Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A leads to a higher contrast between the accretion and photospheric emission relative to higher-mass counterparts. The mass accretion rates measured here are systematically 4-7 times larger than those from H-alpha emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with H-alpha 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for accretors in the Taurus Molecular Cloud.
We report on searches for radio emission from three of the nearest known young brown dwarfs using the Very Large Array. We have obtained sensitive upper limits on 3.6cm emission from 2MASSW J1207334-393254, TWA~5B and SSSPM J1102-3431, all of which are likely members of the $\sim$8-Myr-old TW Hydrae association. We derive constraints on the magnetic field strength and the number density of accelerated electrons, under the assumption that young brown dwarf atmospheres are able to produce gyrosynchrotron emission, as seems to be indicated in older brown dwarfs. For the young brown dwarf TWA~5B, …
We review our the status of the photometric searches of brown dwarfs and vely low mass stars in several young open clusters and present some results on the spectroscopic follow-up, including new cluster ages based on the position of the lithium depletion boundary.
We report the discovery of a population of late-M and L field dwarfs with unusual optical and near-infrared spectral features that we attribute to low gravity -- likely uncommonly young, low-mass brown dwarfs. Many of these new-found young objects have southerly declinations and distance estimates within 60 parsecs. Intriguingly, these are the same properties of recently discovered, nearby, intermediate-age (5-50 Myr), loose associations such as Tucana/Horologium, the TW Hydrae association, and the Beta Pictoris moving group. We describe our efforts to confirm cluster membership and to further…
Studies of disks around young brown dwarfs are of paramount importance to our understanding of the origin, diversity and early evolution of sub-stellar objects. Here we present first results from a systematic search for disk emission in a spectroscopically confirmed sample of young objects near or below the sub-stellar boundary in a variety of star-forming regions. Our VLT and Keck L'-band observations of the sigma Orionis and TW Hydrae associations suggest that if a majority of brown dwarfs are born with disks, at least the inner regions of those disks evolve rapidly, possibly clearing out wi…
We have identified a large sample of probable low mass members of the young open cluster IC2391 based on optical (VRIZ) and Infrared (JHK$_s$) photometry. Our sample includes 50 probable members and 82 possible members, both very low mass stars and brown dwarfs. We also provide accurate positions for these stars and brown dwarf candidates derived from red UK Schmidt plates measured using the microdensitometer SuperCOSMOS. Assuming an age of 53 Myr, we estimate that we have reached a mass of 0.025 M$_\odot$, if the identified objects are indeed members of IC2391.
Recent surveys have identified sub-stellar objects down to planetary masses in nearby star-forming regions. Reliable determination of the disk frequency in young brown dwarfs is of paramount importance to understanding their origin. Here we report the results of a systematic study of infrared L'-band (3.8-micron) disk excess in ~50 spectroscopically confirmed objects near and below the sub-stellar boundary in several young clusters. Our observations, using the ESO Very Large Telescope, Keck I and the NASA Infrared Telescope Facility, reveal that a significant fraction of brown dwarfs harbor di…
A photometric monitoring campaign of brown dwarfs in the ChaI star forming region in the i and R band revealed significant periodic variations of the three M6.5-M7 type brown dwarf candidates ChaHa2, 3 and 6 (Joergens et al. 2002). These are the first rotational periods for very young (1-5 Myr) brown dwarfs and among the first rotational periods for brown dwarfs at all. The relatively long periods of 2.2 to 3.4 days as well as vsini values (Joergens & Guenther 2001) indicate that our targets are moderately fast rotators in contrast to very rapidly rotating old brown dwarfs. The periods for the…
Very low-mass stars and brown dwarfs can undergo pulsational instability excited by central deuterium burning during the initial phases of their evolution. We present the results of evolutionary and nonadiabatic linear stability models that show the presence of unstable fundamental modes. The pulsation periods vary bewteen ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 M$_\odot$ brown dwarf. The results are rather insensitive to variations in the input physics of the models. We show the location of the instability strip in the HR and c-m diagrams and discuss the observational searches for youn…
We review recent theoretical progress aimed at understanding the formation and the early stages of evolution of giant planets, low-mass stars and brown dwarfs. Calculations coupling giant planet formation, within a modern version of the core accretion model, and subsequent evolution yield consistent determinations of the planet structure and evolution. Because of the uncertainties in the initial conditions, however, it is not possible to say whether young planets are faint or bright compared with low-mass young brown dwarfs. We review the effects of irradiation and evaporation on the evolution…
We present some results from a systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars. We find that ~75% of our sample show intrinsic IR excesses, indicative of circum(sub)stellar disks. The observed excesses are correlated with Halpha emission, consistent with a common disk accretion origin. Because the excesses are modest, conventional analyses using only IR colors would have missed most of the sources with disks. In the same star-forming regions, we find that disks around brown dwarfs and T Tauri stars are contemporaneous; assuming coevali…
We have studied twelve very young (1-5Myr) bona fide and candidate brown dwarfs in the ChaI 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 (8.2m), 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 spect…
We have imaged two bona-fide brown dwarfs with TReCS/GEMINI-S and find mid-infrared excess emission that can be explained by optically thick dust disk models. In the case of the young ($\approx$2Myr) Cha H$α$1 we measure fluxes at 10.4$μ$m and 12.3$μ$m that are fully consistent with a standard flared disk model and prominent silicate emission. For the $\approx$ 10Myr old brown dwarf 2MASS1207-3932 located in the TW Hydrae association we find excess emission at 8.7$μ$m and 10.4$μ$m with respect to its photosphere, and confirm disk accretion as likely cause of its strong activity. Disks around b…
Studying the earliest stages in the birth of stars is crucial for understanding how they form. Brown dwarfs with masses between that of stars and planets are not massive enough to maintain stable hydrogen-burning fusion reactions during most of their lifetime. Their origins are subject to much debate in recent literature because their masses are far below the typical mass where core collapse is expected to occur. We present the first confirmed evidence that brown dwarfs undergo a phase of molecular outflow that is typical of young stars. Using the Submillimeter Array, we have obtained a map of…
How frequent are brown dwarf binaries? Do brown dwarfs have planets? Are current theoretical pre-main-sequence evolutionary tracks valid down to the substellar regime? - Any detection of a companion to a brown dwarf takes us one step forward towards answering these basic questions of star formation. We report here on a search for spectroscopic and visual companions to young brown dwarfs in the Cha I star forming cloud. Based on spectra taken with UVES at the VLT, we found significant radial velocity (RV) variations for five bona-fide and candidate brown dwarfs in Cha I. They can be caused by e…
I review here the results of the first RV survey for spectroscopic companions to very young brown dwarfs (BDs) and (very) low-mass stars in the ChaI star-forming cloud with UVES at the VLT. This survey studies the binary fraction in an as yet unexplored domain not only in terms of primary masses (substellar regime) and ages (a few Myr) but also in terms of companion masses (sensitive down to planetary masses) and separations (< 1 AU). The UVES spectra obtained so far hint at spectroscopic companions of a few Jupiter masses around one BD and around one low-mass star (M4.5) with orbital periods …
We have combined new I, J, H, and Ks imaging of portions of the Chamaeleon II, Lupus I, and Ophiuchus star-forming clouds with 3.6 to 24 micron imaging from the Spitzer Legacy Program, "From Molecular Clouds to Planet Forming Disks", to identify a sample of 19 young stars, brown dwarfs and sub-brown dwarfs showing mid-infrared excess emission. The resulting sample includes sources with luminosities of 0.5>log(L/Lsun)>-3.1. Six of the more luminous sources in our sample have been previously identified by other surveys for young stars and brown dwarfs. Five of the sources in our sample have nomi…
Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc, age~50Myr, solar metallicity). Here we present the results from our photometric survey and from a spectroscopic follow-up of the central part of the survey.
We present infrared H- and K-band spectra of a companion candidate 3" north of the young star GSC 08047-00232, a probable member of the nearby young Horologium association. From previously obtained JHK-band colors and the magnitude difference between primary and companion candidate, the latter could well be substellar (Neuh"auser et al. 2003) with the spectral type being roughly M7-L9 from the JHK colors (Chauvin et al. 2003). With the H- and K-band spectra now obtained with ISAAC at the VLT, the spectral type of the companion candidate is found to be M6-9.5. Assuming the same age and distance…
We present near-infrared (1.0-2.4 micron) spectra confirming the youth and cool effective temperatures of 6 brown dwarfs and low mass stars with circumstellar disks toward the Chamaeleon II and Ophiuchus star forming regions. The spectrum of one of our objects indicates that it has a spectral type of ~L1, making it one of the latest spectral type young brown dwarfs identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and giant stars, we define a 1.49-1.56 micron H2O index capable of determining spectral type to within 1 sub-type, independent of gravity. We have also defin…
We present deep VLT/NACO infrared imaging and spectroscopic observations of the brown dwarf 2MASSWJ1207334-393254, obtained during our on-going adaptive optics survey of southern young, nearby associations. This 25 MJup brown dwarf, located 70 pc from Earth, has been recently identified as a member of the TW Hydrae Association (age 8 Myr). Using adaptive optics infrared wavefront sensing to acquire sharp images of its circumstellar environment, we discovered a very faint and very red object at a close separation of 780 mas (55 AU). Photometry in the H, Ks and L' bands and upper limit in J-band…
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 …
Searches for companions of brown dwarfs by direct imaging mainly probe orbital separations > 3-10 AU. On the other hand, previous radial velocity surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6 AU. It has been speculated that the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. This work extends high-precision radial velocity surveys of brown dwarfs for the first time out to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in ChaI was deter…
We present mid-infrared photometry of three very young brown dwarfs located in the $ρ$ Ophiuchi star-forming region -- GY5, GY11 and GY310 --obtained with the Subaru 8-meter telescope. All three sources were detected at 8.6 and 11.7$μ$m, confirming the presence of significant mid-infrared excess arising from optically thick dusty disks. The spectral energy distributions of both GY310 and GY11 exhibit strong evidence of flared disks; flat disks can be ruled out for these two brown dwarfs. The data for GY5 show large scatter, and are marginally consistent with both flared and flat configurations…
Based on high-resolution UVES spectra we found that the radial velocity (RV) dispersion of nine of twelve known young bona fide and candidate brown dwarfs in the Cha I dark cloud is 2.0 km/s, i.e. significantly smaller than the RV dispersion of T Tauri stars in Cha I (3.6 km/s) and only slightly larger than the dispersion of the surrounding molecular gas (1.2 km/s) (Mizuno et al. 1999). This result indicates that the majority of these brown dwarfs are not ejected with high velocity out of a dense region as proposed by some formation scenarios for brown dwarfs. The mean RV values are consistent…
We present results of a radial velocity (RV) survey for planets and brown dwarf (BD) companions to very young BDs and (very) low-mass stars (VLMSs) in ChaI. High-resolution echelle spectra of ChaHa1-8 and 12 (M6-M8), and B34, CHXR74, Sz23 (M2.5-M5) were taken with UVES / VLT between 2000 and 2004. The achieved precision (40 to 670 m/s) is sufficient to detect Jupiter mass planets around the targets. This first RV survey of very young BDs probes multiplicity which is a key parameter for formation in an as yet unexplored domain, in terms of age, mass, and orbital separation. We find that the sub…
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…
Aims: We look for new regions for the search of substellar objects. Methods: Two circular areas, 45 arcmin-radius each, centred on the young massive star systems Alnilam and Mintaka in the Orion Belt, have been explored. The regions are very young (less than 10 Ma), have low extinction, and are neighbours to sigma Orionis (~3 Ma), a young open cluster very rich in brown dwarfs and planetary-mass objects. We have used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and 2MASS catalogues, 10 control fields at similar galactic latitudes, and X-ray, mid-infrared and spectroscopic da…
We report differential I-band and J-band photometry of S Ori 45, a cool (M8.5), young (1-8 Myr) brown dwarf of the sigma Orionis cluster with a likely mass estimated at around 20 times the mass of Jupiter. We detect variability (amplitudes ranging from 34 to 81 mmag) and observe a modulation at a period of 2.5-3.6 h in both optical and near-infrared light curves. The most recent optical data set, however, presents a modulation at the very short period of 46.4+/-1.5 min, which remains a mystery. The origin of the 2.5-3.6 h modulation is analized in terms of various scenarios: inhomogeneous feat…
We present high-resolution optical spectra of 15 objects near or below the sub-stellar limit in the Upper Scorpius and $ρ$ Ophiuchus star-forming regions. These spectra, obtained with the HIRES instrument on the Keck I telescope, are used to investigate disk accretion, rotation and activity in young very low mass objects. We report the detection of a broad, asymmetric H$α$ emission line in the $ρ$ Oph source GY 5 which is also known to harbor mid-infrared excess, consistent with the presence of an accreting disk. The H$α$ profiles of the Upper Sco objects suggest little or no on-going accretio…
We obtained medium-resolution spectra of the bona-fide brown dwarf Cha H$α$ 1, the five brown dwarf candidates Cha H$α$ 2 to 6, two additional late M-type brown dwarf candidates, all originally selected by H$α$ emission, and four previously known T Tauri stars, all located in Chamaeleon I. The spectral types of our targets range down to M8. We show their spectra and also list their IJK magnitudes from DENIS. All objects have radial velocities consistent with kinematic membership to Cha I and show Li 6708Åabsorption. Our Cha I brown dwarf candidates with lithium are young or sub-stellar or both…
We define a sample designed to select low-mass T Tauri stars and young brown dwarfs using DENIS data in the Chamaeleon II molecular cloud. We use a star count method to construct an extinction map of the Chamaeleon II cloud. We select our low-mass T Tauri star and young brown dwarf candidates by their strong infrared colour excess in the I-J/J-K_s colour-colour dereddened diagram. We retain only objects with colours I-J>2, and spatially distributed in groups around the cloud cores. This provides a sample of 70 stars of which 4 are previously known T Tauri stars. We have carefully checked the r…
In the poster presented in Cool Star 15, we analyzed the effect of disk accretion on the evolution of very low mass pre-main sequence stars and young brown dwarfs and the resulting uncertainties on the determination of masses and ages. We use the Lyon evolutionary 1-D code assuming a magnetospheric accretion process, i.e., the material falls covering a small area of the radiative surface, and we take into account the internal energy added from the accreted material as a free parameter $ε$. Even if the approach to this problem is phenomenological, our formalism provides important hints about ch…
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project (COUP) to study the X-ray properties of 34 spectroscopically-identified brown dwarfs with near-infrared spectral types between M6 and M9 in the core of the Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray sources. The apparently low detection rate is in many cases related to the substantial extinction of these brown dwarfs; considering only the BDs with $A_V \leq 5$ mag, nearly half of the objects (7 out of 16) are detected in X-rays. Our 10-day long X-ray lightcurves of these objects exhibit str…
We present first results from the Palomar Adaptive Optics Survey of Young Stars conducted at the Hale 5 m telescope. Through direct imaging we have discovered a brown dwarf and two low-mass stellar companions to the young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Per, and confirmed a previously suspected companion to RX J0329.1+0118 (Sterzik et al. 1997), at respective separations of 0.95" (43 AU), 0.74" (25 AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of each binary system is established through common proper motion and/or low-resolution infrared spectroscopy. B…
The white dwarf Giclas 29-38 has attracted much attention due to its large infrared excess and the suggestion that excess might be due to a companion brown dwarf. We observed this object using speckle interferometry at the Keck telescope, obtaining diffraction-limited resolution (55 milliarcseconds) at K band, and found it unresolved. Assuming the entire K band excess is due to a single point-like companion, we place an upper limit on the binary separation of 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. This result, combined with astroseismological data and other images of…
We examine 14 hot white dwarfs for signs of infrared excess using the Spitzer Space Telescope. Twelve of the objects are massive white dwarfs which have been suggested to be the result of binary mergers. The remaining two objects are undermassive white dwarfs which again may be the result of mergers or the inspiral of a substellar companion. In no case do we find any evidence for significant infrared excesses out to wavelengths of 8 microns. This places strong constraints on the presence of orbiting dust and weaker constraints on the presence of close substellar companions.
The birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows. Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs amongst newborn brown dwarfs: these are the so-called 'failed stars', with masses between stars and planets, that never manage to reach temperatures high enough for normal hydrogen fusion to occur. Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-…
We have studied the photometric variability of very young brown dwarfs and very low-mass stars (masses well below 0.2 M_sun) in the ChaI star forming region. We have determined photometric periods in the Gunn i and R band for the three M6.5-M7 type brown dwarf candidates ChaHa2, ChaHa3 and ChaHa6 of 2.2 to 3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced they correspond to moderately fast rotational velocities, which is fully consistent with their v sini values and their relatively large radii. We have also determined …
We review the current state of our knowledge concerning the rotation and angular momentum evolution of young stellar objects and brown dwarfs from a primarily observational view point. Periods are typically accurate to 1% and available for about 1700 stars and 30 brown dwarfs in young clusters. Discussion of angular momentum evolution also requires knowledge of stellar radii, which are poorly known for pre-main sequence stars. It is clear that rotation rates at a given age depend strongly on mass; higher mass stars (0.4-1.2 M$_\odot$) have longer periods than lower mass stars and brown dwarfs.…
Over the past year, we have conducted a multi-faceted program to investigate the origin and early evolution of brown dwarfs. Using high-resolution Keck optical spectra of ~30 objects near and below the sub-stellar boundary in several star-forming regions, we present compelling evidence for a T Tauri-like accretion phase in young brown dwarfs. Our systematic study of infrared L'-band (3.8-micron) disk excess in ~50 spectroscopically confirmed young very low mass objects reveal that a significant fraction of brown dwarfs harbor disks at a very young age. Their inner disk lifetimes do not appear …
We have carried out an extensive search for X-ray emission from young, very low-mass objects near and beyond the substellar limit, making use of archived ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma Ori we identify three Brown Dwarf candidates with X-ray sources; in Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list published earlier. We combine this data with all previously X-ray detected Brown Dwarfs and Brown Dwarf candidates in young stellar associations and …
Forty new low mass members with spectral types ranging from M4-M9 have been confirmed in the Orion Molecular Cloud 2/3 region. Through deep, I, z', J, H, K photometry of a 20' x 20' field in OMC 2/3, we selected brown dwarf candidates for follow-up spectroscopy. Low resolution far-red and near-infrared spectra were obtained for the candidates, and 19 young brown dwarfs in the OMC 2/3 region are confirmed. They exhibit spectral types of M6.5-M9, corresponding to approximate masses of 0.075-0.015 M_solar using the evolutionary models of Baraffe et al. (1998). At least one of these bona fide youn…
We present the results of a nonadiabatic, linear stability analysis of models of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium burning phase in the center. We find unstable fundamental modes with periods varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The growth time of the instability decreases with decreasing mass and remains well below the deuterium burning time scale in the mass range considered (0.1--0.02 Msun). These results are robust against variations of the relevant input physics in the evolutionary models. We identify possible candid…
We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show (H-K) colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrast…
We report on the first dedicated monitoring campaign of spectroscopic variability in young brown dwarfs. High-resolution optical spectra of six targets in nearby star-forming regions were obtained over 11 nights between 2005 January-March on the Magellan 6.5m telescope. We find significant variability in Halpha and a number of other emission lines related to accretion and outflow processes on a variety of timescales ranging from hours to weeks to years. The most dramatic changes are seen for 2M1207, 2M1101 and ChaI-ISO217. We observe possible accretion rate changes by about an order of magnitu…
I present the results of a search for new low-mass stars and brown dwarfs in the Eta Cha and Eps Cha young associations. Within radii of 1.5 and 0.5 deg surrounding Eta Cha and Eps Cha, respectively, I have constructed color-magnitude diagrams from DENIS and 2MASS photometry and have obtained spectra of the candidate low-mass members therein. The five candidates in Eta Cha are classified as four field M dwarfs and one carbon star. No new members are found in this survey, which is complete for M_sun=0.015-0.15 according to the evolutionary models of Chabrier and Baraffe. Thus, an extended popul…
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 analyse pre-Main Sequence evolutionary tracks for low mass stars with masses $m \le 1.4 \msol$ based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages $\simle$ 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravi…
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…
I report here the detection of two bona-fide brown dwarfs by XMM-Newton: [GY92] 141 in the rho-Ophiuchus star forming region and DENIS-P J155601-233809 in the Upper Scorpius OB association. The two objects have been detected with luminosities of Lx=8.35\pm2.86*10^28 erg/s and Lx=6.54\pm1.35*10^28 erg/s respectively, corresponding to luminosity ratios of log(Lx/Lbol)= -2.07 and log(Lx/Lbol)= -2.69. The two sources were close to the limit of detection of the instruments at only 2~3-sigma above the background level, and no significant flare or variation could be detected during the 48.3ks and 33.…
We investigate the sensitivity to temperature and gravity of the strong absorption features in the J- and K-band spectra of substellar objects. We compare the spectra of giants and young M dwarfs (of low gravity) to field M and L dwarfs (of high gravity) and to model spectra from the Lyon group. We find that low-resolution spectra of M4 - M9 stars and young brown dwarfs at R ~ 350 and S/N > 70 can determine the spectral type to a precision of pm 1 subtype, using the H2O and CO bands, and can measure the surface gravity to pm 0.5 dex, using the atomic lines of KI and NaI. This result points tow…
We present the detection of a young brown dwarf companion DH Tau B associated with the classical T Tauri star DH Tau. Near-infrared coronagraphic observations with CIAO on the Subaru Telescope have revealed DH Tau B with H = \~15 mag located at 2.3" (330 AU) away from the primary DH Tau A. Comparing its position with a Hubble Space Telescope archive image, we confirmed that DH Tau A and B share the common proper motion, suggesting that they are physically associated with each other. The near-infrared color of DH Tau B is consistent with those of young stellar objects. The near-infrared spectra…
As part of a multi-faceted program to investigate the origin and early evolution of sub-stellar objects, we present high-resolution Keck optical spectra of 14 very low mass sources in the IC 348 young cluster and the Taurus star-forming cloud. All of our targets, which span a range of spectral types from M5 to M8, exhibit moderate to very strong H$α$ emission. In half of the IC 348 objects, the H$α$ profiles are broad and asymmetric, indicative of on-going accretion. Of these, IC348-355 (M8) is the lowest mass object to date to show accretion-like H$α$. Three of our ~M6 IC 348 targets with bro…
Multi-band (0.9 to 1.6 um) images of the TW Hydrae Association (TWA) brown dwarf, 2MASS J1207334-393254 (also known as 2M1207), and its candidate planetary mass companion (2M1207b) were obtained on 2004 Aug 28 and 2005 Apr 26 with HST/NICMOS. The images from these two epochs unequivocally confirm the two objects as a common proper motion pair (16.0 sigma confidence). A new measurement of the proper motion of 2M1207 implies a distance to the system of 59+-7 pc and a projected separation of 46+-5 AU. The NICMOS and previously published VLT photometry of 2M1207b, extending overall from 0.9 to 3.8…
Following the recent classification of the X-ray detected object V410 x-ray 3 with a young brown dwarf candidate (Briceno et al. 1998) and the identification of an X-ray source in Chamaeleon as young bona-fide brown dwarf (Neuhaeuser & Comeron 1998), we investigate all ROSAT All-Sky Survey and archived ROSAT PSPC and HRI pointed observations with bona-fide or candidate brown dwarfs in the field of view with exposure times ranging from 0.13 to 221 ks, including dedicated 64 ks and 42 ks deep ROSAT HRI pointed observations on the low-mass star BRI0021-0214 and the brown dwarf Calar 3, respective…
We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project, to study gravity sensitive features as indicators of youth in brown dwarfs. Low-resolution (R~2000) J-band and optical (R~1000) observations using NIRSPEC and LRIS at the W.M. Keck Observatory reveal transitions of TiO, VO, K I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type giants and in old field dwarfs we show that they are sensitive to the gravity (g = GM/R^2) of the object. Using low-gravity spectral signatures as age indicators, we observed and analyzed J-band and optical spec…
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 …
I present new results from a continuing program to identify and characterize the low-mass stellar and substellar populations in the young cluster IC 348 (1-10~Myr). Optical spectroscopy has revealed young objects with spectral types as late as M8.25. The intrinsic J-H and H-K colors of these sources are dwarf-like, whereas the R-I and I-J colors appear intermediate between the colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are reproduced well with averages of standard dwarf and giant spectra, suggesting that such averages should be used in the classification of young…
Brown dwarfs occupy the important region in the mass range between stars and planets. Their existence, ambigious 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, Gl 503.2 (G2V), HD 102982 (G3V), and Gl 577 (G5V). The companion to Gl 577 is a possible binary brown dwarf, according to evolution ary models. A dynamical determination of the components' masses will be achievable in the near future and be…
We report the results of deep infrared observations of brown dwarf candidates in the Trapezium cluster in Orion. Analysis of the JHK color-color diagram indicates that a large fraction (approx 65% +/- 15%) of the observed sources exhibit infrared excess emission. This suggests the extreme youth of these objects and in turn, provides strong independent confirmation of the existence of a large population of substellar objects in the cluster. Moreover, this suggests that the majority of these substellar objects are presently surrounded by circumstellar disks similar to the situation for the stell…
