1,849 research outputs found
On continuum modeling of sputter erosion under normal incidence: interplay between nonlocality and nonlinearity
Under specific experimental circumstances, sputter erosion on semiconductor
materials exhibits highly ordered hexagonal dot-like nanostructures. In a
recent attempt to theoretically understand this pattern forming process, Facsko
et al. [Phys. Rev. B 69, 153412 (2004)] suggested a nonlocal, damped
Kuramoto-Sivashinsky equation as a potential candidate for an adequate
continuum model of this self-organizing process. In this study we theoretically
investigate this proposal by (i) formally deriving such a nonlocal equation as
minimal model from balance considerations, (ii) showing that it can be exactly
mapped to a local, damped Kuramoto-Sivashinsky equation, and (iii) inspecting
the consequences of the resulting non-stationary erosion dynamics.Comment: 7 pages, 2 Postscript figures, accepted by Phys. Rev. B corrected
typos, few minor change
How ripples turn into dots: modeling ion-beam erosion under oblique incidence
Pattern formation on semiconductor surfaces induced by low energetic ion-beam
erosion under normal and oblique incidence is theoretically investigated using
a continuum model in form of a stochastic, nonlocal, anisotropic
Kuramoto-Sivashinsky equation. Depending on the size of the parameters this
model exhibits hexagonally ordered dot, ripple, less regular and even rather
smooth patterns. We investigate the transitional behavior between such states
and suggest how transitions can be experimentally detected.Comment: 11 pages, 4 figures, submitted for publication, revised versio
Kinematic and Thermal Structure at the onset of high-mass star formation
We want to understand the kinematic and thermal properties of young massive
gas clumps prior to and at the earliest evolutionary stages of high-mass star
formation. Do we find signatures of gravitational collapse? Do we find
temperature gradients in the vicinity or absence of infrared emission sources?
Do we find coherent velocity structures toward the center of the dense and cold
gas clumps? To determine kinematics and gas temperatures, we used ammonia,
because it is known to be a good tracer and thermometer of dense gas. We
observed the NH(1,1) and (2,2) lines within seven very young high-mass
star-forming regions with the VLA and the Effelsberg 100m telescope. This
allows us to study velocity structures, linewidths, and gas temperatures at
high spatial resolution of 3-5, corresponding to 0.05 pc. We find on
average cold gas clumps with temperatures in the range between 10 K and 30 K.
The observations do not reveal a clear correlation between infrared emission
peaks and ammonia temperature peaks. We report an upper limit for the linewidth
of 1.3 km s, at the spectral resolution limit of our VLA
observation. This indicates a relatively low level of turbulence on the scale
of the observations. Velocity gradients are present in almost all regions with
typical velocity differences of 1 to 2 km s and gradients of 5 to 10 km
s pc. These velocity gradients are smooth in most cases, but
there is one exceptional source (ISOSS23053), for which we find several
velocity components with a steep velocity gradient toward the clump centers
that is larger than 30 km s pc. This steep velocity gradient is
consistent with recent models of cloud collapse. Furthermore, we report a
spatial correlation of ammonia and cold dust, but we also find decreasing
ammonia emission close to infrared emission sources.Comment: 20 pages, 10 figure
Hierarchical fragmentation and collapse signatures in a high-mass starless region
Aims: Understanding the fragmentation and collapse properties of the dense
gas during the onset of high-mass star formation. Methods: We observed the
massive (~800M_sun) starless gas clump IRDC18310-4 with the Plateau de Bure
Interferometer (PdBI) at sub-arcsecond resolution in the 1.07mm continuum
andN2H+(3-2) line emission. Results: Zooming from a single-dish low-resolution
map to previous 3mm PdBI data, and now the new 1.07mm continuum observations,
the sub-structures hierarchically fragment on the increasingly smaller spatial
scales. While the fragment separations may still be roughly consistent with
pure thermal Jeans fragmentation, the derived core masses are almost two orders
of magnitude larger than the typical Jeans mass at the given densities and
temperatures. However, the data can be reconciled with models using
non-homogeneous initial density structures, turbulence and/or magnetic fields.
While most sub-cores remain (far-)infrared dark even at 70mum, we identify weak
70mum emission toward one core with a comparably low luminosity of ~16L_sun,
re-enforcing the general youth of the region. The spectral line data always
exhibit multiple spectral components toward each core with comparably small
line widths for the individual components (in the 0.3 to 1.0km/s regime). Based
on single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio
<=0.25. We discuss that the likely origin of these spectral properties may be
the global collapse of the original gas clump that results in multiple spectral
components along each line of sight. Even within this dynamic picture the
individual collapsing gas cores appear to have very low levels of internal
turbulence.Comment: 8 pages, 4 figures, A&A in pres
A QUBO formulation for the Tree Containment problem
Phylogenetic (evolutionary) trees and networks are leaf-labeled graphs that
are widely used to represent the evolutionary relationships between entities
such as species, languages, cancer cells, and viruses. To reconstruct and
analyze phylogenetic networks, the problem of deciding whether or not a given
rooted phylogenetic network embeds a given rooted phylogenetic tree is of
recurring interest. This problem, formally know as Tree Containment, is
NP-complete in general and polynomial-time solvable for certain classes of
phylogenetic networks. In this paper, we connect ideas from quantum computing
and phylogenetics to present an efficient Quadratic Unconstrained Binary
Optimization formulation for Tree Containment in the general setting. For an
instance (N,T) of Tree Containment, where N is a phylogenetic network with n_N
vertices and T is a phylogenetic tree with n_T vertices, the number of logical
qubits that are required for our formulation is O(n_N n_T).Comment: final version accepted for publication in Theoretical Computer
Scienc
Influence of the Dufour effect on convection in binary gas mixtures
Linear and nonlinear properties of convection in binary fluid layers heated
from below are investigated, in particular for gas parameters. A Galerkin
approximation for realistic boundary conditions that describes stationary and
oscillatory convection in the form of straight parallel rolls is used to
determine the influence of the Dufour effect on the bifurcation behaviour of
convective flow intensity, vertical heat current, and concentration mixing. The
Dufour--induced changes in the bifurcation topology and the existence regimes
of stationary and traveling wave convection are elucidated. To check the
validity of the Galerkin results we compare with finite--difference numerical
simulations of the full hydrodynamical field equations. Furthermore, we report
on the scaling behaviour of linear properties of the stationary instability.Comment: 14 pages and 10 figures as uuencoded Postscript file (using uufiles
Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions
Context: How do molecular clouds form out of the atomic phase? And what are
the relative fractions of carbon in the ionized, atomic and molecular phase?
These are questions at the heart of cloud and star formation. Methods: Using
multiple observatories from Herschel and SOFIA to APEX and the IRAM 30m
telescope, we mapped the ionized, atomic and molecular carbon ([CII]@1900GHz,
[CI]@492GHz and C18O(2-1)@220GHz) at high spatial resolution (12"-25") in four
young massive infrared dark clouds (IRDCs). Results: The three carbon phases
were successfully mapped in all four regions, only in one source the [CII] line
remained a non-detection. Both the molecular and atomic phases trace the dense
structures well, with [CI] also tracing material at lower column densities.
[CII] exhibits diverse morphologies in our sample, from compact to diffuse
structures probing the cloud environment. In at least two out of the four
regions, we find kinematic signatures strongly indicating that the dense gas
filaments have formed out of a dynamically active and turbulent
atomic/molecular cloud, potentially from converging gas flows. The
atomic-to-molecular carbon gas mass ratios are low between 7% and 12% with the
lowest values found toward the most quiescent region. In the three regions
where [CII] is detected, its mass is always higher by a factor of a few than
that of the atomic carbon. The ionized carbon emission depends as well on the
radiation field, however, we also find strong [CII] emission in a region
without significant external sources, indicating that other processes, e.g.,
energetic gas flows can contribute to the [CII] excitation as well.Comment: 15 pages, 18 figures, accepted by Astronomy & Astrophysics, a higher
resolution version can be found at
http://www.mpia.de/homes/beuther/papers.htm
Kinematic structure of massive star-forming regions - I. Accretion along filaments
The mid- and far-infrared view on high-mass star formation, in particular
with the results from the Herschel space observatory, has shed light on many
aspects of massive star formation. However, these continuum studies lack
kinematic information.
We study the kinematics of the molecular gas in high-mass star-forming
regions.
We complemented the PACS and SPIRE far-infrared data of 16 high-mass
star-forming regions from the Herschel key project EPoS with N2H+ molecular
line data from the MOPRA and Nobeyama 45m telescope. Using the full N2H+
hyperfine structure, we produced column density, velocity, and linewidth maps.
These were correlated with PACS 70micron images and PACS point sources. In
addition, we searched for velocity gradients.
For several regions, the data suggest that the linewidth on the scale of
clumps is dominated by outflows or unresolved velocity gradients. IRDC18454 and
G11.11 show two velocity components along several lines of sight. We find that
all regions with a diameter larger than 1pc show either velocity gradients or
fragment into independent structures with distinct velocities. The velocity
profiles of three regions with a smooth gradient are consistent with gas flows
along the filament, suggesting accretion flows onto the densest regions.
We show that the kinematics of several regions have a significant and complex
velocity structure. For three filaments, we suggest that gas flows toward the
more massive clumps are present.Comment: accepted by A&
Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC18310-4
Aims: We study the fragmentation and dynamical properties of a massive
starless gas clump at the onset of high-mass star formation. Methods: Based on
Herschel continuum data we identify a massive gas clump that remains
far-infrared dark up to 100mum wavelengths. The fragmentation and dynamical
properties are investigated by means of Plateau de Bure Interferometer and
Nobeyama 45m single-dish spectral line and continuum observations. Results: The
massive gas reservoir fragments at spatial scales of ~18000AU in four cores.
Comparing the spatial extent of this high-mass region with intermediate- to
low-mass starless cores from the literature, we find that linear sizes do not
vary significantly over the whole mass regime. However, the high-mass regions
squeeze much more gas into these similar volumes and hence have orders of
magnitude larger densities. The fragmentation properties of the presented
low-to high-mass regions are consistent with gravitational instable Jeans
fragmentation. Furthermore, we find multiple velocity components associated
with the resolved cores. Recent radiative transfer hydrodynamic simulations of
the dynamic collapse of massive gas clumps also result in multiple velocity
components along the line of sight because of the clumpy structure of the
regions. This result is supported by a ratio between viral and total gas mass
for the whole region <1. Conclusions: This apparently still starless high-mass
gas clump exhibits clear signatures of early fragmentation and dynamic collapse
prior to the formation of an embedded heating source. A comparison with regions
of lower mass reveals that the linear size of star-forming regions does not
necessarily have to vary much for different masses, however, the mass
reservoirs and gas densities are orders of magnitude enhanced for high-mass
regions compared to their lower-mass siblings.Comment: 11 pages, 10 figures, accepted to Astronomy and Astrophysics,
high-resolution version with all figures included can be found at
http://www.mpia.de/homes/beuther/papers.htm
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