10,561 research outputs found
The Membership and Distance of the Open Cluster Collinder 419
The young open cluster Collinder 419 surrounds the massive O star, HD 193322,
that is itself a remarkable multiple star system containing at least four
components. Here we present a discussion of the cluster distance based upon new
spectral classifications of the brighter members, UBV photometry, and an
analysis of astrometric and photometric data from the UCAC3 and 2MASS catalogs.
We determine an average cluster reddening of E(B-V)=0.37 +- 0.05 mag and a
cluster distance of 741 +- 36 pc. The cluster probably contains some very young
stars that may include a reddened M3 III star, IRAS~20161+4035
On the breaking of collinear factorization in QCD
We investigate the breakdown of collinear factorization for non-inclusive
observables in hadron-hadron collisions. For pure QCD processes, factorization
is violated at the three-loop level and it has a structure identical to that
encountered previously in the case of super-leading logarithms. In particular,
it is driven by the non-commutation of Coulomb/Glauber gluon exchanges with
other soft exchanges. Beyond QCD, factorization may be violated at the two-loop
level provided that the hard subprocess contains matrix element contributions
with phase differences between different colour topologies.Comment: Version 2: minor improvements for journal publicatio
Adaptive mesh refinement approach to construction of initial data for black hole collisions
The initial data for black hole collisions is constructed using a
conformal-imaging approach and a new adaptive mesh refinement technique, a
fully threaded tree (FTT). We developed a second-order accurate approach to the
solution of the constraint equations on a non-uniformly refined high resolution
Cartesian mesh including second-order accurate treatment of boundary conditions
at the black hole throats. Results of test computations show convergence of the
solution as the numerical resolution is increased. FTT-based mesh refinement
reduces the required memory and computer time by several orders of magnitude
compared to a uniform grid. This opens up the possibility of using Cartesian
meshes for very high resolution simulations of black hole collisions.Comment: 13 pages, 11 figure
Extracting the Distribution Amplitudes of the rho meson from the Color Glass Condensate
We extract the leading twist-2 and subleading twist-3 Distribution Amplitudes
(DAs) of the rho meson using the HERA data on diffractive rho photoproduction.
We do so using several Colour Glass Condensate (CGC) inspired and a Regge
inspired dipole models. We find that our extracted twist-2 DA is not much model
dependent and is consistent with QCD Sum Rules and lattice predictions. The
extracted twist-3 DA is more model dependent but is still consistent with the
Sum Rules prediction.Comment: 21 pages, 10 figures, 3 tables. Section 6 revised, figures 8 and 9
and table 3 updated. Conclusions essentially unchange
Chondrules and Opaque Phases in Unequilibrated R Chondrites: A Comprehensive Assessment of Their Formation
Equilibrated Rumuruti (R) chondrites record an oxygen fugacity between 0 and 3.5 log units below the fayalite-magnetite-quartz buffer, and a sulfur fugacity (fS2) 2 log units above the iron-troilite buffer. They are more than an order of magnitude more oxidized than the ordinary chondrites [1], and orders of magnitude more sulfidized than solar values. Although the R chondrites have the highest (delta)O-17 value of any meteorites, analyses of unequilibrated R chondrites indicate chondrule formation in an oxygen isotope reservoir similar to that of the ordinary chondrite chondrules. We present the relationship of the R chondrite parent body to pre-accretionary volatiles O and S based on our analyses of unequilibrated R chondrite material in two thin sections from the meteorite Mount Prestrud (PRE) 95404
Directed self-organization of graphene nanoribbons on SiC
Realization of post-CMOS graphene electronics requires production of
semiconducting graphene, which has been a labor-intensive process. We present
tailoring of silicon carbide crystals via conventional photolithography and
microelectronics processing to enable templated graphene growth on
4H-SiC{1-10n} (n = 8) crystal facets rather than the customary {0001} planes.
This allows self-organized growth of graphene nanoribbons with dimensions
defined by those of the facet. Preferential growth is confirmed by Raman
spectroscopy and high-resolution transmission electron microscopy (HRTEM)
measurements, and electrical characterization of prototypic graphene devices is
presented. Fabrication of > 10,000 top-gated graphene transistors on a 0.24 cm2
SiC chip demonstrates scalability of this process and represents the highest
density of graphene devices reported to date.Comment: 13 pages, 5 figure
First Results from the CHARA Array. II. A Description of the Instrument
The CHARA Array is a six 1-m telescope optical/IR interferometric array
located on Mount Wilson California, designed and built by the Center for High
Angular Resolution Astronomy of Georgia State University. In this paper we
describe the main elements of the Array hardware and software control systems
as well as the data reduction methods currently being used. Our plans for
upgrades in the near future are also described
Neutralino versus axion/axino cold dark matter in the 19 parameter SUGRA model
We calculate the relic abundance of thermally produced neutralino cold dark
matter in the general 19 parameter supergravity (SUGRA-19) model. A scan over
GUT scale parameters reveals that models with a bino-like neutralino typically
give rise to a dark matter density \Omega_{\tz_1}h^2\sim 1-1000, i.e. between 1
and 4 orders of magnitude higher than the measured value. Models with higgsino
or wino cold dark matter can yield the correct relic density, but mainly for
neutralino masses around 700-1300 GeV. Models with mixed bino-wino or
bino-higgsino CDM, or models with dominant co-annihilation or A-resonance
annihilation can yield the correct abundance, but such cases are extremely hard
to generate using a general scan over GUT scale parameters; this is indicative
of high fine-tuning of the relic abundance in these cases. Requiring that
m_{\tz_1}\alt 500 GeV (as a rough naturalness requirement) gives rise to a
minimal probably dip in parameter space at the measured CDM abundance. For
comparison, we also scan over mSUGRA space with four free parameters. Finally,
we investigate the Peccei-Quinn augmented MSSM with mixed axion/axino cold dark
matter. In this case, the relic abundance agrees more naturally with the
measured value. In light of our cumulative results, we conclude that future
axion searches should probe much more broadly in axion mass, and deeper into
the axion coupling.Comment: 23 pages including 17 .eps figure
Numerical Investigation of Cosmological Singularities
Although cosmological solutions to Einstein's equations are known to be
generically singular, little is known about the nature of singularities in
typical spacetimes. It is shown here how the operator splitting used in a
particular symplectic numerical integration scheme fits naturally into the
Einstein equations for a large class of cosmological models and thus allows
study of their approach to the singularity. The numerical method also naturally
singles out the asymptotically velocity term dominated (AVTD) behavior known to
be characteristic of some of these models, conjectured to describe others, and
probably characteristic of a subclass of the rest. The method is first applied
to the unpolarized Gowdy T cosmology. Exact pseudo-unpolarized solutions
are used as a code test and demonstrate that a 4th order accurate
implementation of the numerical method yields acceptable agreement. For generic
initial data, support for the conjecture that the singularity is AVTD with
geodesic velocity (in the harmonic map target space) < 1 is found. A new
phenomenon of the development of small scale spatial structure is also
observed. Finally, it is shown that the numerical method straightforwardly
generalizes to an arbitrary cosmological spacetime on with one
spacelike U(1) symmetry.Comment: 37 pp +14 figures (not included, available on request), plain Te
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