1,352 research outputs found
Probability distribution function of dipolar field in two-dimensional spin ensemble
We theoretically determine the probability distribution function of the net
field of the random planar structure of dipoles which represent polarized
particles. At small surface concentrations c of the point dipoles this
distribution is expressed in terms of special functions. At the surface
concentrations of the dipoles as high as 0.6 the dipolar field obey the
Gaussian law. To obtain the distribution function within transitional region
c<0.6, we propose the method based on the cumulant expansion. We calculate the
parameters of the distributions for some specific configurations of the
dipoles. The distribution functions of the ordered ensembles of the dipoles at
the low and moderate surface concentrations have asymmetric shape with respect
to distribution medians. The distribution functions allow to calculate various
physical parameters of two-dimensional interacting nanoparticle ensembles.Comment: 9 pages, 3 figure
On the Dynamics of Proto-Neutron Star Winds and r-Process Nucleosynthesis
We study here the formation of heavy r-process nuclei in the high-entropy
environment of rapidly expanding neutrino-driven winds from compact objects. In
particular, we explore the sensitivity of the element creation in the A>130
region to the low-temperature behavior of the outflows. For this purpose we
employ a simplified model of the dynamics and thermodynamical evolution for
radiation dominated, adiabatic outflows. It consists of a first stage of fast,
exponential cooling, followed by a second phase of slower evolution, either
assuming constant density and temperature or a power-law decay of these
quantities. These cases are supposed to capture the most relevant effects of a
strong deceleration or decreasing acceleration of the transsonic outflows,
respectively, e.g. in a wind termination shock caused by the collision with the
slower, preceding supernova ejecta. We find that not only the transition
temperature between the two expansion phases can make a big difference in the
formation of the platinum peak, but also the detailed cooling law during the
later phase. Unless the transition temperature and corresponding (free neutron)
density become too small (T < 2*10^8 K), a lower temperature or faster
temperature decline during this phase allow for a stronger appearance of the
third abundance peak. Since the nuclear photodisintegration rates between
~2*10^8 K and ~10^9 K are more sensitive to the temperature than the n-capture
rates are to the free neutron density, a faster cooling in this temperature
regime shifts the r-process path closer to the n-drip line. With low (gamma,n)-
but high beta-decay rates, the r-processing then does not proceed through a
(gamma,n)-(n,gamma) equilibrium but through a quasi-equilibrium of
(n,gamma)-reactions and beta-decays, as recently also pointed out by Wanajo.Comment: 18 pages, 14 figures with 25 eps plots; referee comments included;
accepted by Astronomy & Astrophysic
- …