9,808 research outputs found
Dark Energy and the mass of galaxy clusters
Up to now, Dark Energy evidences are based on the dynamics of the universe on
very large scales, above 1 Gpc. Assuming it continues to behave like a
cosmological constant on much smaller scales, I discuss its effects
on the motion of non-relativistic test-particles in a weak gravitational field
and I propose a way to detect evidences of at the scale of
about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.Comment: 5 pages, no figures, references adde
Abundance analysis, spectral variability, and search for the presence of a magnetic field in the typical PGa star HD19400
The aim of this study is to carry out an abundance determination, to search
for spectral variability and for the presence of a weak magnetic field in the
typical PGa star HD19400. High-resolution, high signal-to-noise HARPS
spectropolarimetric observations of HD19400 were obtained at three different
epochs in 2011 and 2013. For the first time, we present abundances of various
elements determined using an ATLAS12 model, including the abundances of a
number of elements not analysed by previous studies, such as Ne I, Ga II, and
Xe II. Several lines of As II are also present in the spectra of HD19400. To
study the variability, we compared the behaviour of the line profiles of
various elements. We report on the first detection of anomalous shapes of line
profiles belonging to Mn and Hg, and the variability of the line profiles
belonging to the elements Hg, P, Mn, Fe, and Ga. We suggest that the
variability of the line profiles of these elements is caused by their
non-uniform surface distribution, similar to the presence of chemical spots
detected in HgMn stars. The search for the presence of a magnetic field was
carried out using the moment technique and the SVD method. Our measurements of
the magnetic field with the moment technique using 22 Mn II lines indicate the
potential existence of a weak variable longitudinal magnetic field on the first
epoch. The SVD method applied to the Mn II lines indicates =-76+-25G on
the first epoch, and at the same epoch the SVD analysis of the observations
using the Fe II lines shows =-91+-35G. The calculated false alarm
probability values, 0.008 and 0.003, respectively, are above the value 10^{-3},
indicating no detection.Comment: 13+6 pages, 14 figures, 6+1 tables, including the online-only
material, accepted for publication in MNRA
Boundary fields and renormalization group flow in the two-matrix model
We analyze the Ising model on a random surface with a boundary magnetic field
using matrix model techniques. We are able to exactly calculate the disk
amplitude, boundary magnetization and bulk magnetization in the presence of a
boundary field. The results of these calculations can be interpreted in terms
of renormalization group flow induced by the boundary operator. In the
continuum limit this RG flow corresponds to the flow from non-conformal to
conformal boundary conditions which has recently been studied in flat space
theories.Comment: 31 pages, Late
Entanglement of Dirac fields in non-inertial frames
We analyze the entanglement between two modes of a free Dirac field as seen
by two relatively accelerated parties. The entanglement is degraded by the
Unruh effect and asymptotically reaches a non-vanishing minimum value in the
infinite acceleration limit. This means that the state always remains entangled
to a degree and can be used in quantum information tasks, such as
teleportation, between parties in relative uniform acceleration. We analyze our
results from the point of view afforded by the phenomenon of entanglement
sharing and in terms of recent results in the area of multi-qubit
complementarity.Comment: 15 pages, with 8 figures (Mar 2006); accepted to Physical Review A,
July 2006 - slightly revise
Codimension Two Compactifications and the Cosmological Constant Problem
We consider solutions of six dimensional Einstein equations with two compact
dimensions. It is shown that one can introduce 3-branes in this background in
such a way that the effective four dimensional cosmological constant is
completely independent of the brane tensions. These tensions are completely
arbitrary, without requiring any fine tuning. We must, however, fine tune bulk
parameters in order to obtain a sufficiently small value for the observable
cosmological constant. We comment in the effective four dimensional description
of this effect at energies below the compactification scale.Comment: 4 pages, rextex
Gauge Theory for the Rate Equations: Electrodynamics on a Network
Systems of coupled rate equations are ubiquitous in many areas of science,
for example in the description of electronic transport through quantum dots and
molecules. They can be understood as a continuity equation expressing the
conservation of probability. It is shown that this conservation law can be
implemented by constructing a gauge theory akin to classical electrodynamics on
the network of possible states described by the rate equations. The properties
of this gauge theory are analyzed. It turns out that the network is maximally
connected with respect to the electromagnetic fields even if the allowed
transitions form a sparse network. It is found that the numbers of degrees of
freedom of the electric and magnetic fields are equal. The results shed light
on the structure of classical abelian gauge theory beyond the particular
motivation in terms of rate equations.Comment: 4 pages, 2 figures included, v2: minor revision, as publishe
B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid Nebula, one of the youngest star forming regions
Recent magnetic field surveys in O- and B-type stars revealed that about 10%
of the core-hydrogen-burning massive stars host large-scale magnetic fields.
The physical origin of these fields is highly debated. To identify and model
the physical processes responsible for the generation of magnetic fields in
massive stars, it is important to establish whether magnetic massive stars are
found in very young star-forming regions or whether they are formed in close
interacting binary systems.
In the framework of our ESO Large Program, we carried out low-resolution
spectropolarimetric observations with FORS2 in 2013 April of the three most
massive central stars in the Trifid nebula, HD164492A, HD164492C, and
HD164492D. These observations indicated a strong longitudinal magnetic field of
about 500-600G in the poorly studied component HD164492C. To confirm this
detection, we used HARPS in spectropolarimetric mode on two consecutive nights
in 2013 June.
Our HARPS observations confirmed the longitudinal magnetic field in
HD164492C. Furthermore, the HARPS observations revealed that HD164492C cannot
be considered as a single star as it possesses one or two companions. The
spectral appearance indicates that the primary is most likely of spectral type
B1-B1.5V. Since in both observing nights most spectral lines appear blended, it
is currently unclear which components are magnetic. Long-term monitoring using
high-resolution spectropolarimetry is necessary to separate the contribution of
each component to the magnetic signal. Given the location of the system
HD164492C in one of the youngest star formation regions, this system can be
considered as a Rosetta Stone for our understanding of the origin of magnetic
fields in massive stars.Comment: Five pages, six figures, accepted for publication in A&
The algebraic and Hamiltonian structure of the dispersionless Benney and Toda hierarchies
The algebraic and Hamiltonian structures of the multicomponent dispersionless
Benney and Toda hierarchies are studied. This is achieved by using a modified
set of variables for which there is a symmetry between the basic fields. This
symmetry enables formulae normally given implicitly in terms of residues, such
as conserved charges and fluxes, to be calculated explicitly. As a corollary of
these results the equivalence of the Benney and Toda hierarchies is
established. It is further shown that such quantities may be expressed in terms
of generalized hypergeometric functions, the simplest example involving
Legendre polynomials. These results are then extended to systems derived from a
rational Lax function and a logarithmic function. Various reductions are also
studied.Comment: 29 pages, LaTe
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