833,561 research outputs found
The relevance of ambipolar diffusion for neutron star evolution
We study ambipolar diffusion in strongly magnetised neutron stars, with
special focus on the effects of neutrino reaction rates and the impact of a
superfluid/superconducting transition in the neutron star core. For
axisymmetric magnetic field configurations, we determine the deviation from
equilibrium induced by the magnetic force and calculate the velocity of
the slow, quasi-stationary, ambipolar drift. We study the temperature
dependence of the velocity pattern and clearly identify the transition to a
predominantly solenoidal flow. For stars without superconducting/superfluid
constituents and with a mixed poloidal-toroidal magnetic field of typical
magnetar strength, we find that ambipolar diffusion proceeds fast enough to
have a significant impact on the magnetic field evolution only at low core
temperatures, K. The ambipolar diffusion timescale
becomes appreciably shorter when fast neutrino reactions are present, because
the possibility to balance part of the magnetic force with pressure gradients
is reduced. We also find short ambipolar diffusion timescales in the case of
superconducting cores for K, due to the reduced interaction
between protons and neutrons. In the most favourable scenario, with fast
neutrino reactions and superconducting cores, ambipolar diffusion results in
advection velocities of several km/kyr. This velocity can substantially
reorganize magnetic fields in magnetar cores, in a way that can only be
confirmed by dynamical simulations.Comment: 14 pages, 11 figures, version accepted for publication in MNRA
Some Computational Aspects of Essential Properties of Evolution and Life
While evolution has inspired algorithmic methods of heuristic optimisation, little has been done in the way of using concepts of computation to advance our understanding of salient aspects of biological evolution. We argue that under reasonable assumptions, interesting conclusions can be drawn that are of relevance to behavioural evolution. We will focus on two important features of life--robustness and fitness optimisation--which, we will argue, are related to algorithmic probability and to the thermodynamics of computation, subjects that may be capable of explaining and modelling key features of living organisms, and which can be used in understanding and formulating algorithms of evolutionary computation
Modeling The Nucleosynthesis Of Massive Stars
This overview discusses issues relevant to modeling nucleosynthesis in type
II supernovae and implications of detailed studies of the ejecta. After a brief
presentation of the most common approaches to stellar evolution and
parameterized explosions, the relevance of a number of nuclei to obtain
information on the evolution and explosion mechanisms is discussed. The paper
is concluded by an outlook on multi-dimensional simulations.Comment: Invited talk at the workshop "Astronomy with Radioactivities IV",
Seeon, Germany, June 2003; 6 pages, to appear in New Astronomy Review
Galactic Open Clusters
The study of open clusters has a classic feel to it since the subject
predates anyone alive today. Despite the age of this topic, I show via an ADS
search that its relevance and importance in astronomy has grown faster in the
last few decades than astronomy in general. This is surely due to both
technical reasons and the interconnection of the field of stellar evolution to
many branches of astronomy. In this review, I outline what we know today about
open clusters and what they have taught us about a range of topics from stellar
evolution to Galactic structure to stellar disk dissipation timescales. I argue
that the most important astrophysics we have learned from open clusters is
stellar evolution and that its most important product has been reasonably
precise stellar ages. I discuss where open cluster research is likely to go in
the next few years, as well as in the era of 20m telescopes, SIM, and GAIA. Age
will continue to be of wide relevance in astronomy, from cosmology to planet
formation timescales, and with distance errors soon no longer a problem,
improved ages will be critically important to many of the most fascinating
astrophysical questions.Comment: 14 pages, to appear in Resolved Stellar Populations, ASP Conference
in Cancu
Dynamics of Interacting Scalar Fields in Expanding Space-Time
The effective equation of motion is derived for a scalar field interacting
with other fields in a Friedman-Robertson-Walker background space-time. The
dissipative behavior reflected in this effective evolution equation is studied
both in simplified approximations as well as numerically. The relevance of our
results to inflation are considered both in terms of the evolution of the
inflaton field as well as its fluctuation spectrum. A brief examination also is
made of supersymmetric models that yield dissipative effects during inflation.Comment: 36 pages, 12 figures. Version published in the Physical Review
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