31,304 research outputs found
The Perils of Clumpfind: The Mass Spectrum of Sub-structures in Molecular Clouds
We study the mass spectrum of sub-structures in the Perseus Molecular Cloud
Complex traced by 13CO (1-0), finding that for the
standard Clumpfind parameters. This result does not agree with the classical
. To understand this discrepancy we study the robustness
of the mass spectrum derived using the Clumpfind algorithm. Both 2D and 3D
Clumpfind versions are tested, using 850 m dust emission and 13CO
spectral-line observations of Perseus, respectively. The effect of varying
threshold is not important, but varying stepsize produces a different effect
for 2D and 3D cases. In the 2D case, where emission is relatively isolated
(associated with only the densest peaks in the cloud), the mass spectrum
variability is negligible compared to the mass function fit uncertainties. In
the 3D case, however, where the 13CO emission traces the bulk of the molecular
cloud, the number of clumps and the derived mass spectrum are highly correlated
with the stepsize used. The distinction between "2D" and "3D" here is more
importantly also a distinction between "sparse" and "crowded" emission. In any
"crowded" case, Clumpfind should not be used blindly to derive mass functions.
Clumpfind's output in the "crowded" case can still offer a statistical
description of emission useful in inter-comparisons, but the clump-list should
not be treated as a robust region decomposition suitable to generate a
physically-meaningful mass function. We conclude that the 13CO mass spectrum
depends on the observations resolution, due to the hierarchical structure of
MC.Comment: 5 pages, 3 figures. Accepted for publication in ApJ Letter
The Dynamics of Dense Cores in the Perseus Molecular Cloud II: The Relationship Between Dense Cores and the Cloud
We utilize the extensive datasets available for the Perseus molecular cloud
to analyze the relationship between the kinematics of small-scale dense cores
and the larger structures in which they are embedded. The kinematic measures
presented here can be used in conjunction with those discussed in our previous
work as strong observational constraints that numerical simulations (or
analytic models) of star formation should match. We find that dense cores have
small motions with respect to the 13CO gas, about one third of the 13CO
velocity dispersion along the same line of sight. Within each extinction
region, the core-to-core velocity dispersion is about half of the total (13CO)
velocity dispersion seen in the region. Large-scale velocity gradients account
for roughly half of the total velocity dispersion in each region, similar to
what is predicted from large-scale turbulent modes following a power spectrum
of P(k) ~ k^{-4}.Comment: Accepted for publication in ApJ. 47 pages (preprint format), 20
figures, 5 table
A^2-Net: Molecular Structure Estimation from Cryo-EM Density Volumes
Constructing of molecular structural models from Cryo-Electron Microscopy
(Cryo-EM) density volumes is the critical last step of structure determination
by Cryo-EM technologies. Methods have evolved from manual construction by
structural biologists to perform 6D translation-rotation searching, which is
extremely compute-intensive. In this paper, we propose a learning-based method
and formulate this problem as a vision-inspired 3D detection and pose
estimation task. We develop a deep learning framework for amino acid
determination in a 3D Cryo-EM density volume. We also design a sequence-guided
Monte Carlo Tree Search (MCTS) to thread over the candidate amino acids to form
the molecular structure. This framework achieves 91% coverage on our newly
proposed dataset and takes only a few minutes for a typical structure with a
thousand amino acids. Our method is hundreds of times faster and several times
more accurate than existing automated solutions without any human intervention.Comment: 8 pages, 5 figures, 4 table
The GALFA-HI Compact Cloud Catalog
We present a catalog of 1964 isolated, compact neutral hydrogen clouds from
the Galactic Arecibo L-Band Feed Array Survey Data Release One (GALFA-HI DR1).
The clouds were identified by a custom machine-vision algorithm utilizing
Difference of Gaussian kernels to search for clouds smaller than 20'. The
clouds have velocities typically between |VLSR| = 20-400 km/s, linewidths of
2.5-35 km/s, and column densities ranging from 1 - 35 x 10^18 cm^-2. The
distances to the clouds in this catalog may cover several orders of magnitude,
so the masses may range from less than a Solar mass for clouds within the
Galactic disc, to greater than 10^4 Solar Masses for HVCs at the tip of the
Magellanic Stream. To search for trends, we separate the catalog into five
populations based on position, velocity, and linewidth: high velocity clouds
(HVCs); galaxy candidates; cold low velocity clouds (LVCs); warm, low
positive-velocity clouds in the third Galactic Quadrant; and the remaining warm
LVCs. The observed HVCs are found to be associated with previously-identified
HVC complexes. We do not observe a large population of isolated clouds at high
velocities as some models predict. We see evidence for distinct histories at
low velocities in detecting populations of clouds corotating with the Galactic
disc and a set of clouds that is not corotating.Comment: 34 Pages, 9 Figures, published in ApJ (2012, ApJ, 758, 44), this
version has the corrected fluxes and corresponding flux histogram and masse
Using airborne LiDAR Survey to explore historic-era archaeological landscapes of Montserrat in the eastern Caribbean
This article describes what appears to be the first archaeological application of airborne LiDAR survey to historic-era landscapes in the Caribbean archipelago, on the island of Montserrat. LiDAR is proving invaluable in extending the reach of traditional pedestrian survey into less favorable areas, such as those covered by dense neotropical forest and by ashfall from the past two decades of active eruptions by the Soufrière Hills volcano, and to sites in localities that are inaccessible on account of volcanic dangers. Emphasis is placed on two aspects of the research: first, the importance of ongoing, real-time interaction between the LiDAR analyst and the archaeological team in the field; and second, the advantages of exploiting the full potential of the three-dimensional LiDAR point cloud data for purposes of the visualization of archaeological sites and features
From Multiview Image Curves to 3D Drawings
Reconstructing 3D scenes from multiple views has made impressive strides in
recent years, chiefly by correlating isolated feature points, intensity
patterns, or curvilinear structures. In the general setting - without
controlled acquisition, abundant texture, curves and surfaces following
specific models or limiting scene complexity - most methods produce unorganized
point clouds, meshes, or voxel representations, with some exceptions producing
unorganized clouds of 3D curve fragments. Ideally, many applications require
structured representations of curves, surfaces and their spatial relationships.
This paper presents a step in this direction by formulating an approach that
combines 2D image curves into a collection of 3D curves, with topological
connectivity between them represented as a 3D graph. This results in a 3D
drawing, which is complementary to surface representations in the same sense as
a 3D scaffold complements a tent taut over it. We evaluate our results against
truth on synthetic and real datasets.Comment: Expanded ECCV 2016 version with tweaked figures and including an
overview of the supplementary material available at
multiview-3d-drawing.sourceforge.ne
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