485 research outputs found
The energy dependence of p_t angular correlations inferred from mean-p_t fluctuation scale dependence in heavy ion collisions at the SPS and RHIC
We present the first study of the energy dependence of pt angular correlations inferred from event-wisemean transverse momentum (pt) fluctuations in heavy ion collisions. We compare our large-acceptancemeasurements at CM energies â^sNN = 19.6, 62.4, 130 and 200 GeV to SPS measurements at 12.3 and 17.3 GeV. p_t angular correlation structure suggests that the principal source of p_t correlations and fluctuations is minijets (minimum-bias parton fragments). We observe a dramatic increase in correlations and fluctuations from SPS to RHIC energies, increasing linearly with ln â^sNN from the onset of observable jet-related (p_t) fluctuations near 10 GeV
Two-particle correlations on transverse momentum and momentum dissipation in AuâAu collisions at âsNN = 130 GeV
Measurements of two-particle correlations on transverse momentum p_t for AuâAu collisions at â^sNN = 130 GeV are presented. Significant largemomentum-scale correlations are observed for charged primary hadrons with 0.15 †p_t †2 GeV/c and pseudorapidity |η| †1.3. Such correlations
were not observed in a similar study at lower energy and are not predicted by theoretical collision models. Their direct relation to mean-p_t fluctuations measured in the same angular acceptance is demonstrated. Positive correlations are observed for pairs of particles which have large pt values while negative correlations occur for pairs in which one particle has large p_t and the other has
much lower p_t . The correlation amplitudes per final state particle increase with collision centrality. The observed correlations are consistent with a scenario in which the transverse momentum of hadrons associated with initial-stage
semi-hard parton scattering is dissipated by the medium to lower p_t
Modeling Diel Vertical Migration with Membrane Computing
Diel vertical migration (DVM) is an important ecological phenomenon in which zooplankton migrate vertically to deal with
trade-offs associated with greater food availability in shallow waters and lower predator risk in deep waters due to lower
light availability. Because of these trade-offs, DVM dynamics are particularly sensitive to changes in light intensity at the
water surface. Therefore, changes in the proportion of cloudy and sunny days have the potential to disrupt DVM dynamics.
We propose a new membrane computing model that captures the effect of cloud cover on DVM in Daphnia, and we use
it to explore the impacts of an increased proportion of cloudy days that are predicted to occur with climate change. Our
2-dimensional, spatially explicit model integrates multiple trophic levels from abiotic nutrients to Daphnia predators. We
analyzed the effect that different proportions of cloudy and sunny days throughout the summer have on our model. The model
simulations suggest that an increase in sunny days promotes a high phytoplankton concentration near the surface but does
not necessarily promote an increased abundance of Daphnia. Our model also suggests that a higher proportion of cloudy
days would increase Daphnia abundance due to a shift in the vertical distribution of Daphnia populations towards superficial
waters. Our results highlight that climate changes in multiple regions will affect animal migrations leading to altered food
web dynamics in freshwater ecosystems, and emphasize the potential of membrane computing as a modeling framework for
spatially and temporally explicit ecological processes
Direct Observation of Dijets in Central Au + Au Collisions at âs_(NN) = 200âGeV
The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum (p_T) charged hadrons in Au + Au collisions at higher pT than reported previously. As p_T is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter
Stellar model atmospheres with magnetic line blanketing
Model atmospheres of A and B stars are computed taking into account magnetic
line blanketing. These calculations are based on the new stellar model
atmosphere code LLModels which implements direct treatment of the opacities due
to the bound-bound transitions and ensures an accurate and detailed description
of the line absorption. The anomalous Zeeman effect was calculated for the
field strengths between 1 and 40 kG and a field vector perpendicular to the
line of sight. The model structure, high-resolution energy distribution,
photometric colors, metallic line spectra and the hydrogen Balmer line profiles
are computed for magnetic stars with different metallicities and are discussed
with respect to those of non-magnetic reference models. The magnetically
enhanced line blanketing changes the atmospheric structure and leads to a
redistribution of energy in the stellar spectrum. The most noticeable feature
in the optical region is the appearance of the 5200 A depression. However, this
effect is prominent only in cool A stars and disappears for higher effective
temperatures. The presence of a magnetic field produces opposite variation of
the flux distribution in the optical and UV region. A deficiency of the UV flux
is found for the whole range of considered effective temperatures, whereas the
``null wavelength'' where flux remains unchanged shifts towards the shorter
wavelengths for higher temperatures.Comment: accepted by Astronomy & Astrophysic
Inter-Cohort Competition Drives Density Dependence and Selective Mortality in a Marine Fish
For organisms with complex life cycles, the transition between life stages and between habitats can act as a significant demographic and selective bottleneck. In particular, competition with older and larger conspecifics and heterospecifics may influence the number and characteristics of individuals successfully making the transition. We investigated whether the availability of enemy-free space mediated the interaction between adult goldspot gobies (Gnatholepis thompsoni), a. common tropical reef fish, and juvenile conspecifics that had recently settled from the plankton. We added rocks, which provide refuge from predators, to one-half of each of five entire coral reefs in the Bahamas and measured the survival and growth of recent settlers in relation to adult goby densities. We also evaluated whether mortality was selective with respect to three larval traits (age at settlement, size at settlement, and presettlement growth rate) and measured the influence of refuge availability and adult goby density on selection intensity. Selective mortality was measured by comparing larval traits of newly settled gobies (†5 postsettlement) with those of survivors (2-3 week postsettlement juveniles). We detected a negative relationship between juvenile survival and adult goby density in both low- and high-refuge habitats, though experimental refuge addition reduced the intensity of this density dependence. Juvenile growth also declined with increasing adult goby density, but this effect was similar in both low- and high-refuge habitats. Refuge availability had no consistent effect on selective mortality, but adult goby density was significantly related to the intensity of size-selective mortality: bigger juveniles were favored where adults were abundant, and smaller juveniles were favored where adults were rare. Given the typically large difference in sizes of juveniles and adults, similar stage-structured interactions may be common but underappreciated in many marine species
Resolving the compact HII regions in N160A with HST
Using high-resolution imaging with the Hubble Space Telescope, we study the
Large Magellanic Cloud HII region N160A and uncover several striking features
of this complex massive star-forming site. The two compact high excitation HII
blobs (HEBs) A1 and A2 are for the first time resolved and their stellar
content and morphology is revealed. A1, being of higher excitation, is powered
by a single massive star whose strong wind has created a surrounding bubble. A2
harbors several exciting stars enshrouded inside large quantities of dust. The
whole N160A nebula is energized by three star clusters for which we obtain
photometry and study their color-magnitude diagram. The HII region is
particularly dusty, with extinction values reaching an A_v~2.5 mag in the
visible, and it is separated from the molecular cloud by an outstanding
ionization front. A previously detected infrared young stellar object is also
accurately located with respect to the HII region.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysics. A version of the paper with higher quality images is available
at http://wwwusr.obspm.fr/~heydari/projects/N16
STIS spectroscopy of newborn massive stars in SMC N81
Using Hubble Space Telescope observations with STIS, we study the main
exciting stars of N81, a high excitation compact Hii region in the Small
Magellanic Cloud (SMC). These far UV observations are the first spectroscopic
measurements of stars in such a region and reveal features characteristic of an
O6-O8 stellar type. The astonishing weakness of their wind profiles and their
sub-luminosity (up to ~ 2 mag fainter in Mv than the corresponding dwarfs) make
these stars a unique stellar population in the Magellanic Clouds. Our analysis
suggests that they are probably in the Hertzsprung-Russell diagram locus of a
particularly young class of massive stars, the so-called Vz luminosity class,
as they are arriving on the zero age main sequence.Comment: 9 pages, 3 figure
Costs of Inducible Defence along a Resource Gradient
In addition to having constitutive defence traits, many organisms also respond to predation by phenotypic plasticity. In order for plasticity to be adaptive, induced defences should incur a benefit to the organism in, for example, decreased risk of predation. However, the production of defence traits may include costs in fitness components such as growth, time to reproduction, or fecundity. To test the hypothesis that the expression of phenotypic plasticity incurs costs, we performed a common garden experiment with a freshwater snail, Radix balthica, a species known to change morphology in the presence of molluscivorous fish. We measured a number of predator-induced morphological and behavioural defence traits in snails that we reared in the presence or absence of chemical cues from fish. Further, we quantified the costs of plasticity in fitness characters related to fecundity and growth. Since plastic responses may be inhibited under limited resource conditions, we reared snails in different densities and thereby levels of competition. Snails exposed to predator cues grew rounder and thicker shells, traits confirmed to be adaptive in environments with fish. Defence traits were consistently expressed independent of density, suggesting strong selection from predatory molluscivorous fish. However, the expression of defence traits resulted in reduced growth rate and fecundity, particularly with limited resources. Our results suggest full defence in predator related traits regardless of resource availability, and costs of defence consequently paid in traits related to fitness
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