Discovery of 11 New T Dwarfs in the Two Micron All-Sky Survey, Including a Possible L/T Transition Binary

We present the discovery of 11 new T dwarfs, found during the course of a photometric survey for mid-to-late T dwarfs in the 2MASS Point Source Catalog and from a proper motion selected sample of ultracool dwarfs in the 2MASS Working Database. Using the NASA Infrared Telescope Facility SpeX spectrograph, we obtained low-resolution (R~150) spectroscopy, allowing us to derive near-infrared spectral types of T2-T8. One of these new T dwarfs, 2MASS J13243559+6358284, was also discovered independently by Metchev et al., in prep. This object is spectroscopically peculiar and possibly a binary and/or very young (<300 Myr). We specifically attempted to model the spectrum of this source as a composite binary to reproduce its peculiar spectral characteristics. The latest-type object in our sample is a T8 dwarf, 2MASS J07290002-3954043, now one of the four latest-type T dwarfs known. All 11 T dwarfs are nearby given their spectrophotometric distance estimates, with 1 T dwarf within 10 pc and 8 additional T dwarfs within 25 pc, if single. These new additions increase the 25 pc census of T dwarfs by ~14%. Their proximity offers an excellent opportunity to probe for companions at closer separations than are possible for more distant T dwarfs.Comment: 45 pages, 9 figures, 9 tables. Published in AJ. Replacement: Fixed typos in 3 tables (some reported photometry was from the 2MASS Working Database instead of the 2MASS All Sky Catalog) and updated Fig.

A Candidate Wide Brown Dwarf Binary in the Argus Association: 2MASS J14504216-7841413 and 2MASS J14504113-7841383

[not part of Research Note] We report the discovery of a widely-separated low-mass binary as a candidate member of the $\sim$40 Myr Argus Association. Resolved imaging and astrometry with 2MASS and LDSS-3 reveal a common proper motion pair of red sources separated by 4.23''$\pm$0.11'', with the secondary roughly one magnitude fainter at $i$, $z$ and $J$. Resolved spectroscopy indicates component types of M8pec and M9pec, the peculiarities arising from weak Na I and strong VO absorption characteristic of low gravity sources. With its small proper motion and estimated 75$\pm$25 pc distance, the BANYAN II tool indicates a membership probability of 93% in Argus, which would be consistent with a pair of brown dwarfs of mass $\sim$0.04 M$_{\odot}$ separated by $\sim$300 AU.Comment: 3 pages, 1 figure, accepted to Research Notes of the AA

Sherlock: An Automated Follow-Up Telescope for Wide-Field Transit Searches

The most significant challenge currently facing photometric surveys for transiting gas-giant planets is that of confusion with eclipsing binary systems that mimic the photometric signature. A simple way to reject most forms of these false positives is high-precision, rapid-cadence monitoring of the suspected transit at higher angular resolution and in several filters. We are currently building a system that will perform higher-angular-resolution, multi-color follow-up observations of candidate systems identified by Sleuth (our wide-field transit survey instrument at Palomar), and its two twin system instruments in Tenerife and northern Arizona.Comment: 4 pages, 3 figures, to appear in AIP Conf Proc: The Search for Other Worlds, eds. S. S. Holt & D. Demin

Clouds, Gravity, and Metallicity in Blue L Dwarfs: The Case of 2MASS J11263991–5003550

Optical and near-infrared spectroscopy of the newly discovered peculiar L dwarf 2MASS J11263991–5003550 are presented. Folkes et al. classified this source as a high proper motion L9±1 dwarf based on its strong H2O absorption at 1.4 μ m . We find that the optical spectrum of 2MASS J1126–5003 is in fact consistent with that of a normal L4.5 dwarf with notably enhanced FeH absorption at 9896 Å. However, its near-infrared spectrum is unusually blue, with strong H2O and weak CO bands similar in character to several recently identified "blue L dwarfs." Using 2MASS J1126–5003 as a case study, and guided by trends in the condensate cloud models of Burrows et al. and Marley et al., we find that the observed spectral peculiarities of these sources can be adequately explained by the presence of thin and/or large-grained condensate clouds as compared to normal field L dwarfs. Atypical surface gravities or metallicities alone cannot reproduce the observed peculiarities, although they may be partly responsible for the unusual condensate properties. We also rule out unresolved multiplicity as a cause for the spectral peculiarities of 2MASS J1126–5003. Our analysis is supported by examination of Spitzer mid-infrared spectral data from Cushing et al. which show that bluer L dwarfs tend to have weaker 10 μ m absorption, a feature tentatively associated with silicate oxide grains. With their unique spectral properties, blue L dwarfs like 2MASS J1126–5003 should prove useful in studying the formation and properties of condensates and condensate clouds in low-temperature atmospheres

Protoplanetary Disk Masses from Stars to Brown Dwarfs

We present SCUBA-2 850um observations for 7 very low mass stars (VLMS) and brown dwarfs (BDs): 3 in Taurus, 4 in the TWA, and all classical T Tauri (cTT) analogs. We detect 2 of the 3 Taurus disks, but none of the TWA ones. Our 3sigma limits correspond to a dust mass of 1.2 MEarth in Taurus and a mere 0.2 MEarth in the TWA (3--10x deeper than previous work). We combine our data with other sub-mm/mm surveys of Taurus, rho Oph and the TWA to investigate trends in disk mass and grain growth during the cTT phase. We find : (1) The minimum disk outer radius required to explain the upper envelope of sub-mm/mm fluxes is 100 AU for intermediate-mass stars, solar-types and VLMS, and 20 AU for BDs. (2) While the upper envelope of disk masses increases with Mstar from BDs to VLMS to solar-types, no increase is seen from solar-type to intermediate-mass stars. We propose this is due to enhanced photoevaporation around intermediate masses. (3) Many disks around Taurus and rho Oph intermediate-mass and solar-type stars evince an opacity index beta of 0--1, indicating large grains. Of the only four VLMS/BDs in these regions with multi-wavelength data, three are consistent with large grains, though optically thick disks are not ruled out. (4) For the TWA VLMS (TWA 30A, B), combining our fluxes with Mdot and ages suggests substantial grain growth by 10 Myr. The degree of grain growth in the TWA BDs (2M1207A, SSPM1102) remains largely unknown. (5) A Bayesian analysis shows that mean(log[Mdisk/Mstar]) = -2.4, roughly constant all the way from intermediate-mass stars to VLMS/BDs, and (6) the disk mass in close solar-type Taurus binaries is significantly lower than in singles (by a factor of 10), while that in wide solar-type Taurus binaries is closer to that in singles (lower by a factor of 3). (7) We discuss the implications for planet formation, and for the dependence of Mdot on Mstar.Comment: ApJ, accepted (in press

Discovery of an M9.5 Candidate Brown Dwarf in the TW Hydrae Association - DENIS J124514.1-442907

We report the discovery of a fifth candidate substellar system in the ~5-10 Myr TW Hydrae Association - DENIS J124514.1-442907. This object has a NIR spectrum remarkably similar to that of 2MASS J1139511-315921, a known TW Hydrae brown dwarf, with low surface gravity features such as a triangular-shaped H-band, deep H2O absorption, weak alkali lines, and weak hydride bands. We find an optical spectral type of M9.5 and estimate a mass of <24 M_Jup, assuming an age of ~5-10 Myr. While the measured proper motion for DENIS J124514.1-442907 is inconclusive as a test for membership, its position in the sky is coincident with the TW Hydrae Association. A more accurate proper motion measurement, higher resolution spectroscopy for radial velocity, and a parallax measurement are needed to derive the true space motion and to confirm its membership.Comment: 8 pages - emulateapj style, 2 figures, 3 tables. Accepted to ApJL. Fixed typos, added reference, added footnot

Discovery of a High Proper Motion L Dwarf Binary: 2MASS J15200224-4422419AB

We report the discovery of the wide L1.5+L4.5 binary 2MASS J15200224-4422419AB, identified during spectroscopic followup of high proper motion sources selected from the Two Micron All Sky Survey. This source was independently identified by Kendall et al. in the SuperCOSMOS Sky Survey. Resolved JHK photometry and low resolution near-infrared spectroscopy demonstrate that this system is composed of two well-separated (1"174+/-0"016) L dwarfs. Component classifications are derived using both spectral ratios and comparison to near-infrared spectra of previously classified field L dwarfs. Physical association for the pair is deduced from the large (mu = 0"73+/-0"03 /yr) common proper motion of the components and their similar spectrophotometric distances (19+/-2 pc). The projected separation of the binary, 22+/-2 AU, is consistent with maximum separation/total system mass trends for very low mass binaries. The 2MASS J1520-4422 system exhibits both large tangential (66+/-7 km/s) and radial velocities (-70+/-18 km/s), and its motion in the local standard of rest suggests that it is an old member of the Galactic disk population. This system joins a growing list of well-separated (>0"5), very low mass binaries, and is an excellent target for resolved optical spectroscopy to constrain its age as well as trace activity/rotation trends near the hydrogen-burning limit.Comment: 35 pages, 8 figures; accepted for publication to ApJ; see also Kendall et al. astro-ph/060939

Astrophysical False Positives Encountered in Wide-Field Transit Searches

Wide-field photometric transit surveys for Jupiter-sized planets are inundated by astrophysical false positives, namely systems that contain an eclipsing binary and mimic the desired photometric signature. We discuss several examples of such false alarms. These systems were initially identified as candidates by the PSST instrument at Lowell Observatory. For three of the examples, we present follow-up spectroscopy that demonstrates that these systems consist of (1) an M-dwarf in eclipse in front of a larger star, (2) two main-sequence stars presenting grazing-incidence eclipses, and (3) the blend of an eclipsing binary with the light of a third, brighter star. For an additional candidate, we present multi-color follow-up photometry during a subsequent time of eclipse, which reveals that this candidate consists of a blend of an eclipsing binary and a physically unassociated star. We discuss a couple indicators from publicly-available catalogs that can be used to identify which candidates are likely giant stars, a large source of the contaminants in such surveys.Comment: 10 pages, 9 figures, to appear in AIP Conf Proc: The Search for Other Worlds, eds. S. S. Holt & D. Demin