6,643 research outputs found
Spectral densities for hot QCD plasmas in a leading log approximation
We compute the spectral densities of and in high
temperature QCD plasmas at small frequency and momentum,\, . The leading log Boltzmann equation is reformulated as a Fokker Planck
equation with non-trivial boundary conditions, and the resulting partial
differential equation is solved numerically in momentum space. The spectral
densities of the current, shear, sound, and bulk channels exhibit a smooth
transition from free streaming quasi-particles to ideal hydrodynamics. This
transition is analyzed with conformal and non-conformal second order
hydrodynamics, and a second order diffusion equation. We determine all of the
second order transport coefficients which characterize the linear response in
the hydrodynamic regime.Comment: 39 pages, 6 figures. v3 contains an analysis of the bulk channel with
non-conformal hydrodynamics. Otherwise no significant change
Anomalous fluctuations of active polar filaments
Using a simple model, we study the fluctuating dynamics of inextensible,
semiflexible polar filaments interacting with active and directed force
generating centres such as molecular motors. Taking into account the fact that
the activity occurs on time-scales comparable to the filament relaxation time,
we obtain some unexpected differences between both the steady-state and
dynamical behaviour of active as compared to passive filaments. For the
statics, the filaments have a {novel} length-scale dependent rigidity.
Dynamically, we find strongly enhanced anomalous diffusion.Comment: 5 pages, 3 figure
The Hydrogen Atom in Strong Electric Fields: Summation of the Weak Field Series Expansion
The order dependent mapping method, its convergence has recently been proven
for the energy eigenvalue of the anharmonic oscillator, is applied to re-sum
the standard perturbation series for Stark effect of the hydrogen atom. We
perform a numerical experiment up to the fiftieth order of the perturbation
expansion. A simple mapping suggested by the analytic structure and the strong
field behavior gives an excellent agreement with the exact value for an
intermediate range of the electric field, . The imaginary
part of the energy (the decay width) as well as the real part of the energy is
reproduced from the standard perturbation series.Comment: 14 pages, 8 figure
Organics in comet 67P – a first comparative analysis of mass spectra from ROSINA–DFMS, COSAC and Ptolemy
The ESA Rosetta spacecraft followed comet 67P at a close distance for more than 2 yr. In addition, it deployed the lander Philae on to the surface of the comet. The (surface) composition of the comet is of great interest to understand the origin and evolution of comets. By combining measurements made on the comet itself and in the coma, we probe the nature of this surface material and compare it to remote sensing observations. We compare data from the double focusing mass spectrometer (DFMS) of the ROSINA experiment on ESA's Rosetta mission and previously published data from the two mass spectrometers COSAC (COmetary Sampling And Composition) and Ptolemy on the lander. The mass spectra of all three instruments show very similar patterns of mainly CHO-bearing molecules that sublimate at temperatures of 275 K. The DFMS data also show a great variety of CH-, CHN-, CHS-, CHO2- and CHNO-bearing saturated and unsaturated species. Methyl isocyanate, propanal and glycol aldehyde suggested by the earlier analysis of the measured COSAC spectrum could not be confirmed. The presence of polyoxymethylene in the Ptolemy spectrum was found to be unlikely. However, the signature of the aromatic compound toluene was identified in DFMS and Ptolemy data. Comparison with remote sensing instruments confirms the complex nature of the organics on the surface of 67P, which is much more diverse than anticipated
On the existence and structure of a mush at the inner core boundary of the Earth
It has been suggested about 20 years ago that the liquid close to the inner
core boundary (ICB) is supercooled and that a sizable mushy layer has developed
during the growth of the inner core. The morphological instability of the
liquid-solid interface which usually results in the formation of a mushy zone
has been intensively studied in metallurgy, but the freezing of the inner core
occurs in very unusual conditions: the growth rate is very small, and the
pressure gradient has a key role, the newly formed solid being hotter than the
adjacent liquid. We investigate the linear stability of a solidification front
under such conditions, pointing out the destabilizing role of the thermal and
solutal fields, and the stabilizing role of the pressure gradient. The main
consequence of the very small solidification rate is the importance of
advective transport of solute in liquid, which tends to remove light solute
from the vicinity of the ICB and to suppress supercooling, thus acting against
the destabilization of the solidification front. For plausible phase diagrams
of the core mixture, we nevertheless found that the ICB is likely to be
morphologically unstable, and that a mushy zone might have developed at the
ICB. The thermodynamic thickness of the resulting mushy zone can be
significant, from km to the entire inner core radius, depending on
the phase diagram of the core mixture. However, such a thick mushy zone is
predicted to collapse under its own weight, on a much smaller length scale
( km). We estimate that the interdendritic spacing is probably
smaller than a few tens of meter, and possibly only a few meters
The Statistical Mechanics of Membranes
The fluctuations of two-dimensional extended objects membranes is a rich and
exciting field with many solid results and a wide range of open issues. We
review the distinct universality classes of membranes, determined by the local
order, and the associated phase diagrams. After a discussion of several
physical examples of membranes we turn to the physics of crystalline (or
polymerized) membranes in which the individual monomers are rigidly bound. We
discuss the phase diagram with particular attention to the dependence on the
degree of self-avoidance and anisotropy. In each case we review and discuss
analytic, numerical and experimental predictions of critical exponents and
other key observables. Particular emphasis is given to the results obtained
from the renormalization group epsilon-expansion. The resulting renormalization
group flows and fixed points are illustrated graphically. The full technical
details necessary to perform actual calculations are presented in the
Appendices. We then turn to a discussion of the role of topological defects
whose liberation leads to the hexatic and fluid universality classes. We finish
with conclusions and a discussion of promising open directions for the future.Comment: 75 LaTeX pages, 36 figures. To appear in Physics Reports in the
Proceedings of RG2000, Taxco, 199
Evidence for Transgenerational Transmission of Epigenetic Tumor Susceptibility in Drosophila
Transgenerational epigenetic inheritance results from incomplete erasure of parental epigenetic marks during epigenetic reprogramming at fertilization. The significance of this phenomenon, and the mechanism by which it occurs, remains obscure. Here, we show that genetic mutations in Drosophila may cause epigenetic alterations that, when inherited, influence tumor susceptibility of the offspring. We found that many of the mutations that affected tumorigenesis induced by a hyperactive JAK kinase, HopTum-l, also modified the tumor phenotype epigenetically, such that the modification persisted even in the offspring that did not inherit the modifier mutation. We analyzed mutations of the transcription repressor Krüppel (Kr), which is one of the hopTum-l enhancers known to affect ftz transcription. We demonstrate that the Kr mutation causes increased DNA methylation in the ftz promoter region, and that the aberrant ftz transcription and promoter methylation are both transgenerationally heritable if HopTum-l is present in the oocyte. These results suggest that genetic mutations may alter epigenetic markings in the form of DNA methylation, which are normally erased early in the next generation, and that JAK overactivation disrupts epigenetic reprogramming and allows inheritance of epimutations that influence tumorigenesis in future generations
Systematic calculation of molecular vibrational spectra through a complete Morse expansion
We propose an accurate and efficient method to compute vibrational spectra of
molecules, based on exact diagonalization of an algebraically calculated matrix
based on powers of Morse coordinate. The present work focuses on the 1D
potential of diatomic molecules: as typical examples, we apply this method to
the standard Lennard-Jones oscillator, and to the ab initio potential of the H2
molecule. Global cm-1 accuracy is exhibited through the H2 spectrum, obtained
through the diagonalization of a 30 x 30 matrix. This theory is at the root of
a new method to obtain globally accurate vibrational spectral data in the
context of the multi-dimensional potential of polyatomic molecules, at an
affordable computational cost.Comment: 30 pages including 6 figure
Polycyclic aromatic hydrocarbon processing in a hot gas
Context: PAHs are thought to be a ubiquitous and important dust component of
the interstellar medium. However, the effects of their immersion in a hot
(post-shock) gas have never before been fully investigated. Aims: We study the
effects of energetic ion and electron collisions on PAHs in the hot post-shock
gas behind interstellar shock waves. Methods: We calculate the ion-PAH and
electron-PAH nuclear and electronic interactions, above the carbon atom loss
threshold, in H II regions and in the hot post-shock gas, for temperatures
ranging from 10^3 to 10^8 K. Results: PAH destruction is dominated by He
collisions at low temperatures (T < 3x10^4 K), and by electron collisions at
higher temperatures. Smaller PAHs are destroyed faster for T < 10^6 K, but the
destruction rates are roughly the same for all PAHs at higher temperatures. The
PAH lifetime in a tenuous hot gas (n_H ~ 0.01 cm^-3, T ~ 10^7 K), typical of
the coronal gas in galactic outflows, is found to be about thousand years,
orders of magnitude shorter than the typical lifetime of such objects.
Conclusions: In a hot gas, PAHs are principally destroyed by electron
collisions and not by the absorption of X-ray photons from the hot gas. The
resulting erosion of PAHs occurs via C_2 loss from the periphery of the
molecule, thus preserving the aromatic structure. The observation of PAH
emission from a million degree, or more, gas is only possible if the emitting
PAHs are ablated from dense, entrained clumps that have not yet been exposed to
the full effect of the hot gas.Comment: 16 pages, 11 figures, 3 tables, typos corrected and PAH acronym in
the title substituted with full name to match version published in Astronomy
and Astrophysic
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