762 research outputs found
Neutrino-driven wind and wind termination shock in supernova cores
The neutrino-driven wind from a nascent neutron star at the center of a
supernova expands into the earlier ejecta of the explosion. Upon collision with
this slower matter the wind material is decelerated in a wind termination
shock. By means of hydrodynamic simulations in spherical symmetry we
demonstrate that this can lead to a large increase of the wind entropy,
density, and temperature, and to a strong deceleration of the wind expansion.
The consequences of this phenomenon for the possible r-process nucleosynthesis
in the late wind still need to be explored in detail. Two-dimensional models
show that the wind-ejecta collision is highly anisotropic and could lead to a
directional dependence of the nucleosynthesis even if the neutrino-driven wind
itself is spherically symmetric.Comment: 6 pages, 3 figures, International Symposium on Nuclear Astrophysics -
Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
Global Anisotropies in Supernova Explosions and Pulsar Recoil
We show by two-dimensional and first three-dimensional simulations of
neutrino-driven supernova explosions that low (l=1,2) modes can dominate the
flow pattern in the convective postshock region on timescales of hundreds of
milliseconds after core bounce. This can lead to large global anisotropy of the
supernova explosion and pulsar kicks in excess of 500 km/s.Comment: 3 pages, 2 figures, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Instability of a stalled accretion shock: evidence for the advective-acoustic cycle
We analyze the linear stability of a stalled accretion shock in a perfect gas
with a parametrized cooling function L ~ rho^{beta-alpha} P^alpha. The
instability is dominated by the l=1 mode if the shock radius exceeds 2-3 times
the accretor radius, depending on the parameters of the cooling function. The
growth rate and oscillation period are comparable to those observed in the
numerical simulations of Blondin & Mezzacappa (2006). The instability mechanism
is analyzed by separately measuring the efficiencies of the purely acoustic
cycle and the advective-acoustic cycle. These efficiencies are estimated
directly from the eigenspectrum, and also through a WKB analysis in the high
frequency limit. Both methods prove that the advective-acoustic cycle is
unstable, and that the purely acoustic cycle is stable. Extrapolating these
results to low frequency leads us to interpret the dominant mode as an
advective-acoustic instability, different from the purely acoustic
interpretation of Blondin & Mezzacappa (2006). A simplified characterization of
the instability is proposed, based on an advective-acoustic cycle between the
shock and the radius r_nabla where the velocity gradients of the stationary
flow are strongest. The importance of the coupling region in this mechanism
calls for a better understanding of the conditions for an efficient
advective-acoustic coupling in a decelerated, nonadiabatic flow, in order to
extend these results to core-collapse supernovae.Comment: 29 pages, 18 figures, to appear in ApJ (1 new Section, 2 new Figures
Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows: I. Spherically symmetric hydrodynamic simulations
We investigate the behavior and consequences of the reverse shock that terminates the supersonic expansion of the baryonic wind which is driven by neutrino heating off the surface of (non-magnetized) new-born neutron stars in supernova cores. To this end we perform long-time hydrodynamic simulations in spherical symmetry. In agreement with previous relativistic wind studies, we find that the neutrino-driven outflow accelerates to supersonic velocities and in case of a compact, about 1.4 solar mass (gravitational mass) neutron star with a radius of about 10 km, the wind reaches entropies of about 100 k_B per nucleon. The wind, however, is strongly influenced by the environment of the supernova core. It is decelerated and shock-heated abruptly by a termination shock that forms when the supersonic outflow collides with the slower preceding supernova ejecta. The radial position of this reverse shock varies with time and depends on the strength of the neutrino wind and the different conditions in progenitor stars with different masses and structure. Its basic properties and behavior can be understood by simple analytic considerations. We demonstrate that the entropy of matter going through the reverse shock can increase to a multiple of the asymptotic wind value. Seconds after the onset of the explosion it therefore can exceed 400 k_B per nucleon. The temperature of the shocked wind has typically dropped to about or less than 10^9 K, and density and temperature in the shock-decelerated matter continue to decrease only very slowly. Such conditions might strongly affect the important phases of supernova nucleosynthesis in a time and progenitor dependent way. (abridged
Low relaxation rate in a low-Z alloy of iron
The longest relaxation time and sharpest frequency content in ferromagnetic
precession is determined by the intrinsic (Gilbert) relaxation rate \emph{}.
For many years, pure iron (Fe) has had the lowest known value of for all pure ferromagnetic metals or binary alloys. We show that an
epitaxial iron alloy with vanadium (V) possesses values of which are
significantly reduced, to 355 Mhz at 27% V. The result can be understood
as the role of spin-orbit coupling in generating relaxation, reduced through
the atomic number .Comment: 14 pages, 4 figure
Neutrino Mass Implications for Muon Decay Parameters
We use the scale of neutrino mass to derive model-independent naturalness
constraints on possible contributions to muon decay Michel parameters from new
physics above the electroweak symmetry-breaking scale. Focusing on Dirac
neutrinos, we obtain a complete basis of effective dimension four and dimension
six operators that are invariant under the gauge symmetry of the Standard Model
and that contribute to both muon decay and neutrino mass. We show that -- in
the absence of fine tuning -- the most stringent bounds on chirality-changing
operators relevant to muon decay arise from one-loop contributions to neutrino
mass. The bounds we obtain on their contributions to the Michel parameters are
four or more orders of magnitude stronger than bounds previously obtained in
the literature. We also show that there exist chirality-changing operators that
contribute to muon decay but whose flavor structure allows them to evade
neutrino mass naturalness bounds. We discuss the implications of our analysis
for the interpretation of muon decay experiments.Comment: 19 pages, 4 figure
Improving the patient experience through patient portals: Insights from experienced portal users
Background: Patient portals have become part of the ecosystem of care as both patients and providers use them for a range of activities both individually and collaboratively. As patients and providers gain greater experience using portals, their use and needs related to portals may evolve. Objective: This study aimed to learn from experienced patient portal users to improve our understanding of their perspectives on portal use for collaboration and engagement as well as explore how using a portal influenced their experiences with primary care providers. Methods: Qualitative study involving 29 semi-structured interviews with family medicine patients from a large Academic Medical Center (AMC). Interviewees were patients with chronic conditions who had been identified by their providers as experienced portal users. Interview transcripts were analyzed using rigorous qualitative methods. Results: Common themes emerged around both logistical and psychological benefits of portal use. Logistical benefits included increased efficiency, improved ability to track their health information, and better documentation of communications and information during and between office visits. Psychological benefits were a greater sense of collaboration in care, increased trust in providers, and enhanced engagement in health care activities. Conclusion: Experienced portal users discussed ways in which patient portals improved both their ability to manage their care and their relationships with providers. Frequent users described a sense of collaboration with their providers and greater trust in the relationship. These findings suggest that portal use may be a mechanism through which patients can increase patient engagement and improve the patient experience
Multidimensional supernova simulations with approximative neutrino transport. II. Convection and the advective-acoustic cycle in the supernova core
By 2D hydrodynamic simulations including a detailed equation of state and
neutrino transport, we investigate the interplay between different non-radial
hydrodynamic instabilities that play a role during the postbounce accretion
phase of collapsing stellar cores. The convective mode of instability, which is
driven by negative entropy gradients caused by neutrino heating or by time
variations of the shock strength, can be identified clearly by the development
of typical Rayleigh-Taylor mushrooms. However, in cases where the gas in the
postshock region is rapidly advected towards the gain radius, the growth of
such a buoyancy instability can be suppressed. In such a situation the shocked
flow nevertheless can develop non-radial asymmetry with an oscillatory growth
of the amplitude. This phenomenon has been termed ``standing accretion shock
instability'' (SASI). It is shown here that the SASI oscillations can trigger
convective instability and like the latter they lead to an increase of the
average shock radius and of the mass in the gain layer. Both hydrodynamic
instabilities in combination stretch the advection time of matter through the
neutrino-heating layer and thus enhance the neutrino energy deposition in
support of the neutrino-driven explosion mechanism. A rapidly contracting and
more compact nascent NS turns out to be favorable for explosions, because the
accretion luminosity and neutrino heating are larger and the growth rate of the
SASI is higher. Moreover, we show that the oscillation period of the SASI and a
variety of other features in our simulations agree with estimates for the
advective-acoustic cycle (AAC), in which perturbations are carried by the
accretion flow from the shock to the neutron star and pressure waves close an
amplifying global feedback loop. (abridged)Comment: 23 pages, 20 figures; revised version with extended Sect.5, accepted
by Astronomy & Astrophysics; high-resolution images can be obtained upon
reques
Supernova Asymmetries and Pulsar Kicks -- Views on Controversial Issues
Two- and three-dimensional simulations demonstrate that hydrodynamic
instabilities can lead to low-mode (l=1,2) asymmetries of the fluid flow in the
neutrino-heated layer behind the supernova shock. This provides a natural
explanation for aspherical mass ejection and for pulsar recoil velocities even
in excess of 1000 km/s. We propose that the bimodality of the pulsar velocity
distribution might be a consequence of a dominant l=1 mode in case of the fast
component, while higher-mode anisotropy characterizes the postshock flow and SN
ejecta during the birth of the slow neutron stars. We argue that the observed
large asymmetries of supernovae and the measured high velocities of young
pulsars therefore do not imply rapid rotation of the iron core of the
progenitor star, nor do they require strong magnetic fields to play a crucial
role in the explosion. Anisotropic neutrino emission from accretion contributes
to the neutron star acceleration on a minor level, and pulsar kicks do not make
a good case for non-standard neutrino physics in the nascent neutron star.Comment: 10 pages, 5 figures, full resolution figures available on request or
from Preprint P-MPA1651e on MPA web page. In: The Fate of the Most Massive
Stars, Proc. Eta Carinae Science Symposium (Jackson Hole, May 2004); revision
discusses new Cas A observation
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