3,629 research outputs found
The dressed nonrelativistic electron in a magnetic field
We consider a nonrelativistic electron interacting with a classical magnetic
field pointing along the -axis and with a quantized electromagnetic
field. When the interaction between the electron and photons is turned off, the
electronic system is assumed to have a ground state of finite multiplicity.
Because of the translation invariance along the -axis, we consider the
reduced Hamiltonian associated with the total momentum along the -axis
and, after introducing an ultraviolet cutoff and an infrared regularization, we
prove that the reduced Hamiltonian has a ground state if the coupling constant
and the total momentum along the -axis are sufficiently small. Finally
we determine the absolutely continuous spectrum of the reduced Hamiltonian.Comment: typos correction
Recommended from our members
Assessing impacts to groundwater from CO2-flooding of SACROC and Claytonville oil fields in West Texas
Comparison of groundwater above two Permian Basin oil fields (SACROC Unit and
Claytonville Field) near Snyder, Texas should allow us to assess potential impacts of 30 years of
CO2-injection. CO2-flooding for enhanced oil recovery (EOR) has been active at SACROC in
Scurry County since 1972. Approximately 13.5 million tons per year (MtCO2/yr) are injected
with withdrawal/recycling amounting to ~7MtCO2/yr. It is estimated that the site has accumulated
more than 55MtCO2; however, no rigorous investigation of overlying groundwater has
demonstrated that CO2 is trapped in the subsurface. Mineralogy of reservoir rocks at the
Claytonville field in southwestern Fisher County is similar to SACROC. CO2-EOR is scheduled
to begin at Claytonville Field in Fisher County in early 2007. Here we have the opportunity to
characterize groundwater prior to CO2-injection and establish baseline conditions at Claytonville.
Methods of this study will include: (1) examination of existing analyses of saline to fresh
water samples collected within an eight-county area encompassing SACROC and Claytonville,
(2) additional groundwater sampling for analysis of general chemistry plus field-measured pH,
alkalinity, and temperature, stable isotopic ratios of hydrogen (D/H), oxygen (18O/16O), and
carbon (13C/12C), and (3) geochemical equilibrium and flowpath modeling. Existing groundwater
data are available from previous BEG studies, Texas Water Development Board, Kinder Morgan
CO2 Company, and the U. S. Geological Survey. By examining these data we will identify
regional groundwater variability and focus additional sampling efforts. The objective of this study
is to look for potential impacts to shallow groundwater from deep CO2-injection. In the absence
of conduit flow from depth, we don’t expect to see impacts to shallow groundwater, but
methodology to demonstrate this to regulators needs to be established.
This work is a subset of the Southwest Regional Partnership on Carbon Sequestration
Phase 2studies funded by the Department of Energy (DOE) in cooperation with industry and
government partners.Bureau of Economic Geolog
Evolution of Exoplanets and their Parent Stars
Studying exoplanets with their parent stars is crucial to understand their
population, formation and history. We review some of the key questions
regarding their evolution with particular emphasis on giant gaseous exoplanets
orbiting close to solar-type stars. For masses above that of Saturn, transiting
exoplanets have large radii indicative of the presence of a massive
hydrogen-helium envelope. Theoretical models show that this envelope
progressively cools and contracts with a rate of energy loss inversely
proportional to the planetary age. The combined measurement of planetary mass,
radius and a constraint on the (stellar) age enables a global determination of
the amount of heavy elements present in the planet interior. The comparison
with stellar metallicity shows a correlation between the two, indicating that
accretion played a crucial role in the formation of planets. The dynamical
evolution of exoplanets also depends on the properties of the central star. We
show that the lack of massive giant planets and brown dwarfs in close orbit
around G-dwarfs and their presence around F-dwarfs are probably tied to the
different properties of dissipation in the stellar interiors. Both the
evolution and the composition of stars and planets are intimately linked.Comment: appears in The age of stars - 23rd Evry Schatzman School on Stellar
Astrophysics, Roscoff : France (2013
Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks
Planetesimals in gaseous protoplanetary disks may grow by collecting dust
particles. Hydrodynamical studies show that small particles generally avoid
collisions with the planetesimals because they are entrained by the flow around
them. This occurs when , the Stokes number, defined as the ratio of the
dust stopping time to the planetesimal crossing time, becomes much smaller than
unity. However, these studies have been limited to the laminar case, whereas
these disks are believed to be turbulent. We want to estimate the influence of
gas turbulence on the dust-planetesimal collision rate and on the impact
speeds. We used three-dimensional direct numerical simulations of a fixed
sphere (planetesimal) facing a laminar and turbulent flow seeded with small
inertial particles (dust) subject to a Stokes drag. A no-slip boundary
condition on the planetesimal surface is modeled via a penalty method. We find
that turbulence can significantly increase the collision rate of dust particles
with planetesimals. For a high turbulence case (when the amplitude of turbulent
fluctuations is similar to the headwind velocity), we find that the collision
probability remains equal to the geometrical rate or even higher for , i.e., for dust sizes an order of magnitude smaller than in the laminar
case. We derive expressions to calculate impact probabilities as a function of
dust and planetesimal size and turbulent intensity
Twisting algebras using non-commutative torsors
Non-commutative torsors (equivalently, two-cocycles) for a Hopf algebra can
be used to twist comodule algebras. After surveying and extending the
literature on the subject, we prove a theorem that affords a presentation by
generators and relations for the algebras obtained by such twisting. We give a
number of examples, including new constructions of the quantum affine spaces
and the quantum tori.Comment: 27 pages. Masuoka is a new coauthor. Introduction was revised.
Sections 1 and 2 were thoroughly restructured. The presentation theorem in
Section 3 is now put in a more general framework and has a more general
formulation. Section 4 was shortened. All examples (quantum affine spaces and
tori, twisting of SL(2), twisting of the enveloping algebra of sl(2)) are
left unchange
Inverse scattering at fixed energy for layered media
AbstractIn this article we show that exponentially decreasing perturbations of the sound speed in a layered medium can be recovered from the scattering amplitude at fixed energy. We consider the unperturbed equation utt = c02(xn)δu in ℝ×ℝ, where n ≥ 3. The unperturbed sound speed, c0(xn), is assumed to be bounded, strictly positive, and constant outside a bounded interval on the real axis. The perturbed sound speed, c(x), satisfies ¦c.(x) - co(xn)¦ < C exp(−δ¦x¦) for some δ > 0. Our work is related to the recent results of H. Isozaki (J. Diff. Eq. 138) on the case where c0 takes the constant values c+ and c− on the positive and negative half-lines, and R. Weder on the case c0 = c+ for xn > h, c0 = ch, for 0 < xn, < h, and c0 = c− for xn < 0 (IIMAS-UNAM Preprint 70, November, 1997)
A correlation between the heavy element content of transiting extrasolar planets and the metallicity of their parent stars
Nine extrasolar planets with masses between 110 and 430M are known to transit
their star. The knowledge of their masses and radii allows an estimate of their
composition, but uncertainties on equations of state, opacities and possible
missing energy sources imply that only inaccurate constraints can be derived
when considering each planet separately. Aims: We seek to better understand the
composition of transiting extrasolar planets by considering them as an
ensemble, and by comparing the obtained planetary properties to that of the
parent stars. Methods: We use evolution models and constraints on the stellar
ages to derive the mass of heavy elements present in the planets. Possible
additional energy sources like tidal dissipation due to an inclined orbit or to
downward kinetic energy transport are considered. Results: We show that the
nine transiting planets discovered so far belong to a quite homogeneous
ensemble that is characterized by a mass of heavy elements that is a relatively
steep function of the stellar metallicity, from less than 20 earth masses of
heavy elements around solar composition stars, to up to 100M for three times
the solar metallicity (the precise values being model-dependant). The
correlation is still to be ascertained however. Statistical tests imply a
worst-case 1/3 probability of a false positive. Conclusions: Together with the
observed lack of giant planets in close orbits around metal-poor stars, these
results appear to imply that heavy elements play a key role in the formation of
close-in giant planets. The large masses of heavy elements inferred for planets
orbiting metal rich stars was not anticipated by planet formation models and
shows the need for alternative theories including migration and subsequent
collection of planetesimals.Comment: Astronomy and Astrophysics 0 (2006) in pres
Rainy downdrafts in abyssal atmospheres
Results from Juno's microwave radiometer indicate non-uniform mixing of
ammonia vapor in Jupiter's atmosphere down to tens of bars, far beneath the
cloud level. Helioseismic observations suggest solar convection may require
narrow, concentrated downdrafts called entropy rain to accommodate the Sun's
luminosity. Both observations suggest some mechanism of non-local convective
transport. We seek to predict the depth that a concentrated density anomaly can
reach before efficiently mixing with its environment in bottomless atmospheres.
We modify classic self-similar analytical models of entraining thermals to
account for the compressibility of an abyssal atmosphere. We compare these
models to the output of high resolution three dimensional fluid dynamical
simulations to more accurately model the chaotic influence of turbulence. We
find that localized density anomalies propagate down to ~3-8 times their
initial size without substantially mixing with their environment. Our analytic
model accurately predicts the initial flow, but the self-similarity assumption
breaks down after the flow becomes unstable at a characteristic penetration
depth. In the context of Jupiter, our findings suggest that precipitation
concentrated into localized downdrafts of size ~20km can coherently penetrate
to on the order of a hundred kilometers (tens of bars) beneath its initial
vaporization level without mixing with its environment. This finding is
consistent with expected convective storm length-scales, and Juno MWR
measurements of ammonia depletion. Compositional gradients of volatiles beneath
their cloud levels may be common on stormy giant planets. In the context of the
Sun, we find that turbulent downdrafts in abyssal atmospheres cannot maintain
their coherence through the Sun's convective layer, a potential challenge for
the entropy rain hypothesis.Comment: 20 pages, 16 figures. Accepted by A&
- …