A population of very young brown dwarfs and free-floating planets in Orion

view Abstract Citations (79) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Faintest Stars: The Luminosity and Mass Functions at the Bottom of the Main Sequence Tinney, Christopher G. Abstract We present IR K-band photometry of complete samples of VLM candidates constructed from IIIaF and IVN plates in 10 fields taken as part of the POSSII and UKSRC surveys. Using the I-K colors constructed for these stars we estimate a bolometric luminosity function which extends to M(Bol) = 13.75. We find significant evidence for a luminosity function decreasing toward these luminosities. We also find that our results are consistent with those of studies based on the Nearby Star sample, when those data are presented as a bolometric luminosity function. We convert our observed luminosity function into a mass function, which extends with reasonable statistics to 0.08 solar masses - the H-burning minimum mass. We find significant evidence for features in the mass function at these masses. Specifically, the mass function 'turns over' at 0.25 solar mass, goes through a local minimum at about 0.15 solar mass, and may increase again below 0.1 solar mass - none of these features are predicted by any of the current theories of star formation. Lastly, the mass density we observe just above the H-burning minimum mass makes it difficult to envisage brown dwarfs contributing significant quantities of missing mass without invoking either a mass function in this region significantly steeper than that seen for main-sequence stars, or an extremely low cutoff mass to the mass function. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173075 Bibcode: 1993ApJ...414..279T Keywords: Brown Dwarf Stars; Infrared Photometry; Main Sequence Stars; Star Formation; Stellar Luminosity; Stellar Mass; Astronomical Photometry; Bolometers; Extremely High Frequencies; Astrophysics; STARS: LOW-MASS; BROWN DWARFS; STARS: FORMATION; STARS: LUMINOSITY FUNCTION; MASS FUNCTION; TECHNIQUES: PHOTOMETRIC full text sources ADS | data products SIMBAD (432) CDS (1) Related Materials (1) Erratum: 1995ApJ...445.1017T

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 either a (substellar or planetary) companion or stellar activity. A companion causing the detected RV variations would have about a few Jupiter masses. We are planning further UVES observations in order to explore the nature of the detected RV variations. We also found that the RV dispersion is only ~ 2km/s indicating that there is probably no run-away brown dwarf among them. Additionally a search for companions by direct imaging with the HST and SOFI (NTT) has yielded to the detection of a few companion candidates in larger orbits.

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 periods for the two M5-M5.5 type very low-mass stars B34 and CHXR78C. In addition to the Gunn i and R band data, we have analysed JHK_s monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hours. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sini values as well as (R-i) color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand.

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 in the Chal star-forming cloud. Time-resolved high-resolution echelle spectra of Cha Hα 1-8 and Cha Hα 12 (M6-M8), B34 (M5), CHXR74 (M4.5), and Sz23 (M2.5) were taken with UVES at the VLT between 2000 and 2004. The precision achieved for the relative RVs range between 40 and 670 m s -1 and is sufficient to detect Jupiter mass planets around the targets. This is the first RV survey of very young BDs. It 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 subsample of ten BDs and very low-mass stars (VLMSs, M ≤ 0.12 M ○. , spectral types M5-M8) has constant RVs on time scales of 40 days and less. For this group, estimates of upper limits for masses of hypothetical companions range between 0.1 M Jup and 1.5 M Jup for assumed orbital separations of 0.1 AU. This hints at a rather small multiplicity fraction for very young BDs/VLMSs, for orbital separations of ≤0.1 AU and orbital periods of ≤40 days. Furthermore, the non-variable objects demonstrate the lack of any significant RV noise due to stellar activity down to the precision necessary to detect giant planets. Thus, very young BDs/VLMSs are suitable targets for RV surveys for planets. Three objects of the sample exhibit significant RV variations with peak-to-peak RV differences of 2-3 km s -1 . For the highest mass object observed with UVES (Sz 23, ∼0.3 M ○. ), the variations are on time scales of days, which might be explained by rotational modulation. On the other hand, the BD candidate Cha Ha 8 (M6.5) and the low-mass star CHXR 74 (M4.5) both display significant RV variations on times scales of ≥150 days, while they are both RV constant or show only much smaller amplitude variations on time scales of days to weeks, i.e. of the rotation periods. A suggested explanation for the detected RV variations of CHXR 74 and Cha Ha 8 is that they are caused by giant planets or very low-mass BDs of at least a few Jupiter masses orbiting with periods of several months or longer. Thus, the presented RV data indicate that orbital periods of companions to very young BDs and (very) low-mass stars are possibly several months or longer, and that orbital separations are ≥0.2 AU. This parameter range has not been covered for all targets yet, but will be probed by follow-up observations. Furthermore, we show that the scaled down equivalent to the BD desert found around solar-like stars would be a giant planet desert around BD and VLMS primaries, if formed by the same mechanism. The present data test its existence for the targets in the limited separation range of the survey. So far, no hints of companions in a giant planet desert have been found.

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 star forming regions to perform a study of stellar activity parameters on the as yet largest sample of young, very low mass objects. A similar relation between X-ray and bolometric luminosity, and H-alpha emission, respectively, as is known for T Tauri stars seems to hold for young objects down to the substellar limit, too. No signs for a change in X-ray activity are found on the transition to substellar masses.

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 dwarfs are considered to be 'failed stars' in the sense that they are born with masses between the least massive stars (0.072 solar masses, M(o)) and the most massive planets (approximately 0.013M(o)); they therefore serve as a critical link in our understanding of the formation of both stars and planets. Even the most fundamental physical properties of brown dwarfs remain, however, largely unconstrained by direct measurement. Here we report the discovery of a brown-dwarf eclipsing binary system, in the Orion Nebula star-forming region, from which we obtain direct measurements of mass and radius for these newly formed brown dwarfs. Our mass measurements establish both objects as brown dwarfs, with masses of 0.054 +/- 0.005M(o) and 0.034 +/- 0.003M(o). At the same time, with radii relative to the Sun's of 0.669 +/- 0.034R(o) and 0.511 +/- 0.026R(o), these brown dwarfs are more akin to low-mass stars in size. Such large radii are generally consistent with theoretical predictions for young brown dwarfs in the earliest stages of gravitational contraction. Surprisingly, however, we find that the less-massive brown dwarf is the hotter of the pair; this result is contrary to the predictions of all current theoretical models of coeval brown dwarfs.

We present an analysis of low-resolution infrared spectra for 20 brown dwarf candidates in the core of the ρ Ophiuchi molecular cloud. Fifteen of the sources display absorption-line spectra characteristic of late-type stars. By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types that are independent of reddening. Optical spectroscopy of one brown dwarf candidate confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatures and bolometric stellar luminosities are derived, enabling us to place our sample on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf candidates in this diagram with two sets of theoretical models in order to estimate their masses and ages. Considering uncertainties in placing the candidates in the H-R diagram, six objects consistently lie in the brown dwarf regime and another five objects lie in the transition region between stellar and substellar objects. The ages inferred for the sample are consistent with those derived for higher mass association members. Three of the newly identified brown dwarfs display infrared excesses at λ = 2.2 μm, suggesting that young brown dwarfs can have active accretion disks. Comparing our mass estimates of the brown dwarf candidates with those derived from photometric data alone suggests that spectroscopy is an essential component of investigations of the mass functions of young clusters.

view Abstract Citations (54) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radial Velocities of Very Low mass Stars and Candidate Brown Dwarf Members of the Hyades and Pleiades Stauffer, John R. ; Liebert, James ; Giampapa, Mark ; Macintosh, Bruce ; Reid, Neill ; Hamilton, Donald Abstract We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km s-1 for approximately 20 candidate very low mass members of the Hyades and Pleiades clusters. The radial velocities for the Hyades sample suggest that nearly all of these stars are indeed highly probable members of the Hyades. The faintest stars in the Hyades sample have masses of order 0.1 solar mass. We also obtained radial velocities for four candidate very low mass members of the Pleiades and two objects that are candidate BD Pleiads. All of these stars have apparent V magnitudes fainter than the Hyades stars we observed, and the resultant radial velocity accuracy is worse. We believe that the three brighter stars are indeed likely very low mass stellar members of the Pleiades, whereas the status of the two brown dwarf candidates is uncertain. The Hyades stars we have observed and the three Pleiades very low mass stars are the lowest mass members of any open cluster whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromospheric activity at the boundary where stars are expected to become fully convective (M approximately equals 0.3 solar mass) in either cluster. In the Pleiades, however, there may be a decrease in chromospheric activity for stars with (V-I)K greater than 3.5 (M less than or equal to 0.1 solar mass). Publication: The Astronomical Journal Pub Date: July 1994 DOI: 10.1086/117054 Bibcode: 1994AJ....108..160S Keywords: Astronomical Spectroscopy; Dwarf Stars; Radial Velocity; Star Clusters; Stellar Spectra; Charge Coupled Devices; Pleiades Cluster; Stellar Mass; Astrophysics; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: HYADES; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: PLEIADES; STARS: BROWN DWARFS full text sources ADS | data products SIMBAD (33)

Context. Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. Aims. We aim to search for accreting young brown dwarf candidates in a sample of 85 nearby late-M dwarfs. Methods. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late- M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and Hα emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Results. Among the 85 late-M dwarfs, only DENIS-P J1538317−103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of ~1 Myr and a mass of 47 MJ. The Hα emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to 10−7.9 M⊙ yr−1. Conclusions. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.

By combining infrared photometry from the Two Micron All Sky Survey with new optical imaging and spectroscopy, I have performed a search for young low-mass stars and brown dwarfs in two regions encompassing a total area of 4 deg2 in the Taurus star-forming region (τ ~ 1 Myr). From this work I have discovered 15 new members of Taurus. In addition, I present seven new members outside of these areas from the initial stage of a survey of all of Taurus. These 22 objects exhibit spectral types of M4.5-M9.25 and masses of 0.3-0.015 M☉ according to the theoretical evolutionary models of Baraffe & Chabrier, seven of which are likely to be brown dwarfs. Emission in Hα, He I, Ca II, [O I], and [S II] and excess emission in optical and near-infrared bands among some of these objects suggest the presence of accretion, outflows, and circumstellar disks. These results add to the body of work—initiated by the first detections of brown dwarf disks by Comeron and coworkers in 1998 and Luhman in 1999—indicating that disks around young brown dwarfs are relatively common. The results from the 4 deg2 survey have been combined with previous studies of Taurus to arrive at an initial mass function (IMF) for a total area of 12.4 deg2. As in the previous IMFs for Taurus, the updated IMF peaks at a higher mass (0.8 M☉) than the mass functions in IC 348 and Orion (0.1-0.2 M☉). Meanwhile, the deficit of brown dwarfs in Taurus appears to be less significant (×1.4-1.8) than that found in earlier studies (×2) because of a slightly higher brown dwarf fraction in the new IMF for Taurus and a lower brown dwarf fraction in the new spectroscopic IMF for the Trapezium from Slesnick and coworkers. The spatial distribution of the low-mass stars and brown dwarfs discovered in the two new survey areas closely matches that of the more massive members. Thus, on the degree size scales (~3 pc) probed to date, there is no indication that brown dwarfs form through ejection.

We have measured the surface magnetic flux on four accreting young brown dwarfs and one non-accreting young very low-mass star utilizing high resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, 2MASS J1207334$-$393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilo-Gauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3-$\sigma$ upper limit for the magnetic flux in 2MASS J1207334$-$393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred Gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of five or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently have an easy physical explanation.

view Abstract Citations (17) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Proto--Brown Dwarfs. II. Results in the Ophiuchus and Taurus Molecular Clouds Pound, Marc W. ; Blitz, Leo Abstract We expand our proto-brown dwarf search to include two nearby star-forming regions in the Ophiuchus and Taurus molecular clouds. In both molecular line and continuum searches in the Ophiuchus B and Barnard 18 (in Taurus) star-forming regions, we find no clear-cut evidence of any proto-brown dwarfs. This lack of proto-brown dwarfs is surprising; even if the initial mass function (IMF) (given by dN/dM proportional to M-alpha) were flat (alpha = 1), we would expect to have found approximately 10 objects. We do find, however, a few candidate objects near our detection limit (approximately 0.02 solar mass) which deserve further scrutiny. We find 21 gravitationally bound clumps distributed in mass with a spectral index alpha = 1.1 +/- 0.2. However, there are fewer low-mass clumps (approximately less than 0.1 solar mass) than would be expected from extrapolation of any reasonable mass function, including the well-known giant molecular cloud clump mass spectrum. If the IMF follows the clump mass spectrum below 0.08 solar mass, as it does at higher masses, our results in Oph B imply that, unless some undetermined process causes the production of many more low-mass clumps, the IMF is falling at masses below 0.08 solar mass, even if all our candidate objects turn out to be true proto-brown dwarfs. This work presents evidence that there is a mass range (0.1-0.25 solar mass) where the molecular cloud clump mass spectrum has the same slope stellar IMF (alpha approximately 1). This is an indication that there is indeed a direct relationship between clump mass and subsequent stellar mass and subsequent stellar mass at the scale of a few tenths of a solar mass. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175602 Bibcode: 1995ApJ...444..270P Keywords: Brown Dwarf Stars; Clumps; Interstellar Matter; Molecular Clouds; Stellar Mass; Astronomical Maps; Emission Spectra; Mass Spectra; Molecular Spectra; Star Formation; Astronomy; ISM: CLOUDS; STARS: LOW-MASS; BROWN DWARFS; STARS: LUMINOSITY FUNCTION; MASS FUNCTION full text sources ADS | data products SIMBAD (33) Related Materials (1) Part 1: 1993ApJ...418..328P

Rotational studies at a variety of ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the significant lack of known rotational periods in that mass range. We provide for the first time information on rotational periods for a large sample of young VLM stars and BDs. This extensive rotational period study in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2m telescope on La Silla, Chile. Accurate I-band photometry of 2908 stars was obtained, extending three magnitudes deeper than previous studies in the ONC. We found 487 periodic variables with estimated masses between 0.5 Msun and 0.015 Msun, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. In addition, 808 objects show non-periodic brightness variations. We study the dependence of the period distribution on mass and variability level and compare this with known higher mass objects in the ONC (Herbst et al. 2002) and with the 2 Myr old cluster NGC 2264 (Lamm et al., 2004). We find that substellar objects rotate on average faster than the VLM stars. Our rotational data also suggest a dependence of the rotational periods on position within the field, which can be explained by a possible age spread in the ONC. In addition, periodic variables with larger peak-to-peak amplitudes rotate on average slower than those with small peak-to-peak amplitude variations, which can possibly be explained by different magnetic field topologies.

Aims: We investigated the X-ray emission from young stars and brown dwarfs in the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass, presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory. Methods: We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public catalogues. On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and searched for short-term variability. We also looked for long-term variability by comparing with previous X-ray observations. Results: Among the 107 detected X-ray sources, there were 70 cluster stars with known signposts of youth, two young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two galaxies with optical-infrared counterpart. The remaining sources had extragalactic nature. Based on a robust Poisson-chi^2 analysis, nine cluster stars displayed flares or rotational modulation during the HRC-I observations, while other eight stars and one brown dwarf showed long-term X-ray flux variations. We constructed a cluster X-ray luminosity function from O9.5 (~18 Msol) to M6.5 (~0.06 Msol). We found: a tendency of early-type stars in multiple systems or with spectroscopic peculiarities to display X-ray emission, that the two detected brown dwarfs and the least-massive star are among the sigma Orionis objects with the highest L_X/L_J ratios, and that a large fraction of known classical T Tauri stars in the cluster are absent in this and other X-ray surveys. We concluded that dozens X-ray sigma Orionis stars and brown dwarfs are still to be detected [abridged].