62,458 research outputs found
Properties of dust in the detached shells around U Ant, DR Ser, and V644 Sco
Understanding the properties of dust produced during the asymptotic giant
branch phase of stellar evolution is important for understanding the evolution
of stars and galaxies. Recent observations of the carbon AGB star R Scl have
shown that observations at far-infrared and submillimetre wavelengths can
effectively constrain the grain sizes in the shell, while the total mass
depends on the structure of the grains (solid vs. hollow or fluffy). We aim to
constrain the properties of the dust observed in the submillimetre in the
detached shells around the three carbon AGB stars U Ant, DR Ser, and V644 Sco,
and to investigate the constraints on the dust masses and grain sizes provided
by far-infrared and submm observations. We observed the carbon AGB stars U Ant,
DR Ser, and V644 Sco at 870 micron using LABOCA on APEX. Combined with
observations from the optical to far-infrared, we produced dust radiative
transfer models of the spectral energy distributions (SEDs) with contributions
from the stars, present-day mass-loss and detached shells. We tested the effect
of different total dust masses and grain sizes on the SED, and attempted to
consistently reproduce the SEDs from the optical to the submm. We derive dust
masses in the shells of a few 10e-5 Msun, assuming spherical, solid grains. The
best-fit grain radii are comparatively large, and indicate the presence of
grains between 0.1 micron-2 micron. The LABOCA observations suffer from
contamination from 12CO(3-2), and hence gives fluxes that are higher than the
predicted dust emission at submm wavelengths. We investigate the effect on the
best-fitting models by assuming different degrees of contamination and show
that far-infrared and submillimetre observations are important to constrain the
dust mass and grain sizes in the shells.Comment: Accepted by A&
On the particle spectrum and the conformal window
We study the SU(3) gauge theory with twelve flavours of fermions in the
fundamental representation as a prototype of non-Abelian gauge theories inside
the conformal window. Guided by the pattern of underlying symmetries, chiral
and conformal, we analyze the two-point functions theoretically and on the
lattice, and determine the finite size scaling and the infinite volume fermion
mass dependence of the would-be hadron masses. We show that the spectrum in the
Coulomb phase of the system can be described in the context of a universal
scaling analysis and we provide the nonperturbative determination of the
fermion mass anomalous dimension gamma*=0.235(46) at the infrared fixed point.
We comment on the agreement with the four-loop perturbative prediction for this
quantity and we provide a unified description of all existing lattice results
for the spectrum of this system, them being in the Coulomb phase or the
asymptotically free phase. Our results corroborate the view that the fixed
point we are studying is not associated to a physical singularity along the
bare coupling line and estimates of physical observables can be attempted on
either side of the fixed point. Finally, we observe the restoration of the U(1)
axial symmetry in the two-point functions.Comment: 40 pages, 22 figure
One,Two,Zero: Scales of Strong Interactions
We discuss our results on QCD with a number of fundamental fermions ranging
from zero to sixteen. These theories exhibit a wide array of fascinating
phenomena which have been under close scrutiny, especially in recent years,
first and foremost is the approach to conformality. To keep this review
focused, we have chosen scale generation, or lack thereof as a guiding theme,
however the discussion will be set in the general framework of the analysis of
the phases and phase transitions of strong interactions at zero and nonzero
temperature.Comment: 15 pages, prepared for IJMPA Special Issue 'Recent Nonperturbative
Developments in QCD-like Theories
Chiral symmetry restoration in QCD with many flavours
We discuss the phases of QCD in the parameter space spanned by the number of
light flavours and the temperature with respect to the realisation of chiral
and conformal symmetries. The intriguing interplay of these symmetries is best
studied by means of lattice simulations, and some selected results from our
recent work are presented here.Comment: 10 pages, proceedings of the 9th International Workshop on Critical
Point and Onset of Deconfinement, 17-21 November, 2014, ZiF, Bielefeld,
German
Twisting Null Geodesic Congruences, Scri, H-Space and Spin-Angular Momentum
The purpose of this work is to return, with a new observation and rather
unconventional point of view, to the study of asymptotically flat solutions of
Einstein equations. The essential observation is that from a given
asymptotically flat space-time with a given Bondi shear, one can find (by
integrating a partial differential equation) a class of asymptotically
shear-free (but, in general, twistiing) null geodesic congruences. The class is
uniquely given up to the arbitrary choice of a complex analytic world-line in a
four-parameter complex space. Surprisingly this parameter space turns out to be
the H-space that is associated with the real physical space-time under
consideration. The main development in this work is the demonstration of how
this complex world-line can be made both unique and also given a physical
meaning. More specifically by forcing or requiring a certain term in the
asymptotic Weyl tensor to vanish, the world-line is uniquely determined and
becomes (by several arguments) identified as the `complex center-of-mass'.
Roughly, its imaginary part becomes identified with the intrinsic spin-angular
momentum while the real part yields the orbital angular momentum.Comment: 26 pages, authors were relisted alphabeticall
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