152 research outputs found
Direct Simulations of Homogeneous Bubble Nucleation: Agreement with CNT and no Local Hot Spots
We present results from direct, large-scale molecular dynamics (MD)
simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations
contain half a billion Lennard-Jones (LJ) atoms and cover up to 56 million
time-steps. The unprecedented size of the simulated volumes allows us to
resolve the nucleation and growth of many bubbles per run in simple direct
micro-canonical (NVE) simulations while the ambient pressure and temperature
remain almost perfectly constant. We find bubble nucleation rates which are
lower than in most of the previous, smaller simulations. It is widely believed
that classical nucleation theory (CNT) generally underestimates bubble
nucleation rates by very large factors. However, our measured rates are within
two orders of magnitude of CNT predictions - only at very low temperatures does
CNT underestimate the nucleation rate significantly. Introducing a small,
positive Tolman length leads to very good agreement at all temperatures, as
found in our recent vapor-to-liquid nucleation simulations. The critical
bubbles sizes derived with the nucleation theorem agree well with the CNT
predictions at all temperatures. Local hot spots reported in the literature are
not seen: Regions where a bubble nucleation events will occur are not above the
average temperature, and no correlation of temperature fluctuations with
subsequent bubble formation is seen.Comment: 15 pages, 13 figures. Submitted to PRE. Simulation movies available
at http://www.ics.uzh.ch/~diemand/movies
Simple improvements to classical bubble nucleation models
We revisit classical nucleation theory (CNT) for the homogeneous bubble
nucleation rate and improve the classical formula using a new prefactor in the
nucleation rate. Most of the previous theoretical studies have used the
constant prefactor determined by the bubble growth due to the evaporation
process from the bubble surface. However, the growth of bubbles is also
regulated by the thermal conduction, the viscosity, and the inertia of liquid
motion. These effects can decrease the prefactor significantly, especially when
the liquid pressure is much smaller than the equilibrium one. The deviation in
the nucleation rate between the improved formula and the CNT can be as large as
several orders of magnitude. Our improved, accurate prefactor and recent
advances in molecular dynamics simulations and laboratory experiments for argon
bubble nucleation enable us to precisely constrain the free energy barrier for
bubble nucleation. Assuming the correction to the CNT free energy is of the
functional form suggested by Tolman, the precise evaluations of the free energy
barriers suggest the Tolman length is independently of the
temperature for argon bubble nucleation, where is the unit length of
the Lenard-Jones potential. With this Tolman correction and our new prefactor
one gets accurate bubble nucleation rate predictions in the parameter range
probed by current experiments and molecular dynamics simulations.Comment: 10pages, 6figures, Accepted for publication in Physical Review
Free energy of cluster formation and a new scaling relation for the nucleation rate
Recent very large molecular dynamics simulations of homogeneous nucleation
with Lennard-Jones atoms [Diemand et al. J. Chem. Phys. {\bf
139}, 074309 (2013)] allow us to accurately determine the formation free energy
of clusters over a wide range of cluster sizes. This is now possible because
such large simulations allow for very precise measurements of the cluster size
distribution in the steady state nucleation regime. The peaks of the free
energy curves give critical cluster sizes, which agree well with independent
estimates based on the nucleation theorem. Using these results, we derive an
analytical formula and a new scaling relation for nucleation rates: is scaled by , where the supersaturation ratio is ,
is the dimensionless surface energy, and is a dimensionless
nucleation rate. This relation can be derived using the free energy of cluster
formation at equilibrium which corresponds to the surface energy required to
form the vapor-liquid interface. At low temperatures (below the triple point),
we find that the surface energy divided by that of the classical nucleation
theory does not depend on temperature, which leads to the scaling relation and
implies a constant, positive Tolman length equal to half of the mean
inter-particle separation in the liquid phase.Comment: 7 figure
Pure iron grains are rare in the universe
The abundant forms in which the major elements in the universe exist have
been determined from numerous astronomical observations and meteoritic
analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has
been detected in the interstellar medium, suggesting that Fe is condensed into
a solid, possibly the astronomically invisible metal. To determine the primary
form of Fe, we replicated the formation of Fe grains in gaseous ejecta of
evolved stars by means of microgravity experiments. We found that the sticking
probability for formation of Fe grains is extremely small; only several atoms
will stick per hundred thousand collisions, so that homogeneous nucleation of
metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of Type Ia
supernovae. This implies that most Fe is locked up as grains of Fe compounds or
as impurities accreted onto other grains in the interstellar medium
Low-Temperature Crystallization of Amorphous Silicate in Astrophysical Environments
We construct a theoretical model for low-temperature crystallization of
amorphous silicate grains induced by exothermic chemical reactions. As a first
step, the model is applied to the annealing experiments, in which the samples
are (1) amorphous silicate grains and (2) amorphous silicate grains covered
with an amorphous carbon layer. We derive the activation energies of
crystallization for amorphous silicate and amorphous carbon from the analysis
of the experiments. Furthermore, we apply the model to the experiment of
low-temperature crystallization of amorphous silicate core covered with an
amorphous carbon layer containing reactive molecules. We clarify the conditions
of low-temperature crystallization due to exothermic chemical reactions. Next,
we formulate the crystallization conditions so as to be applicable to
astrophysical environments. We show that the present crystallization mechanism
is characterized by two quantities: the stored energy density Q in a grain and
the duration of the chemical reactions \tau . The crystallization conditions
are given by Q > Q_{min} and \tau < \tau _{cool} regardless of details of the
reactions and grain structure, where \tau _{cool} is the cooling timescale of
the grains heated by exothermic reactions, and Q_{min} is minimum stored energy
density determined by the activation energy of crystallization. Our results
suggest that silicate crystallization occurs in wider astrophysical conditions
than hitherto considered.Comment: 9 figures, accepted for publication in Astrophysical
Largely Extended X-ray Emission around the Elliptical Galaxy NGC 4636 Observed with ASCA
ASCA observations of NGC 4636 and a southern region have revealed extended
X-ray emission to a radius of about 300 kpc from the galaxy. The symmetric
nature of the observed surface brightness around NGC 4636 indicates its
association to this galaxy rather than to the Virgo cluster. Model independent
estimation of the gravitational mass profile shows a flattening at a radius of
kpc, where the total mass reaches
and a mass-to-light ratio of 23. The mass still increases to larger radii,
reaching with a mass-to-light ratio of 300 at
300 kpc from NGC 4636. These features suggest presence of a galaxy group
surrounding NGC 4636. If such optically dark groups are common among X-ray
bright ellipticals, it would explain the very large scatter in their X-ray
luminosities with similar optical luminosities.Comment: 14 pages with 3 figures, AAS Latex, accepted to ApJ
The Nature of Ultra-Luminous Compact X-Ray Sources in Nearby Spiral Galaxies
Studies were made of ASCA spectra of seven ultra-luminous compact X-ray
sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6
(Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.
1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &
Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.
1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, they
are thought to represent a class of enigmatic X-ray sources often found in
spiral galaxies. For some of them, the ASCA data are newly processed, or the
published spectra are reanalyzed. For others, the published results are quoted.
The ASCA spectra of all these seven sources have been described successfully
with so called multi-color disk blackbody (MCD) emission arising from
optically-thick standard accretion disks around black holes. Except the case of
M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not
to exceed the Eddington limits, the black holes are inferred to have rather
high masses, up to ~100 solar masses. However, the observed innermost disk
temperatures of these objects, Tin = 1.1--1.8 keV, are too high to be
compatible with the required high black-hole masses, as long as the standard
accretion disks around Schwarzschild black holes are assumed. Similarly high
disk temperatures are also observed from two Galactic transients with
superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high
disk temperature may be explained by the black hole rotation, which makes the
disk get closer to the black hole, and hence hotter.Comment: submitted to ApJ, December 199
The Iron Discrepancy in Elliptical Galaxies after ASCA
We present estimates for the iron content of the stellar and diffused
components of elliptical galaxies, as derived respectively from integrated
optical spectra and from ASCA X-ray observations. A macroscopic discrepancy
emerges between the expected iron abundances in the hot interstellar medium
(ISM) and what is indicated by the X-ray observations, especially when
allowance is made for the current iron enrichment by Type Ia supernovae. This
strong discrepancy, that in some extreme instances may be as large as a factor
of , calls into question our current understanding of supernova
enrichment and chemical evolution of galaxies. We discuss several astrophysical
implications of the inferred low iron abundances in the ISM, including the
chemical evolution of galaxies and cluster of galaxies, the evolution of gas
flows in ellipticals, and the heating of the intracluster medium. Some of the
consequences appear hard to accept, and in the attempt to avoid some of these
difficulties we explore ways of hiding or diluting iron in the ISM of
ellipticals. None of these possibilities appears astrophysically plausible, and
we alternatively rise the question of the reliability of iron-L line
diagonostic tools. Various thin plasma emission models are shown to give iron
abundances that may differ significantly, especially at low temperatures (kT
\lsim 1 keV). From a collection of ASCA and other X-ray observatory data, it
is shown that current thin plasma codes tend to give very low iron abundances
when the temperature of the objects is below keV. We conclude that --
besides rethinking the chemical evolution of galaxies -- one should also
consider the possibility that existing thin plasma models may incorporate
inaccurate atomic physics for the ions responsible for the iron-L complex.Comment: 39 pages, TeX file, 5 figures, Accepted for publication in the Ap
OAZ-t/OAZ3 Is Essential for Rigid Connection of Sperm Tails to Heads in Mouse
Polyamines are known to play important roles in the proliferation and differentiation of many types of cells. Although considerable amounts of polyamines are synthesized and stored in the testes, their roles remain unknown. Ornithine decarboxylase antizymes (OAZs) control the intracellular concentration of polyamines in a feedback manner. OAZ1 and OAZ2 are expressed ubiquitously, whereas OAZ-t/OAZ3 is expressed specifically in germline cells during spermiogenesis. OAZ-t reportedly binds to ornithine decarboxylase (ODC) and inactivates ODC activity. In a prior study, polyamines were capable of inducing a frameshift at the frameshift sequence of OAZ-t mRNA, resulting in the translation of OAZ-t. To investigate the physiological role of OAZ-t, we generated OAZ-tâdisrupted mutant mice. Homozygous OAZ-t mutant males were infertile, although the polyamine concentrations of epididymides and testes were normal in these mice, and females were fertile. Sperm were successfully recovered from the epididymides of the mutant mice, but the heads and tails of the sperm cells were easily separated in culture medium during incubation. Results indicated that OAZ-t is essential for the formation of a rigid junction between the head and tail during spermatogenesis. The detached tails and heads were alive, and most of the headless tails showed straight forward movement. Although the tailless sperm failed to acrosome-react, the heads were capable of fertilizing eggs via intracytoplasmic sperm injection. OAZ-t likely plays a key role in haploid germ cell differentiation via the local concentration of polyamines
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