7,507 research outputs found
Alternating groups and moduli space lifting Invariants
Main Theorem: Spaces of r-branch point 3-cycle covers, degree n or Galois of
degree n!/2 have one (resp. two) component(s) if r=n-1 (resp. r\ge n). Improves
Fried-Serre on deciding when sphere covers with odd-order branching lift to
unramified Spin covers. We produce Hurwitz-Torelli automorphic functions on
Hurwitz spaces, and draw Inverse Galois conclusions. Example: Absolute spaces
of 3-cycle covers with +1 (resp. -1) lift invariant carry canonical even (resp.
odd) theta functions when r is even (resp. odd). For inner spaces the result is
independent of r. Another use appears in,
http://www.math.uci.edu/~mfried/paplist-mt/twoorbit.html, "Connectedness of
families of sphere covers of A_n-Type." This shows the M(odular) T(ower)s for
the prime p=2 lying over Hurwitz spaces first studied by,
http://www.math.uci.edu/~mfried/othlist-cov/hurwitzLiu-Oss.pdf, Liu and
Osserman have 2-cusps. That is sufficient to establish the Main Conjecture: (*)
High tower levels are general-type varieties and have no rational points.For
infinitely many of those MTs, the tree of cusps contains a subtree -- a spire
-- isomorphic to the tree of cusps on a modular curve tower. This makes
plausible a version of Serre's O(pen) I(mage) T(heorem) on such MTs.
Establishing these modular curve-like properties opens, to MTs, modular
curve-like thinking where modular curves have never gone before. A fuller html
description of this paper is at
http://www.math.uci.edu/~mfried/paplist-cov/hf-can0611591.html .Comment: To appear in the Israel Journal as of 1/5/09; v4 is corrected from
proof sheets, but does include some proof simplification in \S
About the morphology of dwarf spheroidal galaxies and their dark matter content
The morphological properties of the Carina, Sculptor and Fornax dwarfs are
investigated using new wide field data with a total area of 29 square degrees.
The stellar density maps are derived, hinting that Sculptor possesses tidal
tails indicating interaction with the Milky Way. Contrary to previous studies
we cannot find any sign of breaks in the density profiles for the Carina and
Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King
limiting radii in Fornax and Carina are used to derive global M/L ratios,
without using kinematic data. By matching those M/L ratios to kinematically
derived values we are able to constrain the orbital parameters of the three
dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its
perigalacticon now. The other extreme is Sculptor that needs to be on an orbit
with an eccentricity bigger than 0.5 to be able to form tidal tails despite its
kinematic M/L.Comment: 9 pages, 7 figures, accepted by A&
Vlasov simulation in multiple spatial dimensions
A long-standing challenge encountered in modeling plasma dynamics is
achieving practical Vlasov equation simulation in multiple spatial dimensions
over large length and time scales. While direct multi-dimension Vlasov
simulation methods using adaptive mesh methods [J. W. Banks et al., Physics of
Plasmas 18, no. 5 (2011): 052102; B. I. Cohen et al., November 10, 2010,
http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142] have recently shown
promising results, in this paper we present an alternative, the Vlasov Multi
Dimensional (VMD) model, that is specifically designed to take advantage of
solution properties in regimes when plasma waves are confined to a narrow cone,
as may be the case for stimulated Raman scatter in large optic f# laser beams.
Perpendicular grid spacing large compared to a Debye length is then possible
without instability, enabling an order 10 decrease in required computational
resources compared to standard particle in cell (PIC) methods in 2D, with
another reduction of that order in 3D. Further advantage compared to PIC
methods accrues in regimes where particle noise is an issue. VMD and PIC
results in a 2D model of localized Langmuir waves are in qualitative agreement
Nonlocal spectral properties of disordered alloys
A general method is proposed for calculating a fully k-dependent, continuous,
and causal spectral function A(k,E) within the recently introduced nonlocal
version of the coherent-potential approximation (NLCPA). The method involves
the combination of both periodic and anti-periodic solutions to the associated
cluster problem and also leads to correct bulk quantities for small cluster
sizes. We illustrate the method by investigating the Fermi surface of a
two-dimensional alloy. Dramatically, we find a smeared electronic topological
transition not predicted by the conventional CPA.Comment: 17 pages, 5 figures, Submitted to: J. Phys.: Condens. Matter
Editorial receipt 25 May 200
Rapid Oscillations in Cataclysmic Variables. XV. HT Camelopardalis (= RX J0757.0+6306)
We present photometry and spectroscopy of HT Camelopardalis, a recently
discovered X-ray-bright cataclysmic variable. The spectrum shows bright lines
of H, He I, and He II, all moving with a period of 0.059712(1) d, which we
interpret as the orbital period. The star's brightness varies with a strict
period of 515.0592(2) s, and a mean full amplitude of 0.11 mag. These
properties qualify it as a /bona fide/ DQ Herculis star (intermediate polar) --
in which the magnetism of the rapidly rotating white dwarf channels accretion
flow to the surface. Normally at V=17.8, the star shows rare and very brief
outbursts to V=12-13. We observed one in December 2001, and found that the 515
s pulse amplitude had increased by a factor of ~100 (in flux units). A
transient orbital signal may also have appeared.Comment: PDF, 19 pages, 3 tables, 6 figures; accepted, in press, to appear
June 2002, PASP; more info at http://cba.phys.columbia.edu
Mesoscopic modelling of the interaction of infrared lasers with composite materials: an application to dental enamel
The mesostructure and composition of composite materials determine their mechanical, optical and thermal properties and, consequently, their response to incident radiation. We have developed general finite element models of porous composite materials under infrared radiation to examine the influence of pore size on one of the determining parameters of the stress distribution in the material: the temperature distribution. We apply them to the specific case of human dental enamel, a material which has nanometer scale pores containing water/organic, and predict the maximum temperature reached after a single 0.35 ÎŒs laser pulse of sub-ablative fluence by two lasers: Er:YAG (2.9 ÎŒm) and CO2 (10.6 ÎŒm). For the Er:YAG laser, the results imply a strong dependence of the maximum temperature reached at the pore on the area-to-volume ratio of the pore, whereas there is little such dependence for CO2 lasers. Thus, CO2 lasers may produce more reproducible results than Er:YAG lasers when it comes to enamel ablation, which may be of significant interest during clinical practice. More generally, when ablating composite materials by infrared lasers researchers should account for the materialâs microstructure and composition when designing experiments or interpreting results, since a more simplistic continuum approach may not be sufficient to explain differences observed during ablation of materials with similar optical properties or of the same material but using different wavelengths
An Analysis of Fundamental Waffle Mode in Early AEOS Adaptive Optics Images
Adaptive optics (AO) systems have significantly improved astronomical imaging
capabilities over the last decade, and are revolutionizing the kinds of science
possible with 4-5m class ground-based telescopes. A thorough understanding of
AO system performance at the telescope can enable new frontiers of science as
observations push AO systems to their performance limits. We look at recent
advances with wave front reconstruction (WFR) on the Advanced Electro-Optical
System (AEOS) 3.6 m telescope to show how progress made in improving WFR can be
measured directly in improved science images. We describe how a "waffle mode"
wave front error (which is not sensed by a Fried geometry Shack-Hartmann wave
front sensor) affects the AO point-spread function (PSF). We model details of
AEOS AO to simulate a PSF which matches the actual AO PSF in the I-band, and
show that while the older observed AEOS PSF contained several times more waffle
error than expected, improved WFR techniques noticeably improve AEOS AO
performance. We estimate the impact of these improved WFRs on H-band imaging at
AEOS, chosen based on the optimization of the Lyot Project near-infrared
coronagraph at this bandpass.Comment: 15 pages, 11 figures, 1 table; to appear in PASP, August 200
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
The Wisconsin H-Alpha Mapper Northern Sky Survey
The Wisconsin H-Alpha Mapper (WHAM) has surveyed the distribution and
kinematics of ionized gas in the Galaxy above declination -30 degrees. The WHAM
Northern Sky Survey (WHAM-NSS) has an angular resolution of one degree and
provides the first absolutely-calibrated, kinematically-resolved map of the
H-Alpha emission from the Warm Ionized Medium (WIM) within ~ +/-100 km/s of the
Local Standard of Rest. Leveraging WHAM's 12 km/s spectral resolution, we have
modeled and removed atmospheric emission and zodiacal absorption features from
each of the 37,565 spectra. The resulting H-Alpha profiles reveal ionized gas
detected in nearly every direction on the sky with a sensitivity of 0.15 R (3
sigma). Complex distributions of ionized gas are revealed in the nearby spiral
arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the
WHAM-NSS provides information about the WIM out to the tangent point down to a
few degrees from the plane. Ionized gas is also detected toward many
intermediate velocity clouds at high latitudes. Several new H II regions are
revealed around early B-stars and evolved stellar cores (sdB/O). This work
presents the details of the instrument, the survey, and the data reduction
techniques. The WHAM-NSS is also presented and analyzed for its gross
properties. Finally, some general conclusions are presented about the nature of
the WIM as revealed by the WHAM-NSS.Comment: 42 pages, 14 figures (Fig 6-9 & 14 are full color); accepted for
publication in 2003, ApJ, 149; Original quality figures (as well as data for
the survey) are available at http://www.astro.wisc.edu/wham
Thermodynamic Modeling of Fluid Polyamorphism in Hydrogen at Extreme Conditions
Fluid polyamorphism, the existence of multiple amorphous fluid states in a
single-component system, has been observed or predicted in a variety of
substances. A remarkable example of this phenomenon is the fluid-fluid phase
transition in high-pressure hydrogen between insulating and conducting
high-density fluids. This transition is induced by the reversible
dimerization/dissociation of the molecular and atomistic states of hydrogen. In
this work, we present the first attempt to thermodynamically model the
fluid-fluid phase transition in hydrogen at extreme conditions. Our predictions
for the phase coexistence and the reaction equilibrium of the two alternative
forms of fluid hydrogen are based on experimental data and supported by the
results of simulations. {Remarkably, we find that the law of corresponding
states can be utilized to construct a unified equation of state combining the
available computational results for different models of hydrogen and the
experimental data.Comment: Manuscript intended for JC
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