748 research outputs found
Nonlocal radiative coupling in non monotonic stellar winds
There is strong observational evidence of shocks and clumping in
radiation-driven stellar winds from hot, luminous stars. The resulting non
monotonic velocity law allows for radiative coupling between distant locations,
which is so far not accounted for in hydrodynamic wind simulations. In the
present paper, we determine the Sobolev source function and radiative line
force in the presence of radiative coupling in spherically symmetric flows,
extending the geometry-free formalism of Rybicki and Hummer (1978) to the case
of three-point coupling, which can result from, e.g., corotating interaction
regions, wind shocks, or mass overloading. For a simple model of an overloaded
wind, we find that, surprisingly, the flow decelerates at all radii above a
certain height when nonlocal radiative coupling is accounted for. We discuss
whether radiation-driven winds might in general not be able to re-accelerate
after a non monotonicity has occurred in the velocity law.Comment: accepted by A&A, 8 pages, 4 figure
Mean-field instability of trapped dilute boson-fermion mixtures
The influence of boson-boson and boson-fermion interactions on the stability
of a binary mixture of bosonic and fermionic atoms is investigated. The density
profiles of the trapped mixture are obtained from direct numerical solution of
a modified Gross-Pitaevskii equation that is self-consistently coupled to the
mean-field generated by the interaction with the fermionic species. The
fermions which in turn feel the mean-field created by the bosons are treated in
Thomas-Fermi approximation. We study the effects of different combinations of
signs of the boson-boson and the boson-fermion scattering lengths and determine
explicit expressions for critical particle numbers as function of these
scattering lengths.Comment: 4 pages, 2 figures (using RevTeX4
Nuclear Structure based on Correlated Realistic Nucleon-Nucleon Potentials
We present a novel scheme for nuclear structure calculations based on
realistic nucleon-nucleon potentials. The essential ingredient is the explicit
treatment of the dominant interaction-induced correlations by means of the
Unitary Correlation Operator Method (UCOM). Short-range central and tensor
correlations are imprinted into simple, uncorrelated many-body states through a
state-independent unitary transformation. Applying the unitary transformation
to the realistic Hamiltonian leads to a correlated, low-momentum interaction,
well suited for all kinds of many-body models, e.g., Hartree-Fock or
shell-model. We employ the correlated interaction, supplemented by a
phenomenological correction to account for genuine three-body forces, in the
framework of variational calculations with antisymmetrised Gaussian trial
states (Fermionic Molecular Dynamics). Ground state properties of nuclei up to
mass numbers A<~60 are discussed. Binding energies, charge radii, and charge
distributions are in good agreement with experimental data. We perform angular
momentum projections of the intrinsically deformed variational states to
extract rotational spectra.Comment: 32 pages, 15 figure
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
Context: The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and mass-loss
determinations. Present models can only account for the inhomogeneity of
stellar winds in the approximation of small-scale structures that are optically
thin. This treatment of ``microclumping'' has led to reducing empirical
mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be
optically thick in spectral lines. We investigate how this ``macroclumping''
impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR)
model atmosphere code is generalized in the ``formal integral'' to account for
clumps that are not necessarily optically thin. Results: Optically thick clumps
reduce the effective opacity. This has a pronounced effect on the emergent
spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the
optically thin H-alpha line is not affected by wind porosity, but that the PV
resonance doublet becomes significantly weaker when macroclumping is taken into
account. The reported discrepancies between resonance-line and
recombination-line diagnostics can be resolved entirely with the macroclumping
modeling without downward revision of the mass-loss rate. Conclusions:
Mass-loss rates inferred from optically thin emission, such as the H-alpha line
in O stars, are not influenced by macroclumping. The strength of optically
thick lines, however, is reduced because of the porosity effects. Therefore,
neglecting the porosity in stellar wind modeling can lead to underestimating
empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex
High resolution X-ray spectroscopy of bright O type stars
Archival X-ray spectra of the four prominent single, non-magnetic O stars
Zeta Pup, Zeta Ori, Ksi Per and Zeta Oph, obtained in high resolution with
Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles
provide information about the shocked, hot gas which emits the X-radiation, and
about the bulk of comparably cool stellar wind material which partly absorbs
this radiation. In this paper, we synthesize X-ray line profiles with a model
of a clumpy stellar wind. We find that the geometrical shape of the wind
inhomogeneities is important: better agreement with the observations can be
achieved with radially compressed clumps than with spherical clumps. The
parameters of the model, i.e. chemical abundances, stellar radius, mass-loss
rate and terminal wind velocity, are taken from existing analyses of UV and
optical spectra of the programme stars. On this basis, we also calculate the
continuum-absorption coefficient of the cool-wind material, using the Potsdam
Wolf-Rayet (PoWR) model atmosphere code. The radial location of X-ray emitting
gas is restricted from analysing the fir line ratios of helium-like ions. The
only remaining free parameter of our model is the typical distance between the
clumps; here, we assume that at any point in the wind there is one clump
passing by per one dynamical time-scale of the wind. The total emission in a
model line is scaled to the observation. There is a good agreement between
synthetic and observed line profiles. We conclude that the X-ray emission line
profiles in O stars can be explained by hot plasma embedded in a cool wind
which is highly clumped in the form of radially compressed shell fragments.Comment: a typo corrected, 14 pages, MNRAS, in pres
A Simple Scaling Analysis of X-ray Emission and Absorption in Hot-Star Winds
We present a simple analysis of X-ray emission and absorption for hot-star
winds, designed to explore the natural scalings of the observed X-ray
luminosity with wind and sstellar properties. We show that an exospheric
approximation, in which all of the emission above the optical depth unity
radius escapes the wind, reproduces very well the detailed expression for
radiation transport through a spherically symmetric wind. Using this
approximation we find that the X-ray luminosity scales naturally with the
wind density parameter \Mdot/\vinf, obtaining L_x \sim (\Mdot/\vinf)^2 for
optically thin winds, and L_x \sim (\Mdot/\vinf)^{1+s} for optically thick
winds with an X-ray filling factor that varies in radius as . These
scalings with wind density contrast with the commonly inferred empirical
scalings of X-ray luminosity with bolometric luminosity . The
empirically derived linear scaling of for thick winds can
however be reproduced, through a delicate cancellation of emission and
absorption, if one assumes modest radial fall-off in the X-ray filling factor
( or , depending on details of the secondary
scaling of wind density with luminosity). We also explore the nature of the
X-ray spectral energy distribution in the context of this model, and find that
the spectrum is divided into a soft, optically thick part and a hard, optically
thin part. Finally, we conclude that the energy-dependent emissivity must have
a high-energy cut-off, corresponding to the maximum shock energy, in order to
reproduce the general trends seen in X-ray spectral energy distributions of hot
stars.Comment: 16 pages, 2 figures, requiress aaspp4.sty, accepted by Astrophysical
Journal, to appear in the Aug 10, 1999 issue. Several minor changes have been
made at the suggestion of the referee. We have added an appendix in which we
consider winds with beta-velocity laws, rather than simply constant
velocitie
A unitary correlation operator method
The short range repulsion between nucleons is treated by a unitary
correlation operator which shifts the nucleons away from each other whenever
their uncorrelated positions are within the replusive core. By formulating the
correlation as a transformation of the relative distance between particle
pairs, general analytic expressions for the correlated wave functions and
correlated operators are given. The decomposition of correlated operators into
irreducible n-body operators is discussed. The one- and two-body-irreducible
parts are worked out explicitly and the contribution of three-body correlations
is estimated to check convergence. Ground state energies of nuclei up to mass
number A=48 are calculated with a spin-isospin-dependent potential and single
Slater determinants as uncorrelated states. They show that the deduced energy-
and mass-number-independent correlated two-body Hamiltonian reproduces all
"exact" many-body calculations surprisingly well.Comment: 43 pages, several postscript figures, uses 'epsfig.cls'. Submitted to
Nucl. Phys. A. More information available at http://www.gsi.de/~fm
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