Magellan Echelle Spectroscopy of TW Hydrae Brown Dwarfs

We present high-resolution optical spectroscopy of four candidate members of the nearby TW Hydrae young association including three brown dwarfs (2MASS 1207-3932, 2MASS 1139-3159 and TWA 5B) and one T Tauri multiple star (TWA 5A). Using echelle spectra from the Magellan Baade 6.5-meter telescope, we confirm the pre-main sequence status and cluster membership of the substellar candidates, through the detection of LiI, NaI consistent with low gravity, and radial velocity. Given their late spectral type (~M8) and the youth of the association (age ~10Myr), cluster membership certifies these three objects as very low-mass young brown dwarfs. One of them (2MASS 1207-3932) shows strong emission both in the Hydrogen Balmer series (H_alpha to H_epsilon) and in HeI (4471, 5876, 6678 and 7065 A), compared to other young brown dwarfs of similar spectral type. The H_alpha line is also relatively broad (10% width ~200 km/s) and asymmetric. These characteristics suggest that 2MASS 1207-3932 is a (weak) accretor. While we cannot rule out activity, comparison to a flaring field dwarf implies that such activity would have to be quite anomalous. The verification of accretion would make 2MASS 1207-3932 the oldest actively accreting brown dwarf known to date, suggesting that inner disk lifetimes in substellar objects can be comparable to those in stars, consistent with a similar formation mechanism. Finally, TWA 5A appears to be a variable accretor: observations separated by two days show broad accretion-like H_alpha (10% width \~270 km/s), with significant changes in the H_alpha profile, as well as in the strengths of HeI, Na D and [OI 6300]. TWA 5A is known to be a close triple; thus, our result implies that long-lived disks can exist even in multiple systems.Comment: accepted for publication in The Astrophysical Journal Letter

The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo

18 pages, 15 figures and 5 tables accepted in A&AWe performed a deep wide field optical survey of the young (~100-150 Myr) open cluster Blanco1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.The survey covers 2.3 square degrees in the I and z-bands down to I~z~24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I,I-z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. We find that our survey should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range, including 30-40 brown dwarfs. The candidate's radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison between the Blanco1 mass function, other young open clusters' MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution

A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II

We have obtained low- and medium-resolution optical spectra of 20 candidate young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud, using the Magellan Baade telescope. We analyze these data in conjunction with near-infrared photometry from the 2-Micron All Sky Survey. We find that one target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of forbidden emission lines. These signatures are usually associated with accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it. Therefore, we propose that C41 is a classical T Tauri object near the sub-stellar boundary. Thirteen other targets in our sample have continuum spectra without intrinsic absorption or emission features, and are difficult to characterize. They may be background giants or foreground field stars not associated with the cloud or embedded protostars, and need further investigation. The six remaining candidates, with moderate reddening, are likely to be older field dwarfs, given their spectral types, lack of lithium and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200

An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters

We present the results of a high-resolution imaging survey for brown dwarf (BD) binaries in two open clusters. The observations were carried out with WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in the Pleiades with separations in the range 0".094--0".058, corresponding to projected separations between 11.7~AU and 7.2~AU. No binaries were found among the alpha Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Halpha in emission and LiI absorption obtained from Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that BD binaries wider than 12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs with confirmed proper motion membership in the Pleiades, corresponding to a binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited to the separation range 7-12~AU and their mass ratios are larger than 0.7. The relatively high binary frequency (>10%), the bias to separations smaller than about 15 AU and the trend to high mass ratios (q>0.7) are fundamental properties of BDs. Current theories of BD formation do not appear to provide a good description of all these properties.Comment: Accepted by ApJ (scheduled publication in volume 594, September 1, 2003

Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC2391: Lithium depletion and Halpha emission

We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC2391. Of these, 26 spectra are totally new, 14 were already analyzed in a previous paper and another four are in common. These spectra, taken at the Cerro Tololo 4-meter and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, presence of Li, and Halpha emission. Among these new cluster members is CTIO-160 (M7), the first IC2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708A doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. Based on comparison to several theoretical models, we derive an age of 50+-5 Myr for IC2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our ages are 85+-10 Myr and 130+-20 Myr, respectively. We derive an estimate of the initial mass function of IC2391 that extends to below the substellar limit, and compare it to those of other well-studied young open clusters. The index of the power law mass function for IC2391 is alpha=0.96+-0.12, valid in the range 0.5 to 0.072 M(sun).Comment: Astrophysical Journal, accepted June 18, 200