341 research outputs found
Topological bound states for quantum charges
We discuss how, in appropriately designed configurations, solenoids carrying
a semifluxon can be used as topological energy barriers for charged quantum
systems. We interpret this phenomenon as a consequence of the fact that such
solenoids induce nodal lines in the wave function describing the charge, which
on itself is a consequence of the Aharonov-Bohm effect. Moreover, we present a
thought experiment with a cavity where two solenoids are sufficient to create
bound states.Comment: Close to published versio
Simulating Metal Mixing of Both Common and Rare Enrichment Sources in a Low-mass Dwarf Galaxy
One-zone models constructed to match observed stellar abundance patterns have been used extensively to constrain the sites of nucleosynthesis with sophisticated libraries of stellar evolution and stellar yields. The metal mixing included in these models is usually highly simplified, although it is likely to be a significant driver of abundance evolution. In this work we use high-resolution hydrodynamics simulations to investigate how metals from individual enrichment events with varying source energies E_(ej) mix throughout the multiphase interstellar medium (ISM) of a low-mass (M_(gas) = 2 Ă 10ⶠM_â), low-metallicity, isolated dwarf galaxy. These events correspond to the characteristic energies of both common and exotic astrophysical sites of nucleosynthesis, including asymptotic giant branch winds (E_(ej) ~ 10âŽâ¶ erg), neutron starâneutron star mergers (E_(ej) ~ 10âŽâč erg), supernovae (E_(ej) ~ 10â”Âč erg), and hypernovae (E_(ej) ~ 10â”ÂČ erg). We find the mixing timescales for individual enrichment sources in our dwarf galaxy to be long (100 Myrâ1 Gyr), with a clear trend of increasing homogeneity for the more energetic events. Given these timescales, we conclude that the spatial distribution and frequency of events are important drivers of abundance homogeneity on large scales; rare, low-E_(ej) events should be characterized by particularly broad abundance distributions. The source energy E_(ej) also correlates with the fraction of metals ejected in galactic winds, ranging anywhere from 60% at the lowest energy to 95% for hypernovae. We conclude by examining how the radial position, local ISM density, and global star formation rate influence these results
Location Games and Bounds for Median Problems
We consider a two-person zero-sum game in which the maximizer selects a point in a given bounded planar region, the minimizer selects K points in that region,.and the payoff is the distance from the maximizer's location to the minimizer's location closest to it. In a variant of this game, the maximizer has the privilege of restricting the game to any subset of the given region. We evaluate/approximate the values (and the saddle point strategies) of these games for K = 1 as well as for K + , thus obtaining tight upper bounds (and worst possible demand distributions) for K-median problems
The Inability of Ambipolar Diffusion to set a Characteristic Mass Scale in Molecular Clouds
We investigate the question of whether ambipolar diffusion (ion-neutral
drift) determines the smallest length and mass scale on which structure forms
in a turbulent molecular cloud. We simulate magnetized turbulence in a mostly
neutral, uniformly driven, turbulent medium, using a three-dimensional,
two-fluid, magnetohydrodynamics (MHD) code modified from Zeus-MP. We find that
substantial structure persists below the ambipolar diffusion scale because of
the propagation of compressive slow MHD waves at smaller scales. Contrary to
simple scaling arguments, ambipolar diffusion thus does not suppress structure
below its characteristic dissipation scale as would be expected for a classical
diffusive process. We have found this to be true for the magnetic energy,
velocity, and density. Correspondingly, ambipolar diffusion leaves the clump
mass spectrum unchanged. Ambipolar diffusion appears unable to set a
characteristic scale for gravitational collapse and star formation in turbulent
molecular clouds.Comment: 16 pages, 5 figures. ApJ accepte
Metal Mixing and Ejection in Dwarf Galaxies is Dependent on Nucleosynthetic Source
Using a high resolution simulation of an isolated dwarf galaxy, accounting
for multi-channel stellar feedback and chemical evolution on a star-by-star
basis, we investigate how each of 15 metal species are distributed within our
multi-phase interstellar medium (ISM) and ejected from our galaxy by galactic
winds. For the first time, we demonstrate that the mass fraction probability
distribution functions (PDFs) of individual metal species in the ISM are well
described by a piecewise log-normal and power-law distribution. The PDF
properties vary within each ISM phase. Hot gas is dominated by recent
enrichment, with a significant power-law tail to high metal fractions, while
cold gas is predominately log-normal. In addition, elements dominated by
asymptotic giant branch (AGB) wind enrichment (e.g. N and Ba) mix less
efficiently than elements dominated by supernova enrichment (e.g.
elements and Fe). This result is driven by the differences in source energetics
and source locations, particularly the higher chance compared to massive stars
for AGB stars to eject material into cold gas. Nearly all of the produced
metals are ejected from the galaxy (only 4% are retained), but over 20% of
metals dominated by AGB enrichment are retained. In dwarf galaxies, therefore,
elements synthesized predominately through AGB winds should be both
overabundant and have a larger spread compared to elements synthesized in
either core collapse or Type Ia supernovae. We discuss the observational
implications of these results, their potential use in developing improved
models of galactic chemical evolution, and their generalization to more massive
galaxies.Comment: 18 pages, 7 figures (plus 2 page, 2 figure appendix). Accepted to Ap
Magnetic Forces in the Absence of a Classical Magnetic Field
It is shown that, in some cases, the effect of discrete distributions of flux lines in quantum mechanics can be associated with the effect of continuous distributions of magnetic fields with special symmetries. In particular, flux lines with an arbitrary value of magnetic flux can be used to create energetic barriers, which can be used to confine quantum systems in specially designed configurations. This generalizes a previous work where such energy barriers arose from flux lines with half-integer fluxons. Furthermore, it is shown how the Landau levels can be obtained from a two-dimensional grid of flux lines. These results suggest that the classical magnetic force can be seen as emerging entirely from the Aharonov-Bohm effect. Finally, the basic elements of a semiclassical theory that models the emergence of classical magnetic forces from fields with special symmetries are introduced
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