36,571 research outputs found
Comprehensive rate coefficients for electron collision induced transitions in hydrogen
Energy-changing electron-hydrogen atom collisions are crucial to regulating
the energy balance in astrophysical and laboratory plasmas and relevant to the
formation of stellar atmospheres, recombination in H-II clouds, primordial
recombination, three-body recombination and heating in ultracold and fusion
plasmas. Computational modeling of electron-hydrogen collision has been
attempted through quantum mechanical scattering state-to-state calculations of
transitions involving low-lying energy levels in hydrogen (with principal
quantum number n < 7) and at large principal quantum numbers using classical
trajectory techniques. Analytical expressions are proposed which interpolates
the current quantum mechanical and classical trajectory results for
electron-hydrogen scattering in the entire range of energy levels, for nearly
all temperature range of interest in astrophysical environments. An asymptotic
expression for the Born cross-section is interpolated with a modified
expression derived previously for electron-hydrogen scattering in the Rydberg
regime using classical trajectory Monte Carlo simulations. The derived formula
is compared to existing numerical data for transitions involving low principal
quantum numbers, and the dependence of the deviations upon temperature is
discussed.Comment: To appear in The Astrophysical Journa
Post-Impact Thermal Evolution of Porous Planetesimals
Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta
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An accurate, fast, mathematically robust, universal, non-iterative algorithm for computing multi-component diffusion velocities
Using accurate multi-component diffusion treatment in numerical combustion
studies remains formidable due to the computational cost associated with
solving for diffusion velocities. To obtain the diffusion velocities, for low
density gases, one needs to solve the Stefan-Maxwell equations along with the
zero diffusion flux criteria, which scales as , when solved
exactly. In this article, we propose an accurate, fast, direct and robust
algorithm to compute multi-component diffusion velocities. To our knowledge,
this is the first provably accurate algorithm (the solution can be obtained up
to an arbitrary degree of precision) scaling at a computational complexity of
in finite precision. The key idea involves leveraging the fact
that the matrix of the reciprocal of the binary diffusivities, , is low
rank, with its rank being independent of the number of species involved. The
low rank representation of matrix is computed in a fast manner at a
computational complexity of and the Sherman-Morrison-Woodbury
formula is used to solve for the diffusion velocities at a computational
complexity of . Rigorous proofs and numerical benchmarks
illustrate the low rank property of the matrix and scaling of the
algorithm.Comment: 16 pages, 7 figures, 1 table, 1 algorith
Global Scale Impacts
Global scale impacts modify the physical or thermal state of a substantial
fraction of a target asteroid. Specific effects include accretion, family
formation, reshaping, mixing and layering, shock and frictional heating,
fragmentation, material compaction, dilatation, stripping of mantle and crust,
and seismic degradation. Deciphering the complicated record of global scale
impacts, in asteroids and meteorites, will lead us to understand the original
planet-forming process and its resultant populations, and their evolution in
time as collisions became faster and fewer. We provide a brief overview of
these ideas, and an introduction to models.Comment: A chapter for Asteroids IV, a new volume in the Space Science Series,
University of Arizona Press (Patrick Michel, Francesca E. DeMeo, William F.
Bottke, Eds.
Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates
Porosity evolution of dust aggregates is crucial in understanding dust
evolution in protoplanetary disks. In this study, we present useful tools to
study the coagulation and porosity evolution of dust aggregates. First, we
present a new numerical method for simulating dust coagulation and porosity
evolution as an extension of the conventional Smoluchowski equation. This
method follows the evolution of the mean porosity for each aggregate mass
simultaneously with the evolution of the mass distribution function. This
method reproduces the results of previous Monte Carlo simulations with much
less computational expense. Second, we propose a new collision model for porous
dust aggregates on the basis of our N-body experiments on aggregate collisions.
We first obtain empirical data on porosity changes between the classical limits
of ballistic cluster-cluster and particle-cluster aggregation. Using the data,
we construct a recipe for the porosity change due to general hit-and-stick
collisions as well as formulae for the aerodynamical and collisional cross
sections. Simple coagulation simulations using the extended Smoluchowski method
show that our collision model explains the fractal dimensions of porous
aggregates observed in a full N-body simulation and a laboratory experiment.
Besides, we discover that aggregates at the high-mass end of the distribution
can have a considerably small aerodynamical cross section per unit mass
compared with aggregates of lower masses. We point out an important implication
of this discovery for dust growth in protoplanetary disks.Comment: 17 pages, 15 figures; v2: version to appear in ApJ (typos corrected
Classical molecular dynamics simulations of fusion and fragmentation in fullerene-fullerene collisions
We present the results of classical molecular dynamics simulations of
collision-induced fusion and fragmentation of C fullerenes, performed by
means of the MBN Explorer software package. The simulations provide information
on structural differences of the fused compound depending on kinematics of the
collision process. The analysis of fragmentation dynamics at different initial
conditions shows that the size distributions of produced molecular fragments
are peaked for dimers, which is in agreement with a well-established mechanism
of C fragmentation via preferential C emission. Atomic trajectories
of the colliding particles are analyzed and different fragmentation patterns
are observed and discussed. On the basis of the performed simulations,
characteristic time of C emission is estimated as a function of collision
energy. The results are compared with experimental time-of-flight distributions
of molecular fragments and with earlier theoretical studies. Considering the
widely explored case study of C--C collisions, we demonstrate
broad capabilities of the MBN Explorer software, which can be utilized for
studying collisions of a broad variety of nanoscale and biomolecular systems by
means of classical molecular dynamics
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