6,654 research outputs found
Dynamical Mass Estimates for the Halo of M31 from Keck Spectroscopy
The last few months have seen the measurements of the radial velocities of
all of the dwarf spheroidal companions to the Andromeda galaxy (M31) using the
spectrographs (HIRES and LRIS) on the Keck Telescope. This paper summarises the
data on the radial velocities and distances for all the companion galaxies and
presents new dynamical modelling to estimate the mass of extended halo of M31.
The best fit values for the total mass of M31 are between 7 and 10 x 10^{11}
solar masses, depending on the details of the modelling. The mass estimate is
accompanied by considerable uncertainty caused by the smallness of the dataset;
for example, the upper bound on the total mass is roughly 24 x 10^{11} solar
masses, while the lower bound is about 3 x 10^{11} solar masses. These values
are less than the most recent estimates of the most likely mass of the Milky
Way halo. Bearing in mind all the uncertainties, a fair conclusion is that the
M31 halo is roughly as massive as that of the Milky Way halo. There is no
dynamical evidence for the widely held belief that M31 is more massive -- it
may even be less massive.Comment: In press, The Astrophysical Journal (Letters
Nuclear suppression of heavy quark production at forward rapidities in relativistic heavy ion collisions
We calculate nuclear suppression of heavy quarks produced from the
initial fusion of partons in nucleus-nucleus collisions at RHIC and LHC
energies. We take the shadowing as well as the energy loss suffered by them
while passing through Quark Gluon Plasma into account. We obtain results for
charm and bottom quarks at several rapidities using different mechanisms for
energy loss, to see if we can distinguish between them.Comment: 21 pages including 13 figures. To appear in J. Phys.
Screening and Anti-Screening Effects in J/psi Production on Nuclei
The nuclear effects in J/psi hadro- and electroproduction on nuclei are
considered in framework of reggeon approach. It is shown that screening regime
which holds for electroproduction at x_F > 0.7 and for hadroproduction at x_F >
-(0.3-0.4) is changed with anti-screening regime for smaller x_F values.Comment: 6 pages, 6 figures. Small changes in wordin
The effects of the frog Eleutherodactylus coqui on invertebrates and ecosystem processes at two scales in the Luquillo Experimental Forest, Puerto Rico
Determining the ubiquity of top-down control effects of predators on their prey and ecosystem processes is important for understanding community and ecosystem-level consequences that may result from predator loss. We conducted experiments at two spatial scales to investigate the effects of terrestrial frogs (Eleutherodactylus coqui) on aerial and litter invertebrates, plant growth and herbivory, and litter decomposition. At both scales, frogs reduced aerial invertebrates and leaf herbivory, but had no effect on litter invertebrates. At the smaller scale, frogs increased foliage production rates, measured as the number of new leaves and new leaf area produced, by 80% and decomposition rates by 20%. The influence of E. coqui on increasing primary productivity and decomposition rates at the smaller scale appeared to be a result of elimination and excretion rather than of controlling prey. While the results provide evidence for frogs controlling herbivorous prey at both scales, species effects on ecosystem processes were only detectable at the smaller scale. The results highlight the difficulties in conducting experiments at large spatial scales. The findings from this study imply that the loss of amphibians and other species of higher trophic levels may affect nutrient cycling rates in tropical forests
Determination of pulsation periods and other parameters of 2875 stars classified as MIRA in the All Sky Automated Survey (ASAS)
We have developed an interactive PYTHON code and derived crucial ephemeris
data of 99.4% of all stars classified as 'Mira' in the ASAS data base,
referring to pulsation periods, mean maximum magnitudes and, whenever possible,
the amplitudes among others. We present a statistical comparison between our
results and those given by the AAVSO International Variable Star Index (VSX),
as well as those determined with the machine learning automatic procedure of
Richards et al. 2012. Our periods are in good agreement with those of the VSX
in more than 95% of the stars. However, when comparing our periods with those
of Richards et al, the coincidence rate is only 76% and most of the remaining
cases refer to aliases. We conclude that automatic codes require still more
refinements in order to provide reliable period values. Period distributions of
the target stars show three local maxima around 215, 275 and 330 d, apparently
of universal validity, their relative strength seems to depend on galactic
longitude. Our visual amplitude distribution turns out to be bimodal, however
1/3 of the targets have rather small amplitudes (A 2.5) and could
refer to semi-regular variables (SR). We estimate that about 20% of our targets
belong to the SR class. We also provide a list of 63 candidates for period
variations and a sample of 35 multiperiodic stars which seem to confirm the
universal validity of typical sequences in the double period and in the
Petersen diagramsComment: 14 pages, 14 figures, and 8 tables. Accepted to The Astrophysical
Journal Supplement Series, September 201
Miniplant-Scale Evaluation of a Semibatch-Continuous Tandem Reactor System for the Hydroformylation of Long-Chain Olefins
Emergence of Zipf's Law in the Evolution of Communication
Zipf's law seems to be ubiquitous in human languages and appears to be a
universal property of complex communicating systems. Following the early
proposal made by Zipf concerning the presence of a tension between the efforts
of speaker and hearer in a communication system, we introduce evolution by
means of a variational approach to the problem based on Kullback's Minimum
Discrimination of Information Principle. Therefore, using a formalism fully
embedded in the framework of information theory, we demonstrate that Zipf's law
is the only expected outcome of an evolving, communicative system under a
rigorous definition of the communicative tension described by Zipf.Comment: 7 pages, 2 figure
Enhanced charm hadroproduction due to nonlinear corrections to the DGLAP equations
We have studied the effects of nonlinear scale evolution of the parton
distribution functions to charm production in collisions at center-of-mass
energies of 5.5, 8.8 and 14 TeV. We find that the differential charm cross
section can be enhanced up to a factor of 4-5 at low . The enhancement is
quite sensitive to the charm quark mass and the renormalization/factorization
scales.Comment: 4 pages, 3 eps-figures. To appear in the proceedings of the
seventeenth international conference on Ultra-Relativistic Nucleus-Nucleus
Collisions (Quark Matter 2004
A spiral structure in the disk of EX Draconis on the rise to outburst maximum
We report on the R-band eclipse mapping analysis of high-speed photometry of
the dwarf nova EX Dra on the rise to the maximum of the November 1995 outburst.
The eclipse map shows a one-armed spiral structure of ~180 degrees in azimuth,
extending in radius from R ~0.2 to 0.43 R_{L1} (where R_{L1} is the distance
from the disk center to the inner Lagrangian point), that contributes about 22
per cent of the total flux of the eclipse map. The spiral structure is
stationary in a reference frame co-rotating with the binary and is stable for a
timescale of at least 5 binary orbits. The comparison of the eclipse maps on
the rise and in quiescence suggests that the outbursts of EX Dra may be driven
by episodes of enhanced mass-transfer from the secondary star. Possible
explanations for the nature of the spiral structure are discussed.Comment: To appear in the Astrophysical Journal Letters; 8 pages, 2 figures;
coded with AAS latex styl
A Critical Appraisal and Evaluation of Modern PDFs
We review the present status of the determination of parton distribution
functions (PDFs) in the light of the precision requirements for the LHC in Run
2 and other future hadron colliders. We provide brief reviews of all currently
available PDF sets and use them to compute cross sections for a number of
benchmark processes, including Higgs boson production in gluon-gluon fusion at
the LHC. We show that the differences in the predictions obtained with the
various PDFs are due to particular theory assumptions made in the fits of those
PDFs. We discuss PDF uncertainties in the kinematic region covered by the LHC
and on averaging procedures for PDFs, such as advocated by the PDF4LHC15 sets,
and provide recommendations for the usage of PDF sets for theory predictions at
the LHC.Comment: 70 pages pdflatex, 19 figures, 17 tables; final versio
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