1,183 research outputs found
Condition for gapless color-antitriplet excitations in NJL models
We present an exact condition for the existence of gapless quasiparticle
excitations in NJL models of color superconducting quark matter with a
quark-quark interaction in the scalar color-antitriplet channel. The condition
can be represented by a rotated ellipse in the plane of mass and chemical
potential differences for the paired quark fields.Comment: Accepted for publication in PRC. 5 pages, 4 figures; Corrected typos
and added one more term to the series expansion in (19
The evolution of planetary nebulae VII. Modelling planetary nebulae of distant stellar systems
By means of hydrodynamical models we do the first investigations of how the
properties of planetary nebulae are affected by their metal content and what
can be learned from spatially unresolved spectrograms of planetary nebulae in
distant stellar systems. We computed a new series of 1D radiation-hydrodynamics
planetary nebulae model sequences with central stars of 0.595 M_sun surrounded
by initial envelope structures that differ only by their metal content. At
selected phases along the evolutionary path, the hydrodynamic terms were
switched off, allowing the models to relax for fixed radial structure and
radiation field into their equilibrium state with respect to energy and
ionisation. The analyses of the line spectra emitted from both the dynamical
and static models enabled us to systematically study the influence of
hydrodynamics as a function of metallicity and evolution. We also recomputed
selected sequences already used in previous publications, but now with
different metal abundances. These sequences were used to study the expansion
properties of planetary nebulae close to the bright cut-off of the planetary
nebula luminosity function. Our simulations show that the metal content
strongly influences the expansion of planetary nebulae: the lower the metal
content, the weaker the pressure of the stellar wind bubble, but the faster the
expansion of the outer shell because of the higher electron temperature. This
is in variance with the predictions of the interacting-stellar-winds model (or
its variants) according to which only the central-star wind is thought to be
responsible for driving the expansion of a planetary nebula. Metal-poor objects
around slowly evolving central stars become very dilute and are prone to depart
from thermal equilibrium because then adiabatic expansion contributes to gas
cooling. ...abridged abstract.Comment: 35 pages, 43 figures, accepted for publication by A&
Weak magnetic fields in central stars of planetary nebulae?
It is not yet clear whether magnetic fields play an essential role in shaping
planetary nebulae (PNe), or whether stellar rotation alone and/or a close
binary companion can account for the variety of the observed nebular
morphologies. In a quest for empirical evidence verifying or disproving the
role of magnetic fields in shaping PNe, we follow up on previous attempts to
measure the magnetic field in a representative sample of PN central stars. We
obtained low-resolution polarimetric spectra with FORS 2 at VLT for a sample of
twelve bright central stars of PNe with different morphology, including two
round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets
are Wolf-Rayet type central stars. For the majority of the observed central
stars, we do not find any significant evidence for the existence of surface
magnetic fields. However, our measurements may indicate the presence of weak
mean longitudinal magnetic fields of the order of 100 Gauss in the central star
of the young elliptical planetary nebula IC 418, as well as in the Wolf-Rayet
type central star of the bipolar nebula Hen2-113 and the weak emission line
central star of the elliptical nebula Hen2-131. A clear detection of a 250 G
mean longitudinal field is achieved for the A-type companion of the central
star of NGC 1514. Some of the central stars show a moderate night-to-night
spectrum variability, which may be the signature of a variable stellar wind
and/or rotational modulation due to magnetic features. We conclude that strong
magnetic fields of the order of kG are not widespread among PNe central stars.
Nevertheless, simple estimates based on a theoretical model of magnetized wind
bubbles suggest that even weak magnetic fields below the current detection
limit of the order of 100 G may well be sufficient to contribute to the shaping
of PNe throughout their evolution.Comment: 16 pages, 11 figures, 3 tables, accepted for publication in A&A;
References updated, minor correction
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
1-2-3-flavor color superconductivity in compact stars
We suggest a scenario where the three light quark flavors are sequentially
deconfined under increasing pressure in cold asymmetric nuclear matter, e.g.,
as in neutron stars. The basis for our analysis is a chiral quark matter model
of Nambu--Jona-Lasinio (NJL) type with diquark pairing in the spin-1 single
flavor (CSL) and spin-0 two/three flavor (2SC/CFL) channels, and a
Dirac-Brueckner Hartree-Fock (DBHF) approach in the nuclear matter sector. We
find that nucleon dissociation sets in at about the saturation density, n_0,
when the down-quark Fermi sea is populated (d-quark dripline) due to the flavor
asymmetry imposed by beta-equilibrium and charge neutrality. At about 3n_0
u-quarks appear forming a two-flavor color superconducting (2SC) phase, while
the s-quark Fermi sea is populated only at still higher baryon density. The
hybrid star sequence has a maximum mass of 2.1 M_sun. Two- and three-flavor
quark matter phases are found only in gravitationally unstable hybrid star
solutions.Comment: 4 pages, 2 figures, to appear in the proceedings of Quark Matter
2008: 20th International Conference on Ultra-Relativistic Nucleus Nucleus
Collisions (QM 2008), Jaipur, India, 4-10 Feb 200
The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes
We used the PMAS integral field spectrograph to obtain large sets of radial
velocities in the central regions of three northern Galactic globular clusters:
M3, M13, and M92. By applying the novel technique of crowded field 3D
spectroscopy, we measured radial velocities for about 80 stars within the
central ~ 10 arcsec of each cluster. These are by far the largest spectroscopic
datasets obtained in the innermost parts of these clusters up to now. To obtain
kinematical data across the whole extent of the clusters, we complement our
data with measurements available in the literature. We combine our velocity
measurements with surface brightness profiles to analyse the internal dynamics
of each cluster using spherical Jeans models, and investigate whether our data
provide evidence for an intermediate-mass black hole in any of the clusters.
The surface brightness profiles reveal that all three clusters are consistent
with a core profile, although shallow cusps cannot be excluded. We find that
spherical Jeans models with a constant mass-to-light ratio provide a good
overall representation of the kinematical data. A massive black hole is
required in none of the three clusters to explain the observed kinematics. Our
1sigma (3sigma) upper limits are 5300 M_sun (12000 M_sun) for M3, 8600 M_sun
(13000 M_sun) for M13, and 980 M_sun (2700 M_sun) for M92. A puzzling
circumstance is the existence of several potential high velocity stars in M3
and M13, as their presence can account for the majority of the discrepancies
that we find in our mass limits compared to M92.Comment: accepted for publication in A&A, 20 pages, 15 figures, tables D1 to
D6 only available at CD
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