82 research outputs found
The temperature dependency of Wolf-Rayet-type mass loss: An exploratory study for winds launched by the hot iron bump
CONTEXT: The mass loss of He-burning stars, which are partially or completely
stripped of their outer hydrogen envelope, is a catalyst of the cosmic matter
cycle and decisive ingredient of massive star evolution. Yet, its theoretical
fundament is only starting to emerge with major dependencies still to be
uncovered.
AIMS: A temperature or radius dependence is usually not included in
descriptions for the mass loss of classical Wolf-Rayet (cWR) stars, despite
being crucial for other hot star wind domains. We thus aim to determine whether
such a dependency will also be necessary for a comprehensive description of
mass loss in the cWR regime.
METHODS: Sequences of dynamically consistent atmosphere models were
calculated with the hydrodynamic branch of the PoWR code along the temperature
domain, using different choices for luminosity, mass, and surface abundances.
For the first time, we allowed nonmonotonic velocity fields when solving the
equation of motion. The resulting velocity structures were then interpolated
for the comoving-frame radiative transfer, ensuring that the main wind
characteristics were preserved.
RESULTS: We find a strong dependence of the mass-loss rate with the
temperature of the critical/sonic point which mainly reflects the different
radii and resulting gravitational accelerations. Moreover, we obtain a relation
between the observed effective temperature and the transformed mass-loss rate
which seems to be largely independent of the underlying stellar parameters. The
relation shifts for different clumping factors in the outer wind. Below a
characteristic value of -4.5, the slope of this relation changes and the winds
become transparent for He II ionizing photons.
CONCLUSIONS: The mass loss of cWR stars is a high-dimensional problem but
also shows inherent scalings which can be used to obtain an approximation of
the observed effective temperature. (...)Comment: 16 pages + 5 page appendix, 17+9 figures, 3+2 tables. Accepted for
publication in A&
Association between low-grade chronic inflammation and depressed left atrial compliance in heart failure with preserved ejection fraction : a retrospective analysis
Dynamically inflated wind models of classical Wolf-Rayet stars
Vigorous mass loss in the classical Wolf-Rayet (WR) phase is important for
the late evolution and final fate of massive stars. We develop spherically
symmetric time-dependent and steady-state hydrodynamical models of the
radiation-driven wind outflows and associated mass loss from classical WR
stars. The simulations are based on combining the opacities typically used in
static stellar structure and evolution models with a simple parametrised form
for the enhanced line-opacity expected within a supersonic outflow. Our
simulations reveal high mass-loss rates initiated in deep and hot optically
thick layers around T\approx 200kK. The resulting velocity structure is
non-monotonic and can be separated into three phases: i) an initial
acceleration to supersonic speeds ii) stagnation and even deceleration, and
iii) an outer region of rapid re-acceleration. The characteristic structures
seen in converged steady-state simulations agree well with the outflow
properties of our time-dependent models. By directly comparing our dynamic
simulations to corresponding hydrostatic models, we demonstrate explicitly that
the need to invoke extra energy transport in convectively inefficient regions
of stellar structure and evolution models is merely an artefact of enforcing a
hydrostatic outer boundary. Moreover, the "dynamically inflated" inner regions
of our simulations provide a natural explanation for the often-found mismatch
between predicted hydrostatic WR radii and those inferred from spectroscopy.
Finally, we contrast our simulations with alternative recent WR wind models
based on co-moving frame radiative transfer for computing the radiation force.
Since CMF transfer currently cannot handle non-monotonic velocity fields, the
characteristic deceleration regions found here are avoided in such simulations
by invoking an ad-hoc very high degree of clumping.Comment: 15 pages, 9 figure
Plasma-spray synthesis and characterization of ti-based nitride and oxide nanogranules
Metal MatrixâMetal Nanoparticle Composites with Tunable Melting Temperature and High Thermal Conductivity for Phase-Change Thermal Storage
Evidence for self-sputtering during pulsed laser deposition of Zn
4 pags. ; 4 figs.Thin films of Zn have been prepared by pulsed
laser deposition with a KrF excimer laser (248 nm). The laser energy density (E.D.) on the target has been varied in the 1 to 5 J/cm2 range. The results show that as the E.D. increases the material distribution changes. For low E.D.(†1.6 J/cm2)the maximum of the distribution is at the substrate center, for intermediate E.D. it is displaced to the side, and a clear minimum
appears at the center of the substrate for the higher E.D.(â„4.5 J/cm2). The growth velocity at the center of the substrate
reaches a maximum value for E.D. of 2.8 J/cm2, and decreases for higher E.D. as a result of the competition between deposition
and self-sputtering. Virtually a zero growth velocity is obtained for E.D. above 4.5 J/cm2. The self-sputtering process is most
likely responsible for the increase of the film surface roughness as a function of the laser E.D. The low cohesive energy formetal
Zn, compared to other metals (Fe, Ag, Cu) is correlated with the high efficiency of the self-sputtering for this material.This work was partially supported
by MCYT (Spain) under TIC2002-03235 project and by Consejo Nacional de Ciencia y Tecnologia de MÂŽexico (CONACYT). J. G. H. acknowledges a scholarship from the I3P program from the CSIC.Peer reviewe
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