230 research outputs found
Bubble Shape Oscillations and the Onset of Sonoluminescence
An air bubble trapped in water by an oscillating acoustic field undergoes
either radial or nonspherical pulsations depending on the strength of the
forcing pressure. Two different instability mechanisms (the Rayleigh--Taylor
instability and parametric instability) cause deviations from sphericity.
Distinguishing these mechanisms allows explanation of many features of recent
experiments on sonoluminescence, and suggests methods for finding
sonoluminescence in different parameter regimes.Comment: Phys. Rev. Lett., in pres
High speed synchrotron X-ray imaging studies of the ultrasound shockwave and enhanced flow during metal solidification processes
The highly dynamic behaviour of ultrasonic bubble implosion in liquid metal, the multiphase liquid metal flow containing bubbles and particles, and the interaction between ultrasonic waves and semisolid phases during solidification of metal were studied in situ using the complementary ultrafast and high speed synchrotron X-ray imaging facilities housed respectively at the Advanced Photon Source, Argonne National Laboratory, US, and Diamond Light Source, UK. Real-time ultrafast X-ray imaging of 135,780 frames per second (fps) revealed that ultrasonic bubble implosion in a liquid Bi-8 wt. %Zn alloy can occur in a single wave period (30 kHz), and the effective region affected by the shockwave at implosion was 3.5 times the original bubble diameter. Furthermore, ultrasound bubbles in liquid metal move faster than the primary particles, and the velocity of bubbles is 70 ~ 100% higher than that of the primary particles present in the same locations close to the sonotrode. Ultrasound waves can very effectively create a strong swirling flow in a semisolid melt in less than one second. The energetic flow can detach solid particles from the liquid-solid interface and redistribute them back into the bulk liquid very effectively
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio
Intermediate-mass star models with different helium and metal contents
We present a comprehensive theoretical investigation of the evolutionary
properties of intermediate-mass stars. The evolutionary sequences were computed
from the Zero Age Main Sequence up to the central He exhaustion and often up to
the phases which precede the carbon ignition or to the reignition of the
H-shell which marks the beginning of the thermal pulse phase. The evolutionary
tracks were constructed by adopting a wide range of stellar masses
(\msun) and chemical compositions. In order to account for
current uncertainties on the He to heavy elements enrichment ratio, the stellar
models were computed by adopting at Z=0.02 two different He contents (Y=0.27,
0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). To
supply a homogeneous evolutionary scenario which accounts for young Magellanic
stellar systems the calculations were also extended toward lower metallicities
(Z=0.004, Z=0.01), by adopting different initial He abundances. We evaluated
for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the
transition mass between the stellar structures igniting carbon and
those which develop a full electron degeneracy inside the CO core. This
evolutionary scenario allows us to investigate in detail the properties of
classical Cepheids. In particular, we find that the range of stellar masses
which perform the blue loop during the central He-burning phase narrows when
moving toward metal-rich and SMR structures.Comment: 25 pages, 10 figures (4 postscript + 6 gif files), 7 postscript
tables. accepted for publication on ApJ (November 2000
An Alternative Method to Deduce Bubble Dynamics in Single Bubble Sonoluminescence Experiments
In this paper we present an experimental approach that allows to deduce the
important dynamical parameters of single sonoluminescing bubbles (pressure
amplitude, ambient radius, radius-time curve) The technique is based on a few
previously confirmed theoretical assumptions and requires the knowledge of
quantities such as the amplitude of the electric excitation and the phase of
the flashes in the acoustic period. These quantities are easily measurable by a
digital oscilloscope, avoiding the cost of expensive lasers, or ultrafast
cameras of previous methods. We show the technique on a particular example and
compare the results with conventional Mie scattering. We find that within the
experimental uncertainties these two techniques provide similar results.Comment: 8 pages, 5 figures, submitted to Phys. Rev.
Investigation of transition frequencies of two acoustically coupled bubbles using a direct numerical simulation technique
The theoretical results regarding the ``transition frequencies'' of two
acoustically interacting bubbles have been verified numerically. The theory
provided by Ida [Phys. Lett. A 297 (2002) 210] predicted the existence of three
transition frequencies per bubble, each of which has the phase difference of
between a bubble's pulsation and the external sound field, while
previous theories predicted only two natural frequencies which cause such phase
shifts. Namely, two of the three transition frequencies correspond to the
natural frequencies, while the remaining does not. In a subsequent paper [M.
Ida, Phys. Rev. E 67 (2003) 056617], it was shown theoretically that transition
frequencies other than the natural frequencies may cause the sign reversal of
the secondary Bjerknes force acting between pulsating bubbles. In the present
study, we employ a direct numerical simulation technique that uses the
compressible Navier-Stokes equations with a surface-tension term as the
governing equations to investigate the transition frequencies of two coupled
bubbles by observing their pulsation amplitudes and directions of translational
motion, both of which change as the driving frequency changes. The numerical
results reproduce the recent theoretical predictions, validating the existence
of the transition frequencies not corresponding to the natural frequency.Comment: 18 pages, 8 figures, in pres
Comparison of the bifurcation scenarios predicted by the single-mode and multimode semiconductor laser rate equations
We present a detailed comparison of the bifurcation scenarios predicted by single-mode and multimode semiconductor laser rate equation models under large amplitude injection current modulation. The influence of the gain model on the predicted dynamics is investigated. Calculations of the dependence of the time averaged longitudinal mode intensities on modulation frequency are compared with experiments performed on an AlxGa1-xAs Fabry-Pérot semiconductor laser.K. A. Corbett and M. W. Hamilto
A New Class of High-Mass X-ray Binaries: Implications for Core Collapse and Neutron-Star Recoil
We investigate an interesting new class of high-mass X-ray binaries (HMXBs)
with long orbital periods (P_orb > 30 days) and low eccentricities (e <~ 0.2).
The orbital parameters suggest that the neutron stars in these systems did not
receive a large impulse, or ``kick,'' at the time of formation. We develop a
self-consistent phenomenological picture wherein the neutron stars born in the
observed wide HMXBs receive only a small kick (<~ 50 km/s), while neutron stars
born in isolation, in the majority of low-mass X-ray binaries, or in many of
the well-known HMXBs with P_orb <~ 30 days receive the conventional large
kicks, with a mean speed of ~ 300 km/s. We propose that the magnitude of the
natal kick to a neutron star born in a binary system depends on the rotation
rate of the pre-collapse core. We further suggest that the rotation rate of the
core is a strong, well-defined function of the evolutionary path of the
progenitor star.Comment: 13 pages, 5 figures (2 color), submitted to Ap
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