15,885 research outputs found
Near-infrared spectropolarimetry of a delta-spot
Sunspots harboring umbrae of both magnetic polarities within a common
penumbra (delta-spots) are often but not always related to flares. We present
first near-infrared (NIR) observations (Fe I 1078.3 nm and Si I 1078.6 nm
spectra) obtained with the Tenerife Infrared Polarimeter (TIP) at the Vacuum
Tower Telescope (VTT) in Tenerife on 2012 June 17, which afford accurate and
sensitive diagnostics to scrutinize the complex fields along the magnetic
neutral line of a delta-spot within active region NOAA 11504. We examine the
vector magnetic field, line-of-sight (LOS) velocities, and horizontal proper
motions of this rather inactive delta-spot. We find a smooth transition of the
magnetic vector field from the main umbra to that of opposite polarity
(delta-umbra), but a discontinuity of the horizontal magnetic field at some
distance from the delta-umbra on the polarity inversion line. The magnetic
field decreases faster with height by a factor of two above the delta-umbra.
The latter is surrounded by its own Evershed flow. The Evershed flow coming
from the main umbra ends at a line dividing the spot into two parts. This line
is marked by the occurrence of central emission in the Ca II 854.2 nm line.
Along this line, high chromospheric LOS-velocities of both signs appear. We
detect a shear flow within the horizontal flux transport velocities parallel to
the dividing line.Comment: 4 pages, will appear as Letter in Astronomy & Astrophysic
Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference
A recent experimental study showed that, distorting a CoPc molecule adsorbed
on a Au(111) surface, a Kondo effect is induced with a temperature higher than
200 K. We examine a model in which an atom with strong Coulomb repulsion (Co)
is surrounded by four atoms on a square (molecule lobes), and two atoms above
and below it representing the apex of the STM tip and an atom on the gold
surface (all with a single, half-filled, atomic orbital). The Hamiltonian is
solved exactly for the isolated cluster, and, after connecting the leads (STM
tip and gold), the conductance is calculated by standard techniques. Quantum
interference prevents the existence of the Kondo effect when the orbitals on
the square do not interact (undistorted molecule); the Kondo resonance shows up
after switching on that interaction. The weight of the Kondo resonance is
controlled by the interplay of couplings to the STM tip and the gold surface,
and between the molecule lobes.Comment: 5 pages, 3 figura
DNA nanotweezers studied with a coarse-grained model of DNA
We introduce a coarse-grained rigid nucleotide model of DNA that reproduces
the basic thermodynamics of short strands: duplex hybridization,
single-stranded stacking and hairpin formation, and also captures the essential
structural properties of DNA: the helical pitch, persistence length and
torsional stiffness of double-stranded molecules, as well as the comparative
flexibility of unstacked single strands. We apply the model to calculate the
detailed free-energy landscape of one full cycle of DNA 'tweezers', a simple
machine driven by hybridization and strand displacement.Comment: 4 pages, 5 figure
Exact Quantum States for all Two-Dimensional Dilaton Gravity Theories
It is shown that the recently obtained quantum wave functionals in terms of
the CJZ variables for generic 2d dilaton gravity are equivalent to the
previously reported exact quantum wave functionals in geometrical variables. A
third representation of these exact quantum states is also presented
Minimal Universal Extra Dimensions
Highly degenerate spectra associated with universal extra dimensions (UED)
provide an interesting phenomenology not only from the point of view of
cosmology and astrophysics, but also for colliders. We study these exotic
signals for the simplest case, called minimal UED, where it is natural to find
slow charged particles, displaced vertices, tracks with non-vanishing impact
parameters, track kinks, and even vanishing charged tracks.Comment: 6 pages, 3 figures. Contributed to XXIII International Symposium on
Lepton and Photon Interactions at High Energy, Aug 13-18, 2007, Daegu, Kore
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
Resolving Cosmic Gamma Ray Anomalies with Dark Matter Decaying Now
Dark matter particles need not be completely stable, and in fact they may be
decaying now. We consider this possibility in the frameworks of universal extra
dimensions and supersymmetry with very late decays of WIMPs to Kaluza-Klein
gravitons and gravitinos. The diffuse photon background is a sensitive probe,
even for lifetimes far greater than the age of the Universe. Remarkably, both
the energy spectrum and flux of the observed MeV gamma ray excess may be
naturally explained by decaying dark matter with MeV mass splittings. Future
observations of continuum and line photon fluxes will test this explanation and
may provide novel constraints on cosmological parameters.Comment: 4 pages, 4 figures. Published Versio
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