624 research outputs found
Light Elements and Cosmic Rays in the Early Galaxy
We derive constraints on the cosmic rays responsible for the Be and part of
the B observed in stars formed in the early Galaxy: the cosmic rays cannot be
accelerated from the ISM; their energy spectrum must be relatively hard (the
bulk of the nuclear reactions should occur at 30 MeV/nucl); and only
10 erg/SNII in high metallicity, accelerated particle kinetic energy
could suffice to produce the Be and B. The reverse SNII shock could accelerate
the particles.Comment: 5 pages LATEX using paspconf.sty file with one embedded eps figure
using psfig. In press, Proc. Goddard High Resolution Spectrograph Symposium,
PASP, 199
Microwave whirlpools in a rectangular-waveguide cavity with a thin ferrite disk
We study a three dimensional system of a rectangular-waveguide resonator with
an inserted thin ferrite disk. The interplay of reflection and transmission at
the disk interfaces together with material gyrotropy effect, gives rise to a
rich variety of wave phenomena. We analyze the wave propagation based on full
Maxwell-equation numerical solutions of the problem. We show that the
power-flow lines of the microwave-cavity field interacting with a ferrite disk,
in the proximity of its ferromagnetic resonance, form whirlpool-like
electromagnetic vortices. Such vortices are characterized by the dynamical
symmetry breaking. The role of ohmic losses in waveguide walls and dielectric
and magnetic losses in a disk is a subject of our investigations
Gamma Ray Lines from the Orion Complex
We show that the 4.44 and 6.13 MeV line emission observed with COMPTEL from
Orion is consistent with gamma ray spectra consisting of a mixture of narrow
and broad lines or spectra containing only broad lines. We employed several
accelerated particle compositions and showed that the current COMPTEL data in
the 3--7 MeV region alone cannot distinguish between the various possibilities.
However, the COMPTEL upper limits in the 1--3 MeV band favor a composition
similar to that of the winds of Wolf-Rayet stars of spectral type WC. The power
dissipated by the accelerated particles at Orion is about 4 10
erg s. These particles are not expected to produce significant amounts
of Al.Comment: 12 pages, uuencoded compressed postscript with 4 figures ApJL in
press 199
Efficient Immunization Strategies for Computer Networks and Populations
We present an effective immunization strategy for computer networks and
populations with broad and, in particular, scale-free degree distributions. The
proposed strategy, acquaintance immunization, calls for the immunization of
random acquaintances of random nodes (individuals). The strategy requires no
knowledge of the node degrees or any other global knowledge, as do targeted
immunization strategies. We study analytically the critical threshold for
complete immunization. We also study the strategy with respect to the
susceptible-infected-removed epidemiological model. We show that the
immunization threshold is dramatically reduced with the suggested strategy, for
all studied cases.Comment: Revtex, 5 pages, 4 ps fig
Dynamic Screening in Thermonuclear Reactions
It has recently been argued that there are no dynamic screening corrections
to Salpeter's enhancement factor in thermonuclear reactions, in the
weak-screening limit. The arguments used were: 1) The Gibbs probability
distribution is factorable into two parts, one of which, (), is independent of velocity space; and
2) The enhancement factor is with
and . We show that both of these
arguments are incorrect.Comment: Accepted for publication in The Astrophysical Journa
Stability and topology of scale-free networks under attack and defense strategies
We study tolerance and topology of random scale-free networks under attack
and defense strategies that depend on the degree k of the nodes. This situation
occurs, for example, when the robustness of a node depends on its degree or in
an intentional attack with insufficient knowledge on the network. We determine,
for all strategies, the critical fraction p_c of nodes that must be removed for
disintegrating the network. We find that for an intentional attack, little
knowledge of the well-connected sites is sufficient to strongly reduce p_c. At
criticality, the topology of the network depends on the removal strategy,
implying that different strategies may lead to different kinds of percolation
transitions.Comment: Accepted in PR
Scale-Free Networks are Ultrasmall
We study the diameter, or the mean distance between sites, in a scale-free
network, having N sites and degree distribution p(k) ~ k^-a, i.e. the
probability of having k links outgoing from a site. In contrast to the diameter
of regular random networks or small world networks which is known to be d ~
lnN, we show, using analytical arguments, that scale free networks with 2<a<3
have a much smaller diameter, behaving as d ~ lnlnN. For a=3, our analysis
yields d ~ lnN/lnlnN, as obtained by Bollobas and Riordan, while for a>3, d ~
lnN. We also show that, for any a>2, one can construct a deterministic scale
free network with d ~ lnlnN, and this construction yields the lowest possible
diameter.Comment: Latex, 4 pages, 2 eps figures, small corrections, added explanation
Pricing Multi-Unit Markets
We study the power and limitations of posted prices in multi-unit markets,
where agents arrive sequentially in an arbitrary order. We prove upper and
lower bounds on the largest fraction of the optimal social welfare that can be
guaranteed with posted prices, under a range of assumptions about the
designer's information and agents' valuations. Our results provide insights
about the relative power of uniform and non-uniform prices, the relative
difficulty of different valuation classes, and the implications of different
informational assumptions. Among other results, we prove constant-factor
guarantees for agents with (symmetric) subadditive valuations, even in an
incomplete-information setting and with uniform prices
- …