The distribution of ejected brown dwarfs in clusters

We examine the spatial distribution of brown dwarfs produced by the decay of small‐N stellar systems as expected from the embryo ejection scenario. We model a cluster of several hundred stars grouped into ‘cores’ of a few stars/brown dwarfs. These cores decay, preferentially ejecting their lowest‐mass members. Brown dwarfs are found to have a wider spatial distribution than stars, however once the effects of limited survey areas and unresolved binaries are taken into account it can be difficult to distinguish between clusters with many or no ejections. A large difference between the distributions probably indicates that ejections have occurred, however similar distributions sometimes arise even with ejections. Thus the spatial distribution of brown dwarfs is not necessarily a good discriminator between ejection and non‐ejection scenarios. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Submillimetre observations of low‐mass cloud cores: forming tiny objects in situ

The origin of very low‐mass objects such as brown dwarfs and ‘isolated planets’ is unclear: can they form in‐situ from very low‐mass cloud cores in a scaled‐down version of star formation? Here I discuss methods of detecting and characterising such faint cores using submillimetre‐wavelength observations. Some data are presented for the Ophiuchus clouds that strongly suggest there is little division between stars and ultra low‐mass objects at the earliest evolutionary stages. Some challenging results have emerged (in the context of current theory), including finding cores of only a few Jupiter masses and a core mass function still rising at the mass detection limit: the implications are briefly discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Very low mass stars in binaries: a theoretical look

We have undertaken a series of hydrodynamic + N ‐body simulations in order to explore the binary properties of young stars. We find that multiple stars are a natural outcome of collapsing turbulent flows, with a high incidence of N > 2 multiples, specially among the higher mass objects. We find a positive correlation of multiplicity with primary mass and a companion frequency that decreases with age, during the first few Myr after formation. Binary brown dwarfs are rarely formed, in conflict with observations. Brown dwarfs as companions are predominantly found orbiting binaries or triples at large separations. The paucity of ultra low mass and low mass ratio binaries has been investigated further, and we tentatively conclude that their formation is intricately related to an appropriate selection of initial conditions and an accurate modelling of disc accretion and evolution. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Searching for proto‐brown dwarfs: Extending near IR spectroscopy of protostars below the hydrogen burning limit

Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto‐brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto‐brown dwarfs (M < 40 M_Jup), however, will prove challenging to observe given current near IR observational capabilities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accretion in brown dwarfs down to nearly planetary masses

We show that in accreting ultra low‐mass stars and brown dwarfs, the CaII λ 8662 emission line flux correlates remarkably well with the mass accretion rate ( ), just as it does in higher mass classical T Tauri stars (CTTs). A straightforward measurement of the CaII flux thus provides an easier determination technique than detailed modeling of the Hα emission line profile (except at the very lowest accretion rates, where CaII does not appear to be in emission for ultra low‐mass objects, and Hα modeling is required). Using optical high‐resolution spectra, we infer from CaII emission for young ultra low‐mass objects down to nearly the deuterium‐burning (planetary‐mass) limit. Our results, in combination with previous determinations of in CTTs, illustrate that the accretion rate declines steeply with mass, roughly as ∝ M_*² (albeit with considerable scatter). A similar relationship has been suggested by previous studies; we extend it down to nearly the planetary regime. The physical reason for this phenomenon is not yet clear; we discuss various possible mechanisms. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rotation and variability of young very low‐mass objects

Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar‐type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low‐mass stars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accretion and dynamical interactions in small-N star-forming clusters: N= 5

We present results from high-resolution hydrodynamical simulations that explore the effects of small-scale clustering in star-forming regions. A large ensemble of small- N clusters with five stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data.
Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s⁻¹, which might deprive low-mass star-forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and multiples is found to follow a distribution similar to that of observed long-period (uncircularized) binaries.
The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low-mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low-mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores.     

Variability in the Lambda Orionis cluster substellar domain

We present the first results on variability of very low mass stars and brown dwarfs belonging to the ∼5 Myr Lambda Orionis cluster (Collinder 69). We have monitored almost continuously in the J filter a small area of the cluster which includes 12 possible members of the cluster during one night. Some members have turned to be short‐term variables. One of them, LOri167, has a mass close to the planetary mass limit and its variability might be due to instabilities produced by the deuterium burning, although other mechanism cannot be ruled out. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low mass companions to white dwarfs

This paper summarizes the results of over 17 years of work searching for low mass stellar and substellar companions to more than 370 nearby white dwarfs. Roughly 60 low mass, unevolved companions were found and studied all together, with over 20 discovered in the last few years, including the first unambiguous brown dwarf companion to a white dwarf, GD 1400B. The resulting spectral type distributions for companions to white dwarfs and nearby cool field dwarfs are compared, and the implications for binary star formation are discussed. A brief analysis of GD 1400B, including new data, is also presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The origin of the initial mass function and its dependence on the mean Jeans mass in molecular clouds

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 9 and 3, respectively.
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars.     

The brown dwarf desert as a consequence of orbital migration

We show that the dearth of brown dwarfs in short-period orbits around Solar-mass stars – the brown dwarf desert – can be understood as a consequence of inward migration within an evolving protoplanetary disc. Brown dwarf secondaries forming at the same time as the primary star have masses which are comparable to the initial mass of the protoplanetary disc. Subsequent disc evolution leads to inward migration, and destruction of the brown dwarf, via merger with the star. This is in contrast with massive planets, which avoid this fate by forming at a later epoch when the disc is close to being dispersed. Within this model, a brown dwarf desert arises because the mass at the hydrogen-burning limit is coincidentally comparable to the initial disc mass for a Solar mass star. Brown dwarfs should be found in close binaries around very low mass stars, around other brown dwarfs, and around Solar-type stars during the earliest phases of star formation.     

ULMSF conference summary

Ten years on from the discovery of the first brown dwarf and the first exoplanet, how well have we progressed in our understanding of these low‐mass objects? In particular how well do we understand their formation? The strong impression from this conference was that the formation of brown dwarfs was just a continuation of the star formation process, no special additional mechanism is indicated. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Widely separated binary systems of very low‐mass stars

In this paper we review some recent detections of wide binary brown dwarf systems and discuss them in the context of the multiplicity properties of very low‐mass stars and brown dwarfs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultra low‐mass star and substellar formation in σ Orionis

The nearby young σ Orionis cluster (∼360 pc, ∼3 Myr) is becoming one of the most important regions for the study of ultra low‐mass star formation and its extension down to the mass regimes of the brown dwarfs and planetarymass objects. Here, I introduce the σ Orionis cluster and present three studies that the JOVIAN group is developing: a pilot programme of near‐infrared adaptive‐optics imaging of stars of the cluster, intermediate‐resolution optical spectroscopy of a large sample of stars of the cluster and a study of the mass function down to the planetary‐mass domain. This paper is a summary of the content of four posters that I presented in the Ultra low‐mass star formation and evolution Workshop, as single author or on behalf of different collaborations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The potential for Earth-mass planet formation around brown dwarfs

Recent observations point to the presence of structured dust grains in the discs surrounding young brown dwarfs, thus implying that the first stages of planet formation take place also in the substellar regime. Here, we investigate the potential for planet formation around brown dwarfs and very low-mass stars according to the sequential core accretion model of planet formation. We find that, for a brown dwarf mass 0.05 M_⊙, our models predict a maximum planetary mass of  ∼5   M_⊕  , orbiting with semimajor axis ∼ 1 au. However, we note that the predictions for the mass–semimajor axis distribution are strongly dependent upon the models chosen for the disc surface density profiles and the assumed distribution of disc masses. In particular, if brown dwarf disc masses are of the order of a few Jupiter masses, Earth-mass planets might be relatively frequent, while if typical disc masses are only a fraction of Jupiter mass, we predict that planet formation would be extremely rare in the substellar regime. As the observational constraints on disc profiles, mass dependencies and their distributions are poor in the brown dwarf regime, we advise caution in validating theoretical models only on stars similar to the Sun and emphasize the need for observational data on planetary systems around a wide range of stellar masses. We also find that, unlike the situation around solar-like stars, Type II migration is totally absent from the planet formation process around brown dwarfs, suggesting that any future observations of planets around brown dwarfs would provide a direct measure of the role of other types of migration.     

The empirical upper limit for mass loss of cool main sequence stars

The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than M_lim have lost an amount of mass equal to ΔM. The actual value of ΔM or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ΔM in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10^–11 M_⊙/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New brown dwarf candidates in the Pleiades

We have performed deep, wide‐field imaging on a ∼0.4 deg² field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ∼ 22 mag and I ∼ 20 mag, sufficient to detect brown dwarf candidates down to 40 M_J. We found 197 objects, whose location in the (I, R – I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co‐add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two‐dimensional χ² fitting (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

VISIR/VLT observations of brown dwarfs in Upper Scorpius

We present mid‐IR observations of a sample of very low‐mass stars and brown dwarfs in the Upper Scorpius association. The main goal of these observations is to derive the disk frequency and the disk properties of the sample in order to shed light on brown dwarf formation theories. The observations have been performed with VISIR at the Very Large Telescope (VLT, Paranal). A total of ten targets were observed and six were detected. In this paper we show preliminary results for two of them: DENIS‐PJ160334 and DENIS‐PJ161939. While the former shows mid‐IR fluxes consistent with pure photospheric emission, the latter displays strong mid‐IR excesses which are probably related with a circumstellar disk. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Status of the physics of substellar objects project

A full understanding of the properties of substellar objects is one of the major challenges facing astrophysics. Since their discovery in 1995, hundreds of brown dwarfs and extrasolar planets have been discovered. While these discoveries have enabled important comparisons with theory, observational progress has been much more rapid than the theoretical understanding of cool atmospheres. The reliable determination of mass, abundances, gravities and temperatures is not yet possible. The key problem is that substellar objects emit their observable radiation in the infrared region of the spectrum where our knowledge of atomic, molecular and line broadening data is poor. Here we report on the status of our PoSSO (Physics of SubStellar Objects) project. In order to understand brown dwarfs and extrasolar planets increasing more like those in our solar system, we are studying a wide range of processes. Here we give an update on the project and sketch an outline of atoms, molecules and processes requiring study. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)