28,961 research outputs found
Self-Avoiding Modes of Motion in a Deterministic Lorentz Lattice Gas
We study the motion of a particle on the two-dimensional honeycomb lattice,
whose sites are occupied by either flipping rotators or flipping mirrors, which
scatter the particle according to a deterministic rule. For both types of
scatterers we find a new type of motion that has not been observed in a Lorentz
Lattice gas, where the particle's trajectory is a self-avoiding walk between
returns to its initial position. We show that this behavior is a consequence of
the deterministic scattering rule and the particular class of initial scatterer
configurations we consider. Since self-avoiding walks are one of the main tools
used to model the growth of crystals and polymers, the particle's motion in
this class of systems is potentially important for the study of these
processes.Comment: 32 pages, 18 figure
Weak compressible magnetohydrodynamic turbulence in the solar corona
This Letter presents a calculation of the power spectra of weakly turbulent
Alfven waves and fast magnetosonic waves ("fast waves") in low-beta plasmas. It
is shown that three-wave interactions transfer energy to high-frequency fast
waves and, to a lesser extent, high-frequency Alfven waves. MHD turbulence is
thus a promising mechanism for producing the high-frequency waves needed to
explain the anisotropic heating of minor ions in the solar corona.Comment: 4 pages, 3 figures, accepted, Phys. Rev. Let
The Panchromatic Hubble Andromeda Treasury. VI. The reliability of far-ultraviolet flux as a star formation tracer on sub-kpc scales
We have used optical observations of resolved stars from the Panchromatic
Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star
formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas
ranged from ~ to pc, which allowed us to test the reliability
of FUV flux as a tracer of recent star formation on sub-kpc scales. The star
formation rates (SFRs) derived from the extinction-corrected observed FUV
fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to
within the 1 scatter. Overall, the scatter was larger than the
uncertainties in the SFRs and particularly evident among the smallest regions.
The scatter was consistent with an even combination of discrete sampling of the
initial mass function and high variability in the SFHs. This result
demonstrates the importance of satisfying both the full-IMF and the
constant-SFR assumptions for obtaining precise SFR estimates from FUV flux.
Assuming a robust FUV extinction correction, we estimate that a factor of 2.5
uncertainty can be expected in FUV-based SFRs for regions smaller than
pc, or a few hundred pc. We also examined ages and masses derived from UV
flux under the common assumption that the regions are simple stellar
populations (SSPs). The SFHs showed that most of the regions are not SSPs, and
the age and mass estimates were correspondingly discrepant from the SFHs. For
those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age
and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like
regions from the others based on integrated FUV flux.Comment: Accepted for publication in The Astrophysical Journa
Karen Rothenberg’s (Not So) Secret Roles and Contributions at the U.S. National Institutes of Health
Weakly Turbulent MHD Waves in Compressible Low-Beta Plasmas
In this Letter, weak turbulence theory is used to investigate interactions
among Alfven waves and fast and slow magnetosonic waves in collisionless
low-beta plasmas. The wave kinetic equations are derived from the equations of
magnetohydrodynamics, and extra terms are then added to model collisionless
damping. These equations are used to provide a quantitative description of a
variety of nonlinear processes, including "parallel" and "perpendicular" energy
cascade, energy transfer between wave types, "phase mixing," and the generation
of back-scattered Alfven waves.Comment: Accepted, Physical Review Letter
- …