6,645 research outputs found
GRB blastwaves through wind-shaped circumburst media
Context:A significant fraction of progenitors for long gamma-ray bursts
(GRBs) are believed to be massive stars. The investigation of long GRBs
therefore requires modeling the propagation of ultra-relativistic blastwaves
through the circumburst medium surrounding massive stars. We simulate the
expansion of an isotropic, adiabatic relativistic fireball into the wind-shaped
medium around a massive GRB progenitor. The circumburst medium is composed of a
realistically stratified stellar wind zone up to its termination shock,
followed by a region of shocked wind characterized by a constant density. Aims:
We followed the evolution of the blastwave through all its stages, including
the extremely rapid acceleration up to a Lorentz factor 75 flow, its
deceleration by interaction with stellar wind, its passage of the wind
termination shock, until its propagation through shocked wind. Methods: We used
the adaptive mesh refinement versatile advection code to follow the evolution
of the fireball. Results: We show that the acceleration from purely thermal to
ultra-relativistic kinetic regimes is abrupt and produces an internally
structured blastwave. We resolved the structure of this ultra-relativistic
shell in all stages, thanks to the adaptive mesh. We comment on the dynamical
roles played by forward and reverse shock pairs in the phase of interaction
with the free stellar wind and clearly identify the complex shock-dominated
structure created when the shell crosses the terminal shock. Conclusion: We
show that in our model where the terminal shock is taken relatively close to
the massive star, the phase of self-similar deceleration of Blandford-McKee
type can only be produced in the constant density, shocked wind zone.Comment: Accepted in A&A Letter (04/04/2007
Comparing the transitional behaviour of kaolinite and bentonite suspension flows
Past research has demonstrated the dramatic effects that variations in suspended clay can have on the properties of flow by producing a range of transitional flows between turbulent and laminar states, depending on clay concentration and fluid shear. Past studies have been restricted to kaolinite flows, a clay mineral that has relatively weak cohesive properties. This paper extends these studies to suspension flows of bentonite, a clay mineral that attains higher viscosities at far lower volumetric concentrations within a flow. The results show that the types of transitional flow behaviour recognized in past studies can also be found in bentonite suspension flows, but at lower suspended sediment concentrations, thus demonstrating an even more dramatic effect on flow properties, and potentially on sediment transport and resulting bed morphology, than kaolinite flows. The paper proposes new stability diagrams for the phase space of bentonite flows and compares these to past work on kaolinite suspension flows. These new data suggest that the transitional-flow Reynolds number can be used to delineate the types of transitional flow across different clay types and assess modern and ancient clay-suspension flows
Dynamic Correlation in Wave Propagation in Random Media
We report time-resolved measurements of the statistics of pulsed transmission
through quasi-one-dimensional dielectric media with static disorder. The
normalized intensity correlation function with displacement and polarization
rotation for an incident pulse of linewidth at delay time t is a
function only of the field correlation function, which is identical to that
found for steady-state excitation, and of , the residual
degree of intensity correlation at points at which the field correlation
function vanishes. The dynamic probability distribution of normalized intensity
depends only upon . Steady-state statistics are recovered
in the limit ->0, in which is the steady-state
degree of correlation.Comment: 4 RevTex pages, 4 figure
Direct measurement of B(D_s^+ â ÏX^+)
The absolute inclusive branching fraction of D_s^+âÏX^+ has been measured from data collected by the BES detector at a center-of-mass energy of 4.03 GeV, corresponding to an integrated luminosity of 22.3 pb^(-1). At this energy, direct pair production e^+e^-âD_s^+D_s^- has been observed. We have selected D_s candidate events by reconstructing five hadronic decay modes D_s^+âÏÏ^+, K^(0*)K^+, K^0K^+, f0^(Ï+) and K^0K^-Ï^+Ï^+ and have searched for inclusive Ïâs in the recoiling D_s^-. We observed three recoiling Ïâs in the 166.4 ± 31.8 D_s candidate events, which leads to the absolute branching fraction B(D_s^+âÏX^+)=(17.8(-7.2 -6.3)^(+15.1+0.6))â% and B(D_s-6.3+âÏÏ-6.3+)=(3.6_(-1.6 -1.3)(^_3.1+0.4)â%. [S0556-2821(97)02423-5
Probing the mechanical properties of graphene using a corrugated elastic substrate
The exceptional mechanical properties of graphene have made it attractive for
nano-mechanical devices and functional composite materials. Two key aspects of
graphene's mechanical behavior are its elastic and adhesive properties. These
are generally determined in separate experiments, and it is moreover typically
difficult to extract parameters for adhesion. In addition, the mechanical
interplay between graphene and other elastic materials has not been well
studied. Here, we demonstrate a technique for studying both the elastic and
adhesive properties of few-layer graphene (FLG) by placing it on deformable,
micro-corrugated substrates. By measuring deformations of the composite
graphene-substrate structures, and developing a related linear elasticity
theory, we are able to extract information about graphene's bending rigidity,
adhesion, critical stress for interlayer sliding, and sample-dependent tension.
The results are relevant to graphene-based mechanical and electronic devices,
and to the use of graphene in composite, flexible, and strain-engineered
materials.Comment: 5 pages, 4 figure
Liquid racism and the Danish Prophet Muhammad cartoons
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2010 The Author.This article examines reactions to the October 2005 publication of the Prophet Muhammad cartoons in the Danish newspaper Jyllands-Posten. It does so by using the concept of âliquid racismâ. While the controversy arose because it is considered blasphemous by many Muslims to create images of the Prophet Muhammad, the article argues that the meaning of the cartoons is multidimensional, that their analysis is significantly more complex than most commentators acknowledge, and that this complexity can best be addressed via the concept of liquid racism. The article examines the liquidity of the cartoons in relation to four readings. These see the cartoons as: (1) a criticism of Islamic fundamentalism; (2) blasphemous images; (3) Islamophobic and racist; and (4) satire and a defence of freedom of speech. Finally, the relationship between postmodernity and the rise of fundamentalism is discussed because the cartoons, reactions to them, and Islamic fundamentalism, all contain an important postmodern dimension.ESR
A hydrodynamical model of the circumstellar bubble created by two massive stars
Numerical models of the wind-blown bubble of massive stars usually only
account for the wind of a single star. However, since massive stars are usually
formed in clusters, it would be more realistic to follow the evolution of a
bubble created by several stars. We develope a two-dimensional (2D) model of
the circumstellar bubble created by two massive stars, a 40 solar mass star and
a 25 solar mass star, and follow its evolution. The stars are separated by
approximately 16 pc and surrounded by a cold medium with a density of 20
particles per cubic cm. We use the MPI-AMRVAC hydrodynamics code to solve the
conservation equations of hydrodynamics on a 2D cylindrical grid using
time-dependent models for the wind parameters of the two stars. At the end of
the stellar evolution (4.5 and 7.0 million years for the 40 and 25 solar mass
stars, respectively), we simulate the supernova explosion of each star. Each
star initially creates its own bubble. However, as the bubbles expand they
merge, creating a combined, aspherical bubble. The combined bubble evolves over
time, influenced by the stellar winds and supernova explosions. The evolution
of a wind-blown bubble created by two stars deviates from that of the bubbles
around single stars. In particular, once one of the stars has exploded, the
bubble is too large for the wind of the remaining star to maintain and the
outer shell starts to disintegrate. The lack of thermal pressure inside the
bubble also changes the behavior of circumstellar features close to the
remaining star. The supernovae are contained inside the bubble, which reflects
part of the energy back into the circumstellar medium.Comment: Accepted for publication in A&A. Six .avi files to be published
online (uploaded to ArXiv DC and available as ancillary files) (updated after
language corrections
Detecting Gluino-Containing Hadrons
When SUSY breaking produces only dimension-2 operators, gluino and photino
masses are of order 1 GeV or less. The gluon-gluino bound state has mass
1.3-2.2 GeV and lifetime > 10^{-5} - 10^{-10} s. This range of mass and
lifetime is largely unconstrained because missing energy and beam dump
techniques are ineffective. With only small modifications, upcoming K^0 decay
experiments can study most of the interesting range. The lightest
gluino-containing baryon (uds-gluino) is long-lived or stable; experiments to
find it and the uud-gluino are also discussed.Comment: 13 pp, 1 figure (uuencoded). Descendant of hep-ph/9504295,
hep-ph/9508291, and hep-ph/9508292, focused on experimental search
techniques. To be published in Phys Rev Let
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