21,782 research outputs found
Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)
Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can
provide a valuable probe of warm molecular hydrogen within the interstellar
medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer
archive, are strikingly similar to spectral line maps of the H2 pure rotational
transitions that we obtained with the Infrared Spectrograph (IRS) instrument on
Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed
dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of
the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)
and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,
show band ratios that are inconsistent with the presence of gas at a single
temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent
with pure H2 emission from shocked material with a power-law distribution of
gas temperatures. CO vibrational emissions do not contribute significantly to
the observed Band 2 intensity. Assuming that the column density of H2 at
temperatures T to T+dT is proportional to T raised to the power -b for
temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The
power-law index, b, shows variations over the range 3 to 6 within the set of
different sight-lines probed by the maps, with the majority of sight-lines
showing b in the range 4 to 5. The observed power-law index is consistent with
the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap
Electrically driven magnetism on a Pd thin film
Using first-principles density functional calculations we demonstrate that
ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd
metal thin-film surface through application of an external electric field. As
free charges are either accumulated or depleted at the Pd surface to screen the
applied electric field there is a corresponding change in the surface density
of states. This change can be made sufficient for the Fermi-level density of
states to satisfy the Stoner criterion, driving a transition locally at the
surface from a paramagnetic state to an itinerant ferromagnetic state above a
critical applied electric field, Ec. Furthermore, due to the second-order
nature of this transition, the surface magnetization of the ferromagnetic state
just above the transition exhibits a substantial dependence on electric field,
as the result of an enhanced magnetoelectric susceptibility. Using a linearized
Stoner model we explain the occurrence of the itinerant ferromagnetism and
demonstrate that the magnetic moment on the Pd surface follows a square-root
variation with electric field consistent with our first-principles
calculations.Comment: 8 pages, 7 figure
The inexorable resistance of inertia determines the initial regime of drop coalescence
Drop coalescence is central to diverse processes involving dispersions of
drops in industrial, engineering and scientific realms. During coalescence, two
drops first touch and then merge as the liquid neck connecting them grows from
initially microscopic scales to a size comparable to the drop diameters. The
curvature of the interface is infinite at the point where the drops first make
contact, and the flows that ensue as the two drops coalesce are intimately
coupled to this singularity in the dynamics. Conventionally, this process has
been thought to have just two dynamical regimes: a viscous and an inertial
regime with a crossover region between them. We use experiments and simulations
to reveal that a third regime, one that describes the initial dynamics of
coalescence for all drop viscosities, has been missed. An argument based on
force balance allows the construction of a new coalescence phase diagram
Heat exchanger test experimental apparatus. Section I - Helium heater. Section II - Helium circulator Summary report
Molybdenum heater tube for heating helium to operating temperature of high temperature heat exchanger facility - performance of helium circulato
Master agreement task order 2 - Analyses and limited evaluations of payload and landing system structures for the survivable soft landing of instrument payloads
Computer programs for analysis and evaluation of payload and landing system structures for soft landing of instrument package
Nonlinear stabilitty for steady vortex pairs
In this article, we prove nonlinear orbital stability for steadily
translating vortex pairs, a family of nonlinear waves that are exact solutions
of the incompressible, two-dimensional Euler equations. We use an adaptation of
Kelvin's variational principle, maximizing kinetic energy penalised by a
multiple of momentum among mirror-symmetric isovortical rearrangements. This
formulation has the advantage that the functional to be maximized and the
constraint set are both invariant under the flow of the time-dependent Euler
equations, and this observation is used strongly in the analysis. Previous work
on existence yields a wide class of examples to which our result applies.Comment: 25 page
A kinetic model of radiating electrons
A kinetic theory is developed to describe radiating electrons whose motion is governed by the Lorentz-Dirac equation. This gives rise to a generalized Vlasov equation coupled to an equation for the evolution of the physical submanifold of phase space. The pathological solutions of the 1-particle theory may be removed by expanding the latter equation in powers of τ ≔ q 2/6πm. The radiation-induced change in entropy is explored and its physical origin is discussed. As a simple demonstration of the theory, the radiative damping rate of longitudinal plasma waves is calculated
Observed Effects of a Changing Step-Edge Density on Thin-Film Growth Dynamics
We grew SrTiO3 on SrTiO3 [001] by pulsed laser deposition, while observing
x-ray diffraction at the (0 0 .5) position. The drop dI in the x-ray intensity
following a laser pulse contains information about plume-surface interactions.
Kinematic theory predicts dI/I = -4sigma(1-sigma), so that dI/I depends only on
the amount of deposited material sigma. In contrast, we observed experimentally
that |dI/I| < 4sigma(1-sigma), and that dI/I depends on the phase of x-ray
growth oscillations. The combined results suggest a fast smoothing mechanism
that depends on surface step-edge density.Comment: 4 figure
- …