equation of state -- our more realistic treatment of the energy equation results in more protostellar o…
Observed variations in the IMF are reviewed with an emphasis on environmental density. The remote field IMF studied in the LMC by several authors is clearly steeper than most cluster IMFs, which have slopes close to the Salpeter value. Local field regions of star formation, like Taurus, may have relatively steep IMFs too. Very dense and massive clusters, like super star clusters, could have flatter IMFs, or inner-truncated IMFs. We propose that these variations are the result of three distinct processes during star formation that affect the mass function in different ways depending on mass ran…
We present intermediate and low resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a CCD-based IZ survey covering an area of 1 sq. deg in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs, having spectral types in the range M6-M9. Five out of the seven objects can be considered as Pleiades members on the basis of their radial velocities, Halpha emission and other gravity sensitive atomic features like the NaI doublet at 818.3 and 819.5 nm. According to current …
We present detailed modeling of the disk around the TW Hydrae Association (TWA) brown dwarf 2MASSW J1207334-393254 (2M1207), using {\it Spitzer} observations from 3.6 to 24 $\micron$. The spectral energy distribution (SED) does not show a high amount of flaring. We have obtained a good fit using a flat disk of mass between $10^{-4}$ and $10^{-6}$ $M_{\sun}$, $\dot{M}$ $\la10^{-11} M_{\sun}$/yr and a large inclination angle between 60$\degr$ and 70$\degr$. We have used three different grain models to fit the 10 $\micron$ Si emission feature, and have found the results to be consistent with ISM-…
We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of nine and three, respectively. We find that the denser cloud, with the lower mea…
Many questions in physical cosmology regarding the thermal history of the intergalactic medium, chemical enrichment, reionization, etc. are thought to be intimately related to the nature and evolution of pregalactic structure. In particular the efficiency of primordial star formation and the primordial IMF are of special interest. We present results from high resolution three-dimensional adaptive mesh refinement simulations that follow the collapse of primordial molecular clouds and their subsequent fragmentation within a cosmologically representative volume. Comoving scales from 128 kpc down …
A number of previous studies of the fragmentation of self-gravitating protostellar discs have modeled radiative cooling with a cooling timescale (t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local dynamical timescale. Such studies have delineated the `fragmentation boundary' in terms of a critical value of beta_{cool} (beta_{crit}), where the disc fragments if beta_{cool} < beta_{crit}. Such an approach however begs the question of how in reality a disc could ever be assembled with beta_{cool} < beta_{crit}. Here we adopt the more realistic approach of gradually reducing b…
We report the statistical properties of stars, brown dwarfs and multiple systems obtained from the largest hydrodynamical simulation of star cluster formation to date that resolves masses down to the opacity limit for fragmentation (a few Jupiter masses). The simulation is essentially identical to that of Bate, Bonnell & Bromm except that the initial molecular cloud is larger and more massive. It produces more than 1250 stars and brown dwarfs, providing unprecedented statistical information that can be compared with observational surveys. We find that hydrodynamical/sink particle simulations c…
The gravitational instability of expanding shells is discussed. Linear and nonlinear terms are included in an analytical solution in the static and homogeneous medium. We discuss the interaction of modes and give the time needed for fragmentation. Masses of individual fragments are also estimated and their formation rates and the initial mass function are derived. Results of simulations are compared to observation.
