1,507 research outputs found
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 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''0.11'', with the secondary
roughly one magnitude fainter at , and . 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 7525 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 0.04 M separated by
300 AU.Comment: 3 pages, 1 figure, accepted to Research Notes of the AA
Biodiversity climate change impacts report card technical paper:10. Implications of climate change for coastal and inter-tidal habitats in the UK
Executive summary - Coastal habitats are complex, dynamic and interdependent. They are important in providing sea defences, areas for recreation, biodiversity and a range of other ecosystem services. - Increased air- and sea-surface temperatures have resulted in changes in the distribution of marine and coastal species. Both warmer- and colder-water species are shifting northwards. However, warmer-water species are shifting northwards faster than colder-water species are retreating, resulting in changes in community composition. Changes in the abundance of keystone taxa can cause a cascade of responses, further altering community composition. - Changes in the phenology of coastal species have been observed, with the rates of change in marine species being considerably greater than those in terrestrial and freshwater systems. Recent advances in the phenology of species have not all occurred at the same rate, in some cases resulting in mismatches of timing of annual cycles of animals and their food organisms. - Changes in precipitation are likely to affect coastal habitats, but the projected increase in winter rainfall and decrease in summer rainfall will tend to have opposing effects; the net result of these is not known. High winter rainfall and milder winter temperatures may extend the growing season and lead to faster succession and dominance by taller competitive plant species. This will be exacerbated by anthropogenic nutrient enrichment. However, increasing frequency and severity of summer droughts may counteract the effects of nutrient enrichment and winter precipitation. Increased drought will have impacts on habitats that are highly dependent on the maintenance of hydrological regimes, such as machair lochs and dune slacks. - Rising sea levels have been associated with the loss of coastal habitats. Predicted future rises will have significant impacts on coastal and intertidal habitats, including changing geomorphological processes, further habitat loss and increasing the vulnerability of infrastructure. However, coastal systems are dynamic and have the potential to adapt to rising sea levels, but only if there is an adequate supply of sediment to allow accretion and if there is landward space for the coast to roll-back into. Sea defences and other coastal management interrupt the movement of sediment between systems and prevent natural coastal realignment. - Managed coastal realignment is beneficial because it offers the potential to create habitat and provide flood defence benefits. Inevitably, there will be conflict between the need to maintain intertidal and other coastal habitats (e.g. saltmarsh, mud flat and sand dune) by realignment, and the need to protect valuable inland coastal habitats, such as grazing marsh and saline lagoons. - Future changes in coastal habitats are hard to predict because it is difficult to separate the impacts of rising sea levels from those of coastal management, including sea defences. Coastal zone management and adaptation, and the interactions with other climate drivers, nutrient deposition and habitat management, will have significant influence on the quantity, quality and location of future coastal habitats
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
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
Three newly-discovered M-dwarf companions of Solar Neighbourhood stars
We present low-resolution spectroscopy of newly-discovered candidate
companions to three stars in the Solar Neighbourhood. All three companions are
M dwarfs, with spectral types ranging from M4 to M9.5. In two cases, G85-55`B'
(M6) and G87-9`B' (M4), we have circumstantial evidence from spectroscopy,
photometry and limited astrometry that the systems are physical binaries; in
the third, G216-7B (M9.5), comparison of POSS II IIIaF plate material and the
2MASS image indicates common proper motion. The primary star in this system,
G216-7A (M0), appears itself to be an unresolved, nearly equal-mass binary. All
three low-mass companions are highly likely to be stellar in nature, although
G216-7B lies very close to the hydrogen-burning limit.Comment: Accepted for publication in PASP; 21 pages, 6 figure
Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5
We present an analysis of 23 L dwarfs whose optical spectra display unusual
features. Twenty-one were uncovered during our search for nearby, late-type
objects using the Two Micron All-Sky Survey while two were identified in the
literature. The unusual spectral features, notably weak FeH molecular
absorption and weak Na I and K I doublets, are attributable to low-gravity and
indicate that these L dwarfs are young, low-mass brown dwarfs. We use these
data to expand the spectral classification scheme for L0 to L5-type dwarfs to
include three gravity classes. Most of the low-gravity L dwarfs have southerly
declinations and distance estimates within 60 pc. Their implied youth, on-sky
distribution, and distances suggest that they are members of nearby,
intermediate-age (~10-100 Myr), loose associations such as the Beta Pictoris
moving group, the Tucana/Horologium association, and the AB Doradus moving
group. At an age of 30 Myr and with effective temperatures from 1500 to 2400 K,
evolutionary models predict masses of 11-30 M_Jupiter for these objects. One
object, 2M 0355+11, with J-K_s=2.52+/-0.03, is the reddest L dwarf found in the
field and its late spectral type and spectral features indicative of a very low
gravity suggest it might also be the lowest-mass field L dwarf. However, before
ages and masses can be confidently adopted for any of these low-gravity L
dwarfs, additional kinematic observations are needed to confirm cluster
membership.Comment: Accepted to A
- …