3,246 research outputs found
Outstanding Issues in Our Understanding of L, T, and Y Dwarfs
Since the discovery of the first L dwarf 19 years ago and the discovery of
the first T dwarf 7 years after that, we have amassed a large list of these
objects, now numbering almost six hundred. Despite making headway in
understanding the physical chemistry of their atmospheres, some important
issues remain unexplained. Three of these are the subject of this paper: (1)
What is the role of "second parameters" such as gravity and metallicity in
shaping the emergent spectra of L and T dwarfs? Can we establish a robust
classification scheme so that objects with unusual values of log(g) or [M/H],
unusual dust content, or unresolved binarity are easily recognized? (2) Which
physical processes drive the unusual behavior at the L/T transition? Which
observations can be obtained to better confine the problem? (3) What will
objects cooler than T8 look like? How will we know a Y dwarf when we first
observe one?Comment: 11 pages including 5 figures. To appear in the conference proceedings
for Cool Stars 1
New spectral types L and T
The establishment of new spectral classes cooler than type M has had a brief, yet already rich, history. Prototypes of the new "L dwarf" and "T dwarf" classes were first found in the late 1980s to mid-1990s, with a flood of new discoveries occurring in the late 1990s with the advent of deep, large-area, digital sky surveys. Over four hundred and fifty L and T dwarfs are now cataloged. This review concentrates on the spectroscopic properties of these objects, beginning with the establishment of classification schemes rooted in the MK Process. The resulting grid of spectral types is then used as a tool to ferret out the underlying physics. The temperature ranges covered by these spectral types, the complex chemical processes responsible for the shape of their emergent spectra, their nature as either true stars or brown dwarfs, and their number density in the Galaxy are discussed. Two promising avenues for future research are also explored: the extension of the classification system to three dimensions to account for gravity- and metallicity-dependent features, and the capability of newer large-area surveys to uncover brown dwarfs cooler than those now recognized
Population status and habitat preferences of critically endangered Dipterocarpus littoralis in West Nusakambangan, Indonesia
The conservation of the endemic tree species Dipterocarpus littoralis (Bl.) Kurz. is hampered by the paucity of information on its population biology and ecology. Consequently, a targeted survey was carried out in the West Nusakambangan Nature Reserve to assess its population size and structure as well as habitat preferences. In total, 676 individuals of D. littoralis were located at 52 locations, with an extent of occurrence of 3.66 km2 and an area of occupancy of 1.71 km2. The population had an inverse-J-shaped distribution of diameter at breast height (DBH), with 63% of individuals in the 0-5 cm class and another 21% in the 5-10 cm class; only 11 (1.6%) mature individuals (DBH≥30) were found. D. littoralis was associated with steep, low, southwest-facing sites and sites that had high litter cover and thickness. Illegal logging and fuel-wood chopping were the main threats to D. littoralis and its habitat. In addition, an invasive shrub, Langkap (Arenga obtusifolia, Arecaceae), was a potential competitor with the seedlings throughout the reserve. In view of its endemism, narrow range and localized distribution, small population, environmental preferences, and the severe threats from anthropogenic activities and invasive species, D. littoralis appears to more than justify its conservation status of Critically Endangered
Analysis of 1995 survey data. Phase 2 post survey appraisal. R&D progress report EMA 036/pr4 for the period from 1st November 1998 to 31st January 1999
Effects of the topology of social networks on information transmission
Social behaviours cannot be fully understood without considering the network structures that underlie them. Developments in network theory provide us with relevant modelling tools. The topology of social networks may be due to selection for information transmission. To investigate this, we generated network topologies with varying proportions of random connections and degrees of preferential attachment. We simulated two social tasks on these networks: a spreading innovation model and a simple market. Results indicated that non-zero levels of random connections and low levels of preferential attachment led to more efficient information transmission. Theoretical and practical implications are discussed
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
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