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.Comment: 2 pages, 1 figure (new version with minor corrections); to appear in the proceedings of the workshop "Ultra-low mass star formation and evolution", to be published in A

DENIS-P J104814.7-395606.1: An M9 dwarfs at about 4 pc

We present the discovery of a previously unknown member of the immediate solar neighbourhood, DENIS-P J104814.7-395606.1 (hereafter DENIS 1048-39), identified while mining the DENIS database for new nearby stars. A HIRES echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9 dwarf. DENIS 1048-39 has a very bright apparent magnitude (I=12.67) for its spectral type and colour (I-J=3.07), and is therefore very nearby. If it is single its distance is only 4.1 +- 0.6pc, ranking it as between our twelfth and fortyth closest neighbour. It is also the closest star or brown dwarf with a spectral type later than M7V. Its proper motion was determined through comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer, with the DENIS images. At 1.53"/yr it further attests the closeness of DENIS 1048-39, and hence its dwarf status. These characteristics make it an obvious target for further detailed studies.Comment: In press in A&A Letter

Spitzer Mid-Infrared Photometry of 500 - 750 K Brown Dwarfs

Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is critical for understanding the cold population of brown dwarfs now being found, objects which have more in common with planets than stars. As effective temperature (T_eff) drops from 800 K to 400 K, the fraction of flux emitted beyond 3 microns increases rapidly, from about 40% to >75%. This rapid increase makes a color like H-[4.5] a very sensitive temperature indicator, and it can be combined with a gravity- and metallicity-sensitive color like H-K to constrain all three of these fundamental properties, which in turn gives us mass and age for these slowly cooling objects. Determination of mid-infrared color trends also allows better exploitation of the WISE mission by the community. We use new Spitzer Cycle 6 IRAC photometry, together with published data, to present trends of color with type for L0 to T10 dwarfs. We also use the atmospheric and evolutionary models of Saumon & Marley to investigate the masses and ages of 13 very late-type T dwarfs, which have H-[4.5] > 3.2 and T_eff ~ 500 K to 750 K.Comment: To be published in the on-line version of the Proceedings of Cool Stars 16 (ASP Conference Series). This is an updated version of Leggett et al. 2010 ApJ 710 1627; a photometry compilation is available at http://www.gemini.edu/staff/slegget

Discovery of very nearby ultracool dwarfs from DENIS

We report new spectroscopic results, obtained with UKIRT/CGS4, of a sample of 14 candidate ultracool dwarfs selected from the DENIS (Deep Near-Infrared Survey of the Southern Sky) database. A further object, selected from the 2MASS Second Incremental Release, was observed at a later epoch with the same instrument. Six objects are already known in the literature; we re-derive their properties. A further four prove to be very nearby (~10 pc) mid-to-late L-dwarfs, three unknown hitherto, two of which are almost certainly substellar. These findings increase the number of L-dwarfs known within ~10 pc by ~25%. The remainder of the objects discussed here are early L or very late M-type dwarfs lying between ~45 and 15 pc and are also new to the literature. Spectral types have been derived by direct comparison with J-,H- and K- band spectra of known template ultracool dwarfs given by Leggett et al. (ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dL.spectra/) For the known objects, we generally find agreement to within ~1 subclass with previously derived spectral types. Distances are determined from the most recent M_J vs. spectral type calibrations, and together with our derived proper motions yield kinematics for most targets consistent with that expected for the disk population; for three probable late M-dwarfs, membership of a dynamically older population is postulated. The very nearby L-type objects discussed here are of great interest for future studies of binarity and parallaxes.Comment: 4 pages, 2 figures, accepted in A&A Letter