976 research outputs found
Nuclear Waste and Native America: The MRS Siting Exercise
Drs. Gowda & Easterling provide cross-cultural perspectives on issues of risk perception, equity and policy as they affect nuclear waste storage on Native American sites
Probing the Large Magellanic Cloud's recent chemical enrichment history through its star clusters
We present Washington system colour-magnitude diagrams (CMDs) for 17
practically unstudied star clusters located in the bar as well as in the inner
disc and outer regions of the Large Magellanic Cloud (LMC). Cluster sizes were
estimated from star counts distributed throughout the entire observed fields.
Based on the best fits of theoretical isochrones to the cleaned
CMDs, as well as on the parameter and the standard giant branch
method, we derive ages and metallicities for the cluster sample. Four objects
are found to be intermediate-age clusters (1.8-2.5 Gyr), with [Fe/H] ranging
from -0.66 to -0.84. With the exception of SL263, a very young cluster (
16 Myr), the remaining 12 objects are aged between 0.32 and 0.89 Gyr, with
their [Fe/H] values ranging from -0.19 to -0.50. We combined our results with
those for other 231 clusters studied in a similar way using the Washington
system. The resulting age-metallicity relationship shows a significant
dispersion in metallicities, whatever age is considered. Although there is a
clear tendency for the younger clusters to be more metal-rich than the
intermediate ones, we believe that none of the chemical evolution models
currently available in the literature reasonably well represents the recent
chemical enrichment processes in the LMC clusters. The present sample of 17
clusters is part of our ongoing project of generating a database of LMC
clusters homogeneously studied using the Washington photometric system and
applying the same analysis procedureComment: 11 pages, 20 figures, 3 tables, Accepted for publication in MNRA
A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry
To enlarge our growing sample of well-studied star clusters in the Large
Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in
the fields of twenty-three mostly unstudied clusters located in the inner disc
and outer regions of the LMC. We estimated cluster radii from star counts.
Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically
cleaned from field star contamination, we derived cluster ages and
metallicities from a comparison with theoretical isochrones of the Padova
group. Whenever possible, we also derived ages using delta_T1 - the magnitude
difference between the red giant clump and the main sequence turn off - and
estimated metallicities from the standard giant branch procedure. We enlarged
our sample by adding clusters with published ages and metallicities determined
on a similar scale by applying the same methods. We examined relationships
between their positions in the LMC, ages and metallicities. We find that the
two methods for age and metallicity determination agree well with each other.
Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with
[Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out
to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23
clusters represent an increase of ~ 30% in the current total amount number of
well-studied LMC clusters using Washington photometry. In agreement with
previous studies, we find no evidence for a metallicity gradient. We also find
that the younger clusters were formed closer to the LMC centre than the older
ones.Comment: 11 pages, 10 figures. A&A, in pres
U.S. Law of the Sea Cruise to Map the Foot of the Slope and 2500-m Isobath of the Northeast U.S. Atlantic Continental Margin: Legs 4 and 5. Cruise Report
U.S. Law of the Sea cruise to map the foot of the slope and 2500-m isobath of the Northeast US Atlantic continental margin: Legs 4 and 5
CRUISES PF05-1 and 2
April 25 to June 30, 2005
Norfolk, VA to Port Canaveral, F
An Emerging Role for the Mammalian Target of Rapamycin in “Pathological” Protein Translation: Relevance to Cocaine Addiction
Complex neuroadaptations within key nodes of the brain’s “reward circuitry” are thought to underpin long-term vulnerability to relapse. A more comprehensive understanding of the molecular and cellular signaling events that subserve relapse vulnerability may lead to pharmacological treatments that could improve treatment outcomes for psychostimulant-addicted individuals. Recent advances in this regard include findings that drug-induced perturbations to neurotrophin, metabotropic glutamate receptor, and dopamine receptor signaling pathways perpetuate plasticity impairments at excitatory glutamatergic synapses on ventral tegmental area and nucleus accumbens neurons. In the context of addiction, much previous work, in terms of downstream effectors to these receptor systems, has centered on the extracellular-regulated MAP kinase signaling pathway. The purpose of the present review is to highlight the evidence of an emerging role for another downstream effector of these addiction-relevant receptor systems – the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 functions to regulate synaptic protein translation and is a potential critical link in our understanding of the neurobiological processes that drive addiction and relapse behavior. The precise cellular and molecular changes that are regulated by mTORC1 and contribute to relapse vulnerability are only just coming to light. Therefore, we aim to highlight evidence that mTORC1 signaling may be dysregulated by drug exposure and that these changes may contribute to aberrant translation of synaptic proteins that appear critical to increased relapse vulnerability, including AMPARs. The importance of understanding the role of this signaling pathway in the development of addiction vulnerability is underscored by the fact that the mTORC1 inhibitor rapamycin reduces drug-seeking in pre-clinical models and preliminary evidence indicating that rapamycin suppresses drug craving in humans
Indications for grain growth and mass decrease in cold dust disks around Classical T Tauri stars in the MBM 12 young association
We report detection of continuum emission at 850 and 450 micron from disks
around four Classical T Tauri stars in the MBM 12 (L1457) young association.
Using a simple model we infer masses of 0.0014-0.012 M_sun for the disk of LkHa
263 ABC, 0.005-0.021 M_sun for S18 ABab, 0.03-0.18 M_sun for LkHa 264 A, and
0.023-0.23 M_sun for LkHa 262. The disk mass found for LkHa 263 ABC is
consistent with the 0.0018 M_sun inferred from the scattered light image of the
edge-on disk around component C. Comparison to earlier 13CO line observations
indicates CO depletion by up to a factor 300 with respect to dark-cloud values.
The spectral energy distributions (SED) suggest grain growth, possibly to sizes
of a few hundred micron, but our spatially unresolved data cannot rule out
opacity as an explanation for the SED shape. Our observations show that these T
Tauri stars are still surrounded by significant reservoirs of cold material at
an age of 1-5 Myr. We conclude that the observed differences in disk mass are
likely explained by binary separation affecting the initial value. With
available accretion rate estimates we find that our data are consistent with
theoretical expectations for viscously evolving disks having decreased their
masses by ~30%.Comment: 15 pages, 3 figures, uses aastex. ApJ Letters, in pres
Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination
of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d)
legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have
applied a method developed for identifying embedded protostars in Perseus to
these datasets and in this way construct a relatively unbiased sample of 27
candidate embedded protostars with envelopes more massive than our sensitivity
limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores -
less than in Perseus (58%). On the other hand the mid-infrared sources in
Ophiuchus have less red mid-infrared colors, possibly indicating that they are
less embedded. We apply a nearest neighbor surface density algorithm to define
the substructure in each of the clouds and calculate characteristic numbers for
each subregion - including masses, star formation efficiencies, fraction of
embedded sources etc. Generally the main clusters in Ophiuchus and Perseus
(L1688, NGC1333 and IC348) are found to have higher star formation efficiencies
than small groups such as B1, L1455 and L1448, which on the other hand are
completely dominated by deeply embedded protostars. We discuss possible
explanations for the differences between the regions in Perseus and Ophiuchus,
such as different evolutionary timescales for the YSOs or differences, e.g., in
the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract
abridged). Version with full-resolution figures available at
http://www.astro.uni-bonn.de/~jes/paper120.pd
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Primary evolving networks and the comparative analysis of robust and fragile structures
In this paper we consider the structure of dynamically evolving networks modelling information and activity moving across a large set of vertices. We adopt the communicability concept that generalizes that of centrality which is defined for static networks. We define the primary network structure within the whole as comprising of the most influential vertices (both as senders and receivers of dynamically sequenced activity). We present a methodology based on successive vertex knockouts, up to a very small fraction of the whole primary network,that can characterize the nature of the primary network as being either relatively robust and lattice-like (with redundancies built in) or relatively fragile and tree-like (with sensitivities and few redundancies). We apply these ideas to the analysis of evolving networks derived from fMRI scans of resting human brains. We show that the estimation of performance parameters via the structure tests of the corresponding primary networks is subject to less variability than that observed across a very large population of such scans. Hence the differences within the population are significant
Population Gradients in Local Group Dwarf Spheroidals
We present a systematic and homogeneous analysis of population gradients for
the Local Group dwarf spheroidals (dSphs) Carina, Sculptor, Sextans, Tucana,
Andromeda I-III, V, and VI. For all of the Milky Way companions studied here we
find significant population gradients. The same is true for the remote dSph
Tucana located at the outskirts of the LG. Among the M 31 dSph companions only
Andromeda I and VI show obvious gradients. In all cases where a HB morphology
gradient is visible, the red HB stars are more centrally concentrated. The
occurence of a HB morphological gradient shows a correlation with a morphology
gradient in the red giant branch. It seems likely that metallicity is the
driver of the gradients in Sextans, Sculptor, Tucana, and Andromeda VI, while
age is an important factor in Carina. We find no evidence that the vicinity of
a nearby massive spiral galaxy influences the formation of the population
gradients.Comment: accepted for publication in AJ; 25 pages; 11 images in jpeg and png
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