Search

Analysis of T dwarfs using model atmospheres has been hampered by the absence of reliable line lists for methane and ammonia. Newly computed high-temperature line lists for both of these important molecules are now available, so it is timely to investigate the appearance of the various absorption features in T dwarfs in order to better understand their atmospheres and validate the new line lists. We present high-quality R ∼ 5000 Gemini/NIFS 1.0-2.4 μm spectra of the T8 standard 2MASS 0415-0935 and the T9 standard UGPS 0722-0540. We use these spectra to identify numerous methane and ammonia features not previously seen and we discuss the implications for our understanding of T dwarf atmospheres. Among our results, we find that ammonia is the dominant opacity source between ∼1.233-1.266 μm in UGPS 0722-0540, and we tentatively identify several absorption features in this wavelength range in the T9's spectrum which may be due entirely to ammonia opacity. Our results also suggest that water rather than methane is the dominant opacity source in the red half of the J band of the T8 dwarf. Water appears to be the main absorber in this wavelength region in the T9 dwarf until ∼1.31 μm, when methane starts to dominate. [ABSTRACT FROM AUTHOR]

We present the discovery of a very cold, very low mass, nearby brown dwarf using data from the NASA Wide-field Infrared Survey Explorer (WISE). The object, WISE J064723.23–623235.5, has a very red WISE color of W1–W2 > 3.77 mag and a very red Spitzer Space Telescope color of ch1–ch2 = 2.82 ± 0.09 mag. In JMKO –ch2 color (7.58 ± 0.27 mag) it is one of the two or three reddest brown dwarfs known. Our grism spectrum from the Hubble Space Telescope (HST) confirms it to be the seventeenth Y dwarf discovered, and its spectral type of Y1 ± 0.5 makes it one of the four latest-type Y dwarfs classified. Astrometric imaging from Spitzer and HST, combined with data from WISE, provides a preliminary parallax of π = 115 ± 12 mas (d = 8.7 ± 0.9 pc) and proper motion of μ = 387 ± 25 mas yr–1 based on 2.5 yr of monitoring. The spectrum implies a blue J–H color, for which model atmosphere calculations suggest a relatively low surface gravity. The best fit to these models indicates an effective temperature of 350-400 K and a mass of ∼5-30 MJup. Kinematic analysis hints that this object may belong to the Columba moving group, which would support an age of ∼30 Myr and thus an even lower mass of <2 MJup, but verification would require a radial velocity measurement not currently possible for a J = 22.7 mag brown dwarf. [ABSTRACT FROM AUTHOR]

We have performed a 970 arcmin2 (1.75 pc2) survey of the young cluster IC 2391 using the High Acuity Wide field K-band Imager on the Very Large Telescope. Observations were made in both the CH4 and H filters, targeting the methane absorption found in T dwarfs. From our survey of the cluster, five candidate T dwarfs were found. This is substantially smaller than the number that would be found by an equivalent near-infrared broad-band imaging survey, and allowed rapid spectroscopy of all five candidates. Follow-up spectroscopy was carried out using the Folded-port Infrared Echellette on Magellan. These spectra confirm that none of these objects are T dwarfs. This negative result emphasizes the critical importance of follow-up spectroscopy for any photometrically selected candidates, even those from methane imaging. Simulations of the photometric data set, combined with data for known IC 2391 members, imply that the power-law mass function in IC 2391 between 0.003 and 0.13 M⊙ has a slope of α < 1.7. [ABSTRACT FROM PUBLISHER]

ABSTRACT We have searched the Wide-field Infrared Survey Explorer first data release for widely separated (≤10 000 au) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J142320.86+011638.1 and the mildly metal poor ([Fe/H] =−0.38 ± 0.06) primary BD +01° 2920 (HIP 70319), a G1 dwarf at a distance of 17.2 pc. This new benchmark has Teff= 680 ± 55 K and a mass of 20-50 MJup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well-determined metallicity of its companion, the properties of BD +01° 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8. [ABSTRACT FROM AUTHOR]

We present i and z photometry for 25 T dwarfs and 1 L dwarf. Combined with published photometry, the data show that the i -- z, z -- Y, and z -- J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with Teff ≈ 600 K. We present new 0.7-1.0 µm and 2.8-.2 µm spectra for the very late type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using new and published data, with Saumon & Marley models, shows that the dwarf has Teff = 505 ± 10 K, a mass of 3-11 MJupiter, and an age between 60 Myr and 1 Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE 4.5 µm photometry and the Saumon & Marley models. The optical spectrum for this 500 K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200 K. [ABSTRACT FROM AUTHOR]

BSTRACT [ABSTRACT FROM AUTHOR]

We have used blue near-infrared colours to select a group of 12 spectroscopically confirmed United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) T dwarfs later than T4. From amongst these, we identify the first two kinematic halo T-dwarf candidates. Blue near-infrared colours have been attributed to collisionally induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. and components are estimated for our objects by assuming . From this, ULAS J0926+0835 is found to have km s and km s, and ULAS J1319+1209 is found to have km s and km s. These values are consistent with potential halo membership. However, these are not the bluest objects in our selection. The bluest is ULAS J1233+1219, with , and surprisingly this object is found to have young disc-like and . Our sample also contains Hip 73786B, companion to the metal-poor K5 dwarf Hip 73786. Hip 73786 is a metal-poor star, with and is located at a distance of 19 pc. space velocity components are calculated for Hip 73786A and B, finding that km s, km s and km s. From the properties of the primary, Hip 73786B is found to be at least -Gyr old. As a metal-poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks. [ABSTRACT FROM AUTHOR]

We have carried out a search for late-type T dwarfs in the UKIRT Infrared Deep Sky Survey Galactic Plane Survey 6th Data Release. The search yielded two persuasive candidates, both of which have been confirmed as T dwarfs. The brightest, UGPS J0521+3640, has been assigned the spectral type T8.5 and appears to lie at a distance of 7-9 pc. The fainter of the two, UGPS J0652+0324, is classified as a T5.5 dwarf and lies at an estimated distance of 28-37 pc. Warm- Spitzer observations in IRAC channels 1 and 2, taken as part of the GLIMPSE360 Legacy Survey, are available for UGPS J0521+3640 and we used these data with the near-infrared spectroscopy to estimate its properties. We find best-fitting solar metallicity BT-Settl models for and 650 K and and 5.0. These parameters suggest a mass between 14 and for an age between 1 and 5 Gyr. The proximity of this very cool T dwarf, and its location in the Galactic plane, makes it an ideal candidate for high-resolution adaptive optics imaging to search for cool companions. [ABSTRACT FROM AUTHOR]

We report Doppler measurements of the stars HD 187085 and HD 20782 which indicate two high eccentricity low-mass companions to the stars. We find HD 187085 has a Jupiter-mass companion with a ∼1000-d orbit. Our formal ‘best-fitting’ solution suggests an eccentricity of 0.47, however, it does not sample the periastron passage of the companion and we find that orbital solutions with eccentricities between 0.1 and 0.8 give only slightly poorer fits (based on rms and ) and are thus plausible. Observations made during periastron passage in 2007 June should allow for the reliable determination of the orbital eccentricity for the companion to HD 187085. Our data set for HD 20782 does sample periastron and so the orbit for its companion can be more reliably determined. We find the companion to HD 20782 has , an orbital period of 595.86 ± 0.03 d and an orbit with an eccentricity of 0.92 ± 0.03. The detection of such high-eccentricity (and relatively low-velocity amplitude) exoplanets appears to be facilitated by the long-term precision of the Anglo-Australian Planet Search. Looking at exoplanet detections as a whole, we find that those with higher eccentricity seem to have relatively higher velocity amplitudes indicating higher mass planets and/or an observational bias against the detection of high-eccentricity systems. [ABSTRACT FROM AUTHOR]

ABSTRACT We report the detection of a new candidate exoplanet around the metal-rich star τ[sup 1] Gruis. With M sin i= 1.26 ± 0.18 M[sub Jup] , a period of 1391 ± 300 d and an orbit with an eccentricity of 0.14 ± 0.14 it adds to the growing population of long-period exoplanets with low-eccentricity orbits. This population now comprises more than 20 per cent of known exoplanets. When the companion to τ[sup 1] Gruis is plotted together with all exoplanets found by radial velocity searches we find evidence for a peak in the number of short-period exoplanets, followed by a minimum of planets between approximately 5 and 50 d and then an apparent rise in the number of planets per unit radius that seems to set in by 100 d, indicating more planets further from the host star. This is very different from the Gaussian-like period distribution found for stellar companions. This lends support to the idea that once a clearing in the inner protoplanetary disc develops, it halts the inward migration of planets. In particular, the smooth distribution of exoplanets arising from planetary migration through a disc is altered by an accumulation of exoplanets at the point where the disc has been cleared out. [ABSTRACT FROM AUTHOR]

ABSTRACT We report the results of observations designed to investigate the spectroscopic signatures of dust clouds on the L2 brown dwarf Kelu-1. Time-resolved medium-resolution spectra show no significant evidence of variability in the dust-sensitive TiO, CrH and FeH bandheads on a time-scale of 1–24 h. We do, however, report periodic variability in the pseudo-equivalent width of Hα consistent with the 1.8-h rotation period previously reported for this object. Near-contemporaneous I-band photometry shows evidence for non-periodic variability at the level of 2 per cent. [ABSTRACT FROM AUTHOR]

ABSTRACT We have used an innovative tunable filter technique to carry out a search for the variability signatures of meteorological processes in the atmospheres of two nearby brown dwarfs. We find no evidence for variability in the L-type brown dwarf DENIS-P J1228-1547 in observations spanning approximately half a rotation period (3 h), and evidence for variability in the M-type brown dwarf LP 944-20 in multiple observations spanning approximately one third of a rotation period (1.5 h). [ABSTRACT FROM AUTHOR]

In recent years, brown dwarfs have been extended to a new Y-dwarf class with effective temperatures colder than 500 K and masses in the range of 5-30 Jupiter masses. They fill a crucial gap in observable atmospheric properties between the much colder gas-giant planets of our own solar system (at around 130 K) and both hotter T-type brown dwarfs and the hotter planets that can be imaged orbiting young nearby stars (both with effective temperatures in the range of 1500-1000 K). Distance measurements for these objects deliver absolute magnitudes that make critical tests of our understanding of very cool atmospheres. Here we report new distances for nine Y dwarfs and seven very late T dwarfs. These reveal that Y dwarfs do indeed represent a continuation of the T-dwarf sequence to both fainter luminosities and cooler temperatures. They also show that the coolest objects display a large range in absolute magnitude for a given photometric color. The latest atmospheric models show good agreement with the majority of these Y-dwarf absolute magnitudes. This is also the case for WISE0855-0714, the coldest and closest brown dwarf to the Sun, which shows evidence for water ice clouds. However, there are also some outstanding exceptions, which suggest either binarity or the presence of condensate clouds. The former is readily testable with current adaptive optics facilities. The latter would mean that the range of cloudiness in Y dwarfs is substantial with most hosting almost no clouds—while others have dense clouds, making them prime targets for future variability observations to study cloud dynamics. [ABSTRACT FROM AUTHOR]

We present the discovery of another seven Y dwarfs from the Wide-field Infrared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we further explore the transition between spectral types T and Y. We find that the T/Y boundary roughly coincides with the spot where the J – H colors of brown dwarfs, as predicted by models, turn back to the red. Moreover, we use preliminary trigonometric parallax measurements to show that the T/Y boundary may also correspond to the point at which the absolute H (1.6 μm) and W2 (4.6 μm) magnitudes plummet. We use these discoveries and their preliminary distances to place them in the larger context of the solar neighborhood. We present a table that updates the entire stellar and substellar constituency within 8 pc of the Sun, and we show that the current census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. This factor will decrease with time as more brown dwarfs are identified within this volume, but unless there is a vast reservoir of cold brown dwarfs invisible to WISE, the final space density of brown dwarfs is still expected to fall well below that of stars. We also use these new Y dwarf discoveries, along with newly discovered T dwarfs from WISE, to investigate the field substellar mass function. We find that the overall space density of late-T and early-Y dwarfs matches that from simulations describing the mass function as a power law with slope –0.5 < α < 0.0; however, a power law may provide a poor fit to the observed object counts as a function of spectral type because there are tantalizing hints that the number of brown dwarfs continues to rise from late-T to early-Y. More detailed monitoring and characterization of these Y dwarfs, along with dedicated searches aimed at identifying more examples, are certainly required. [ABSTRACT FROM AUTHOR]