658 research outputs found
A probabilistic model of insolation for the Mojave Desert area
A discussion of mathematical models of insolation characteristics suitable for use in analysis of solar energy systems is presented and shows why such models are essential for solar energy system design. A model of solar radiation for the Mojave Desert area is presented with probabilistic and deterministic components which reflect the occurrence and density of clouds and haze, and mimic their effects on both direct and indirect radiation. Multiple comparisons were made between measured total energy received per day and the corresponding simulated totals. The simulated totals were all within 11 percent of the measured total. The conclusion is that a useful probabilistic model of solar radiation for the Goldstone, California, area of the Mojave Desert has been constructed
Crystal-field splitting for low symmetry systems in ab initio calculations
In the framework of the LDA+U approximation we propose the direct way of
calculation of crystal-field excitation energy and apply it to La and Y
titanates. The method developed can be useful for comparison with the results
of spectroscopic measurements because it takes into account fast relaxations of
electronic system. For titanates these relaxation processes reduce the value of
crystal-field splitting by as compared with the difference of LDA one
electron energies. However, the crystal-field excitation energy in these
systems is still large enough to make an orbital liquid formation rather
unlikely and experimentally observed isotropic magnetism remains unexplained.Comment: 13 pages, 5 figures, 3 table
Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds
By means of two- and three-dimensional particle-in-cell simulations, we
investigate the process of driven magnetic reconnection at the termination
shock of relativistic striped flows. In pulsar winds and in magnetar-powered
relativistic jets, the flow consists of stripes of alternating magnetic field
polarity, separated by current sheets of hot plasma. At the wind termination
shock, the flow compresses and the alternating fields annihilate by driven
magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the
wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated
flows), shock-driven reconnection transfers all the magnetic energy of
alternating fields to the particles, whose average Lorentz factor increases by
a factor of sigma with respect to the pre-shock value. In the limit
lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind,
the post-shock particle spectrum approaches a flat power-law tail with slope
around -1.5, populated by particles accelerated by the reconnection electric
field. The presence of a current-aligned "guide" magnetic field suppresses the
acceleration of particles only when the guide field is stronger than the
alternating component. Our findings place important constraints on the models
of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at
https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special
issue of Computational Science and Discovery on selected research from the
22nd International Conference on Numerical Simulation of Plasma
Very-high-energy gamma radiation associated with the unshocked wind of the Crab pulsar
We show that the relativistic wind in the Crab pulsar, which is commonly
thought to be invisible in the region upstream of the termination shock at R <
0.1 pc, in fact could be directly observed through its inverse Compton gamm-ray
emission. The search for such specific component of radiation in the gamma-ray
spectrum of the Crab can provide unique information about the unshocked pulsar
wind that is not accessible at other wavelengths.Comment: 11 pages, 11 figures, to appear in one of the April issues of MNRA
A theoretical investigation into the trapping of noble gases by clathrates on Titan
In this paper, we use a statistical thermodynamic approach to quantify the
efficiency with which clathrates on the surface of Titan trap noble gases. We
consider different values of the Ar, Kr, Xe, CH4, C2H6 and N2 abundances in the
gas phase that may be representative of Titan's early atmosphere. We discuss
the effect of the various parameters that are chosen to represent the
interactions between the guest species and the ice cage in our calculations. We
also discuss the results of varying the size of the clathrate cages. We show
that the trapping efficiency of clathrates is high enough to significantly
decrease the atmospheric concentrations of Xe and, to a lesser extent, of Kr,
irrespective of the initial gas phase composition, provided that these
clathrates are abundant enough on the surface of Titan. In contrast, we find
that Ar is poorly trapped in clathrates and, as a consequence, that the
atmospheric abundance of argon should remain almost constant. We conclude that
the mechanism of trapping noble gases via clathration can explain the
deficiency in primordial Xe and Kr observed in Titan's atmosphere by Huygens,
but that this mechanism is not sufficient to explain the deficiency in Ar.Comment: Accepted for publication in Planetary and Space Scienc
Measurement of high-p_T Single Electrons from Heavy-Flavor Decays in p+p Collisions at sqrt(s) = 200 GeV
The momentum distribution of electrons from decays of heavy flavor (charm and
beauty) for midrapidity |y| < 0.35 in p+p collisions at sqrt(s) = 200 GeV has
been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider
(RHIC) over the transverse momentum range 0.3 < p_T < 9 GeV/c. Two independent
methods have been used to determine the heavy flavor yields, and the results
are in good agreement with each other. A fixed-order-plus-next-to-leading-log
pQCD calculation agrees with the data within the theoretical and experimental
uncertainties, with the data/theory ratio of 1.72 +/- 0.02^stat +/- 0.19^sys
for 0.3 < p_T < 9 GeV/c. The total charm production cross section at this
energy has also been deduced to be sigma_(c c^bar) = 567 +/- 57^stat +/-
224^sys micro barns.Comment: 375 authors from 57 institutions, 6 pages, 3 figures. Submitted to
Physical Review Letters. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (or will be) publicly
available at http://www.phenix.bnl.gov/papers.htm
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
Charged hadron multiplicity fluctuations in Au+Au and Cu+Cu collisions from sqrt(s_NN) = 22.5 to 200 GeV
A comprehensive survey of event-by-event fluctuations of charged hadron
multiplicity in relativistic heavy ions is presented. The survey covers Au+Au
collisions at sqrt(s_NN) = 62.4 and 200 GeV, and Cu+Cu collisions sqrt(s_NN) =
22.5, 62.4, and 200 GeV. Fluctuations are measured as a function of collision
centrality, transverse momentum range, and charge sign. After correcting for
non-dynamical fluctuations due to fluctuations in the collision geometry within
a centrality bin, the remaining dynamical fluctuations expressed as the
variance normalized by the mean tend to decrease with increasing centrality.
The dynamical fluctuations are consistent with or below the expectation from a
superposition of participant nucleon-nucleon collisions based upon p+p data,
indicating that this dataset does not exhibit evidence of critical behavior in
terms of the compressibility of the system. An analysis of Negative Binomial
Distribution fits to the multiplicity distributions demonstrates that the heavy
ion data exhibit weak clustering properties.Comment: 464 authors from 60 institutions, 17 pages, 12 figures, 1 table.
Submitted to Physical Review C. Plain text data tables for the points plotted
in figures for this and previous PHENIX publications are (or will be)
publicly available at http://www.phenix.bnl.gov/papers.htm
Cold Nuclear Matter Effects on J/Psi as Constrained by Deuteron-Gold Measurements at sqrt(s_NN) = 200 GeV
We present a new analysis of J/psi production yields in deuteron-gold
collisions at sqrt(s_NN) = 200 GeV using data taken by the PHENIX experiment in
2003 and previously published in [S.S. Adler et al., Phys. Rev. Lett 96, 012304
(2006)]. The high statistics proton-proton J/psi data taken in 2005 is used to
improve the baseline measurement and thus construct updated cold nuclear matter
modification factors R_dAu. A suppression of J/psi in cold nuclear matter is
observed as one goes forward in rapidity (in the deuteron-going direction),
corresponding to a region more sensitive to initial state low-x gluons in the
gold nucleus. The measured nuclear modification factors are compared to
theoretical calculations of nuclear shadowing to which a J/psi (or precursor)
break-up cross-section is added. Breakup cross sections of sigma_breakup =
2.8^[+1.7_-1.4] (2.2^[+1.6_-1.5]) mb are obtained by fitting these calculations
to the data using two different models of nuclear shadowing. These breakup
cross section values are consistent within large uncertainties with the 4.2 +/-
0.5 mb determined at lower collision energies. Projecting this range of cold
nuclear matter effects to copper-copper and gold-gold collisions reveals that
the current constraints are not sufficient to firmly quantify the additional
hot nuclear matter effect.Comment: 453 authors from 59 institutions, 15 pages, 13 figures, 5 tables.
Submitted to Physical Review C. Plain text data tables for the points plotted
in figures for this and previous PHENIX publications are (or will be)
publicly available at http://www.phenix.bnl.gov/papers.htm
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