2MASS J20261584-2943124: An Unresolved L0.5 + T6 Spectral Binary

We identify the L dwarf 2MASS J20261584-2943124 as an unresolved spectral binary, based on low-resolution, near-infrared spectroscopy from IRTF/SpeX. The data reveal a peculiar absorption feature at 1.6 microns, previously noted in the spectra of other very low-mass spectral binaries, which likely arises from overlapping FeH and CH4 absorption bands in the blended light of an L dwarf/T dwarf pair. Spectral template matching analysis indicates component types of L0.5 and T6, with relative brightness Delta H = 4.2+/-0.6. Laser guide star adaptive optics imaging observations with Keck/NIRC2 fail to resolve the source, indicating a maximum separation at the observing epoch of 0.25 arcsec, or a projected separation of 9 AU assuming a distance of 36+/-5 pc. With an age that is likely to be relatively older (>5 Gyr based on the system's large vtan and mass ratio arguments, the relative motion of the potentially "massive" (0.06-0.08 Msun) components of 2MASS J2026-2943 may be detectable through radial velocity variations, like its earlier-type counterpart 2MASS J03202839-0446358 (M8+T5), providing dynamical mass measurements that span the hydrogen burning limit.Comment: 25 pages, 5 figures; AJ in pres

A Search for High Proper Motion T Dwarfs with Pan-STARRS1 + 2MASS + WISE

We have searched ~8200 sq. degs for high proper motion (~0.5-2.7"/year) T dwarfs by combining first-epoch data from the Pan-STARRS1 (PS1) 3-Pi Survey, the 2MASS All-Sky Point Source Catalog, and the WISE Preliminary Data Release. We identified two high proper motion objects with the very red (W1-W2) colors characteristic of T dwarfs, one being the known T7.5 dwarf GJ 570D. Near-IR spectroscopy of the other object (PSO J043.5+02 = WISEP J0254+0223) reveals a spectral type of T8, leading to a photometric distance of 7.2+/-0.7 pc. The 2.56"/yr proper motion of PSO J043.5+02 is the second highest among field T dwarfs, corresponding to an tangential velocity of 87+/-8 km/s. According to the Besancon galaxy model, this velocity indicates its galactic membership is probably in the thin disk, with the thick disk an unlikely possibility. Such membership is in accord with the near-IR spectrum, which points to a surface gravity (age) and metallicity typical of the field population. We combine 2MASS, SDSS, WISE, and PS1 astrometry to derive a preliminary parallax of 171+/-45 mas (5.8{+2.0}{-1.2} pc), the first such measurement using PS1 data. The proximity and brightness of PSO J043+02 will facilitate future characterization of its atmosphere, variability, multiplicity, distance, and kinematics. The modest number of candidates from our search suggests that the immediate (~10 pc) solar neighborhood does not contain a large reservoir of undiscovered T dwarfs earlier than about T8.Comment: ApJL, in press. v3 is the same as v2, except that the arXiv.org metadata has been updated to match the final order of the author lis

SpeX Spectroscopy of Unresolved Very Low-Mass Binaries. I. Identification of Seventeen Candidate Binaries Straddling the L Dwarf/T Dwarf Transition

We report the identification of 17 candidate brown dwarf binaries whose components straddle the L dwarf/T dwarf transition. These sources were culled from a large near-infrared spectral sample of L and T dwarfs observed with the Infrared Telescope Facility SpeX spectrograph. Candidates were selected on the basis of spectral ratios which segregate known (resolved) L dwarf/T dwarf pairs from presumably single sources. Composite templates, constructed by combining 13581 pairs of absolute flux-calibrated spectra, are shown to provide statistically superior fits to the spectra of our seventeen candidates as compared to single templates. Ten of these candidates appear to have secondary components that are significantly brighter than their primaries over the 1.0-1.3 micron band, indicative of rapid condensate depletion at the L dwarf/T dwarf transition. Our results support prior indications of enhanced multiplicity amongst early-type T dwarfs; 53+/-7% of the T0-T4 dwarfs in our spectral sample are found to be either resolved or unresolved (candidate) pairs, although this is consistent with an intrinsic (volume complete) brown dwarf binary fraction of only 15%. If verified, this sample of spectral binaries more than doubles the number of known L dwarf/T dwarf transition pairs, enabling a broader exploration of this poorly-understood phase of brown dwarf atmospheric evolution.Comment: 65 pages (11pt manuscript format), 68 figures, accepted for publication to ApJ; spectral data can be accessed at http://www.browndwarfs.org/spexpris

Trigonometric parallaxes of ten ultracool subdwarfs

We measured absolute trigonometric parallaxes and proper motions with respect to many background galaxies for a sample of ten ultracool subdwarfs. The observations were taken in the H-band with the OMEGA2000 camera at the 3.5m-telescope on Calar Alto, Spain during a time period of 3.5 years. For the first time, the reduction of the astrometric measurements was carried out directly with respect to background galaxies. We obtained absolute parallaxes with mean errors ranging between 1 and 3 mas. With six completely new parallaxes we more than doubled the number of benchmark ultracool (>sdM7) subdwarfs. Six stars in the M_{K_s} vs. J-K_s diagram fit perfectly to model subdwarf sequences from M7 to L4 with [M/H] between -1.0 and -1.5, whereas 4 are consistent with a moderately low metallicity ([M/H]=-0.5) from M7 to T6. All but one of our objects have large tangential velocities between 200 and 320 km/s typical of the Galactic halo population. Our results are in good agreement with recent independent measurements for three of our targets and confirm the previously measured parallax and absolute magnitude M_{K_s} of the nearest and coolest (T-type) subdwarf 2MASS 0937+29 with higher accuracy. For all targets, we also obtained infrared J,H,K_s photometry at a level of a few milli-magnitudes relative to 2MASS standards.Comment: Letter 4 pages 1 figure. accepted by Astronomy and Astrophysic

Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude

We present a sample of 17 newly discovered ultracool dwarf candidates later than ~M8, drawn from 231.90 arcmin2 of {\it Hubble Space Telescope} Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5<J_125<25.5 AB mag to an exponential disk model, we estimate a vertical scale height of z_scl=290 +- 25 (random) +- 30 (systematic) pc for a binarity fraction of f_b=0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J_{125}~24 AB mag due to the exponentially-declining number density at the edge of the disc. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence.Comment: 9 pages, 6 figures, 3 tables, accepted to ApJ (v2 is consistent with Referee changes

A Cross-Match of 2MASS and SDSS. II. Peculiar L Dwarfs, Unresolved Binaries, and the Space Density of T Dwarf Secondaries

We present the completion of a program to cross-correlate the SDSS Data Release 1 and 2MASS Point Source Catalog in search for extremely red L and T dwarfs. The program was initiated by Metchev and collaborators, who presented the findings on all newly identified T dwarfs in SDSS DR1, and estimated the space density of isolated T0--T8 dwarfs in the solar neighbourhood. In the current work we present most of the L dwarf discoveries. Our red-sensitive (z-J > 2.75 mag) cross-match proves to be efficient in detecting peculiarly red L dwarfs, adding two new ones, including one of the reddest known L dwarfs. Our search also nets a new peculiarly blue L7 dwarf and, surprisingly, two M8 dwarfs. We further broaden our analysis to detect unresolved binary L or T dwarfs through spectral template fitting to all L and T dwarfs presented here and in the earlier work by Metchev and collaborators. We identify nine probable binaries, six of which are new and eight harbour likely T dwarf secondaries. We combine this result with current knowledge of the mass ratio distribution and frequency of substellar companions to estimate an overall space density of 0.005--0.05 pc^{-3} for individual T0--T8 dwarfs.Comment: Accepted by Ap

Discovery of a Companion at the L/T Transition with the Wide-field Infrared Survey Explorer

We report the discovery of a substellar companion to the nearby solar-type star HD 46588 (F7V, 17.9 pc, ~3 Gyr). HD 46588 B was found through a survey for common proper motion companions to nearby stars using data from the Wide-field Infrared Survey Explorer and the Two-Micron All-Sky Survey. It has an angular separation of 79.2" from its primary, which corresponds to a projected physical separation of 1420 AU. We have measured a spectral type of L9 for this object based on near-infrared spectroscopy performed with TripleSpec at Palomar Observatory. We estimate a mass of 0.064+0.008/-0.019 Msun from a comparison of its luminosity to the values predicted by theoretical evolutionary models for the age of the primary. Because of its companionship to a well-studied star, HD 46588 B is one of the few known brown dwarfs at the L/T transition for which both age and distance estimates are available. Thus, it offers new constraints on the properties of brown dwarfs during this brief evolutionary phase. The discovery of HD 46588 B also illustrates the value of the Wide-field Infrared Survey Explorer for identifying brown dwarfs in the solar neighborhood via their proper motions.Comment: Astrophysical Journal, in pres

Binarity at the L/T brown dwarf transition

Current atmospheric models cannot reproduce some of the characteristics of the transition between the L dwarfs with cloudy atmospheres and the T dwarfs with dust-depleted photospheres. It has been proposed that a majority of the L/T transition brown dwarfs could actually be a combinaison of a cloudy L dwarf and a clear T dwarf. Indeed binarity seems to occur more frequently among L/T transition brown dwarfs. We aim to refine the statistical significance of the seemingly higher frequency of binaries. Co-eval binaries would also be interesting test-beds for evolutionary models. We obtained high-resolution imaging for six mid-L to late-T dwarfs, with photometric distances between 8 and 33pc, using the adaptive optics systems NACO at the VLT, and the Lick system, both with the laser guide star. We resolve none of our targets. Combining our data with published results, we obtain a frequency of resolved L/T transition brown dwarfs of (31+21-15)%, compared to (21+10-7)% and (14+14-7)% for mid-L and T dwarfs (90% of confidence level). These fractions do not significantly support, nor contradict, the hypothesis of a larger binary fraction in the L/T transition. None of our targets has companions with effective temperatures as low as 360-1000K at separations larger than 0.5".Comment: 6 pages, 4 figures, accepted by A&

ULAS J141623.94+134836.3: a Blue T Dwarf Companion to a Blue L Dwarf

We confirm the substellar nature of ULAS J141623.94+134836.3, a common proper motion companion to the blue L dwarf SDSS J141624.08+134826.7 identified by Burningham et al. and Scholz. Low-resolution 0.8-2.4 micron spectroscopy obtained with IRTF/SpeX shows strong H2O and CH4 absorption bands, consistent with a T7.5 spectral type, and we see possible indications of NH3 absorption in the 1.0-1.3 micron region. More importantly, the spectrum of ULAS J1416+1348 shows a broadened Y-band peak and highly suppressed K-band flux, both indicative of high surface gravity and/or subsolar metallicity. These traits are verified through spectral model fits, from which we derive atmospheric parameters Teff = 650+/-60 K, log g = 5.2+/-0.4 cgs, [M/H] <= -0.3 and Kzz = 10^4 cm^2/s, the temperature being significantly warmer than that estimated by Burningham et al. These fits also indicate a model-dependent spectroscopic distance of 10.6(+3.0,-2.8) pc for ULAS J1416+1348, formally consistent with the 7.9+/-1.7 pc astrometric distance for SDSS J1416+1348 from Scholz. The common peculiarities of these two co-spatial, co-moving sources suggest that their unusual blue colors - and those of other blue L and T dwarfs in general - arise from age or metallicity, rather than cloud properties alone.Comment: 21 pages, 3 figures (manuscript format); submitted to ApJ, constructive comments welcom