12,141 research outputs found
Filamentary structure in the Orion molecular cloud
A large scale 13CO map (containing 33,000 spectra) of the giant molecular cloud located in the southern part of Orion is presented which contains the Orion Nebula, NGC1977, and the LI641 dark cloud complex. The overall structure of the cloud is filamentary, with individual features having a length up to 40 times their width. This morphology may result from the effects of star formation in the region or embedded magnetic fields in the cloud. We suggest a simple picture for the evolution of the Orion-A cloud and the formation of the major filament. A rotating proto-cloud (counter rotating with respect to the galaxy) contians a b-field aligned with the galaxtic plane. The northern protion of this cloud collapsed first, perhaps triggered by the pressure of the Ori I OB association. The magnetic field combined with the anisotropic pressure produced by the OB-association breaks the symmetry of the pancake instability, a filament rather than a disc is produced. The growth of instabilities in the filament formed sub-condensations which are recent sites of star formation
Hamiltonians for curves
We examine the equilibrium conditions of a curve in space when a local energy
penalty is associated with its extrinsic geometrical state characterized by its
curvature and torsion. To do this we tailor the theory of deformations to the
Frenet-Serret frame of the curve. The Euler-Lagrange equations describing
equilibrium are obtained; Noether's theorem is exploited to identify the
constants of integration of these equations as the Casimirs of the euclidean
group in three dimensions. While this system appears not to be integrable in
general, it {\it is} in various limits of interest. Let the energy density be
given as some function of the curvature and torsion, . If
is a linear function of either of its arguments but otherwise arbitrary, we
claim that the first integral associated with rotational invariance permits the
torsion to be expressed as the solution of an algebraic equation in
terms of the bending curvature, . The first integral associated with
translational invariance can then be cast as a quadrature for or for
.Comment: 17 page
Minimum free-energy path of homogenous nucleation from the phase-field equation
The minimum free-energy path (MFEP) is the most probable route of the
nucleation process on the multidimensional free-energy surface. In this study,
the phase-field equation is used as a mathematical tool to deduce the minimum
free-energy path (MFEP) of homogeneous nucleation. We use a simple
square-gradient free-energy functional with a quartic local free-energy
function as an example and study the time evolution of a single nucleus placed
within a metastable environment. The time integration of the phase-field
equation is performed using the numerically efficient cell-dynamics method. By
monitoring the evolution of the size of the nucleus and the free energy of the
system simultaneously, we can easily deduce the free-energy barrier as a
function of the size of the sub- and the super-critical nucleus along the MFEP.Comment: 8 pages, 5 figures, Journal of Chemical Physics accepted for
publicatio
A repulsive atomic gas in a harmonic trap on the border of itinerant ferromagnetism
Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the
most well-characterized phases of correlated Fermi systems. A recent experiment
has reported the first evidence for novel phase behavior on the repulsive side
of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting
recent theoretical studies to the atomic trap geometry, we show that an
adiabatic ferromagnetic transition would take place at a weaker interaction
strength than is observed in experiment. This discrepancy motivates a simple
non-equilibrium theory that takes account of the dynamics of magnetic defects
and three-body losses. The formalism developed displays good quantitative
agreement with experiment.Comment: 4 pages, 2 figure
Localized induction equation and pseudospherical surfaces
We describe a close connection between the localized induction equation
hierarchy of integrable evolution equations on space curves, and surfaces of
constant negative Gauss curvature.Comment: 21 pages, AMSTeX file. To appear in Journal of Physics A:
Mathematical and Genera
Phase-field-crystal modeling of the (2x1)-(1x1) phase-transitions of Si(001) and Ge(001) surfaces
We propose a two-dimensional phase-field-crystal model for the
(21)-(11) phase transitions of Si(001) and Ge(001) surfaces.
The dimerization in the 21 phase is described with a
phase-field-crystal variable which is determined by solving an evolution
equation derived from the free energy. Simulated periodic arrays of
dimerization variable is consistent with scanning-tunnelling-microscopy images
of the two dimerized surfaces. Calculated temperature dependence of the
dimerization parameter indicates that normal dimers and broken ones coexist
between the temperatures describing the charactristic temperature width of the
phase-transition, and , and a first-order phase transition takes
place at a temperature between them. The dimerization over the whole
temperature is determined. These results are in agreement with experiment. This
phase-field-crystal approach is applicable to phase-transitions of other
reconstructed surface phases, especially semiconductor 1 reconstructed
surface phases.Comment: 10 pages with 4 figures include
Atomic oxygen beam source for erosion simulation
A device for the production of low energy (3 to 10 eV) neutral atomic beams for surface modification studies is described that reproduces the flux of atomic oxygen in low Earth orbit. The beam is produced by the acceleration of plasma ions onto a negatively biased plate of high-Z metal; the ions are neutralized and reflected by the surface, retaining some fraction of their incident kinetic energy, forming a beam of atoms. The plasma is generated by a coaxial RF exciter which produces a magnetically-confined (4 kG) plasma column. At the end of the column, ions fall through the sheath to the plate, whose bias relative to the plasma can be varied to adjust the beam energy. The source provides a neutral flux approximately equal to 5 x 10(exp 16)/sq cm at a distance of 9 cm and a fluence approximately equal to 10(exp 20)/sq cm in five hours. The composition and energy of inert gas beams was diagnosed using a mass spectometer/energy analyzer. The energy spectra of the beams demonstrate energies in the range 5 to 15 eV, and qualitatively show expected dependences upon incident and reflecting atom species and potential drop. Samples of carbon film, carbon-based paint, Kapton, mylar, and teflon exposed to atomic O beams show erosion quite similar to that observed in orbit on the space shuttle
Active Carbon and Oxygen Shell Burning Hydrodynamics
We have simulated 2.5 s of the late evolution of a star with full hydrodynamic behavior. We present the first simulations
of a multiple-shell burning epoch, including the concurrent evolution and
interaction of an oxygen and carbon burning shell. In addition, we have evolved
a 3D model of the oxygen burning shell to sufficiently long times (300 s) to
begin to assess the adequacy of the 2D approximation. We summarize striking new
results: (1) strong interactions occur between active carbon and oxygen burning
shells, (2) hydrodynamic wave motions in nonconvective regions, generated at
the convective-radiative boundaries, are energetically important in both 2D and
3D with important consequences for compositional mixing, and (3) a spectrum of
mixed p- and g-modes are unambiguously identified with corresponding adiabatic
waves in these computational domains. We find that 2D convective motions are
exaggerated relative to 3D because of vortex instability in 3D. We discuss the
implications for supernova progenitor evolution and symmetry breaking in core
collapse.Comment: 5 pages, 4 figures in emulateapj format. Accepted for publication in
ApJ Letters. High resolution figure version available at
http://spinach.as.arizona.ed
A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
ABRIDGED: Context: HI and CO large scale surveys of the Milky Way trace the
diffuse atomic clouds and the dense shielded regions of molecular hydrogen
clouds. However, until recently, we have not had spectrally resolved C+ surveys
to characterize the photon dominated interstellar medium, including, the H2 gas
without C, the CO-dark H2, in a large sample of clouds. Aims: To use a sparse
Galactic plane survey of the 1.9 THz [C II] spectral line from the Herschel
Open Time Key Programme, Galactic Observations of Terahertz C+ (GOT C+), to
characterize the H2 gas without CO in a statistically significant sample of
clouds. Methods: We identify individual clouds in the inner Galaxy by fitting
[CII] and CO isotopologue spectra along each line of sight. We combine these
with HI spectra, along with excitation models and cloud models of C+, to
determine the column densities and fractional mass of CO-dark H2 clouds.
Results: We identify 1804 narrow velocity [CII] interstellar cloud components
in different categories. About 840 are diffuse molecular clouds with no CO, 510
are transition clouds containing [CII] and 12CO, but no 13CO, and the remainder
are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are
concentrated between Galactic radii 3.5 to 7.5 kpc and the column density of
the CO-dark H2 layer varies significantly from cloud-to-cloud with an average
9X10^(20) cm-2. These clouds contain a significant fraction of CO-dark H2 mass,
varying from ~75% for diffuse molecular clouds to ~20% for dense molecular
clouds. Conclusions: We find a significant fraction of the warm molecular ISM
gas is invisible in HI and CO, but is detected in [CII]. The fraction of
CO-dark H2 is greatest in the diffuse clouds and decreases with increasing
total column density, and is lowest in the massive clouds.Comment: 21 pages, 19 figures, accepted for publication in A&A (2014
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