27,239 research outputs found
Self-Consistent Analysis of OH-Zeeman Observations: Too Much Noise about Noise
We had recently re-analyzed in a self-consistent way OH-Zeeman observations
in four molecular-cloud envelopes and we had shown that, contrary to claims by
Crutcher et al., there is no evidence that the mass-to-flux ratio decreases
from the envelopes to the cores of these clouds. The key difference between our
data analysis and the earlier one by Crutcher et al. is the relaxation of the
overly restrictive assumption made by Crutcher et al, that the magnetic field
strength is independent of position in each of the four envelopes. In a more
recent paper, Crutcher et al. (1) claim that our analysis is not
self-consistent, in that it misses a cosine factor, and (2) present new
arguments to support their contention that the magnetic-field strength is
indeed independent of position in each of the four envelopes. We show that the
claim of the missing cosine factor is false, that the new arguments contain
even more serious problems than the Crutcher et al. original data analysis, and
we present new observational evidence, independent of the OH-Zeeman data, that
suggests significant variations in the magnetic-field strength in the four
cloud envelopes.Comment: 8 pages, 3 figures, MNRAS in pres
Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping
We present a study of dense molecular gas kinematics in seventeen nearby
protostellar systems using single-dish and interferometric molecular line
observations. The non-axisymmetric envelopes around a sample of Class 0/I
protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and
PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to
construct line-center velocity and linewidth maps for all sources to examine
the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000
AU. The direction of the large-scale velocity gradients from single-dish
mapping is within 45 degrees of normal to the outflow axis in more than half
the sample. Furthermore, the velocity gradients are often quite substantial,
the average being ~2.3 km\s\pc. The interferometric data often reveal
small-scale velocity structure, departing from the more gradual large-scale
velocity gradients. In some cases, this likely indicates accelerating infall
and/or rotational spin-up in the inner envelope; the median velocity gradient
from the interferometric data is ~10.7 km/s/pc. In two systems, we detect
high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\
emission. This enables us to study the infall and rotation close to the disk
and estimate the central object masses. The velocity fields observed on large
and small-scales are more complex than would be expected from rotation alone,
suggesting that complex envelope structure enables other dynamical processes
(i.e. infall) to affect the velocity field.Comment: 85 Pages, 31 Figures, 11 Tables, Accepted to ApJ
The spectral analysis of nonstationary categorical time series using local spectral envelope
Most classical methods for the spectral analysis are based on the assumption that the time
series is stationary. However, many time series in practical problems shows nonstationary
behaviors. The data from some fields are huge and have variance and spectrum which changes
over time. Sometimes,we are interested in the cyclic behavior of the categorical-valued time
series such as EEG sleep state data or DNA sequence, the general method is to scale the
data, that is, assign numerical values to the categories and then use the periodogram to find
the cyclic behavior. But there exists numerous possible scaling. If we arbitrarily assign the
numerical values to the categories and proceed with a spectral analysis, then the results will
depend on the particular assignment. We would like to find the all possible scaling that
bring out all of the interesting features in the data. To overcome these problems, there have
been many approaches in the spectral analysis.
Our goal is to develop a statistical methodology for analyzing nonstationary categorical
time series in the frequency domain. In this dissertation, the spectral envelope methodology
is introduced for spectral analysis of categorical time series. This provides the general
framework for the spectral analysis of the categorical time series and summarizes information
from the spectrum matrix. To apply this method to nonstationary process, I used the
TBAS(Tree-Based Adaptive Segmentation) and local spectral envelope based on the piecewise
stationary process. In this dissertation,the TBAS(Tree-Based Adpative Segmentation)
using distance function based on the Kullback-Leibler divergence was proposed to find the
best segmentation
Adjustment of interaural-time-difference analysis to sound level
To localize low-frequency sound sources in azimuth, the binaural system compares the timing of sound waves at the two ears with microsecond precision. A similarly high precision is also seen in the binaural processing of the envelopes of high-frequency complex sounds. Both for low- and high-frequency sounds, interaural time difference (ITD) acuity is to a large extent independent of sound level. The mechanisms underlying this level-invariant extraction of ITDs by the binaural system are, however, only poorly understood. We use high-frequency pip trains with asymmetric and dichotic pip envelopes in a combined psychophysical, electrophysiological, and modeling approach. Although the dichotic envelopes cannot be physically matched in terms of ITD, the match produced perceptually by humans is very reliable, and it depends systematically on the overall sound level. These data are reflected in neural responses from the gerbil lateral superior olive and lateral lemniscus. The results are predicted in an existing temporal-integration model extended with a level-dependent threshold criterion. These data provide a very sensitive quantification of how the peripheral temporal code is conditioned for binaural analysis
Asymptotic power of sphericity tests for high-dimensional data
This paper studies the asymptotic power of tests of sphericity against
perturbations in a single unknown direction as both the dimensionality of the
data and the number of observations go to infinity. We establish the
convergence, under the null hypothesis and contiguous alternatives, of the log
ratio of the joint densities of the sample covariance eigenvalues to a Gaussian
process indexed by the norm of the perturbation. When the perturbation norm is
larger than the phase transition threshold studied in Baik, Ben Arous and Peche
[Ann. Probab. 33 (2005) 1643-1697] the limiting process is degenerate, and
discrimination between the null and the alternative is asymptotically certain.
When the norm is below the threshold, the limiting process is nondegenerate,
and the joint eigenvalue densities under the null and alternative hypotheses
are mutually contiguous. Using the asymptotic theory of statistical
experiments, we obtain asymptotic power envelopes and derive the asymptotic
power for various sphericity tests in the contiguity region. In particular, we
show that the asymptotic power of the Tracy-Widom-type tests is trivial (i.e.,
equals the asymptotic size), whereas that of the eigenvalue-based likelihood
ratio test is strictly larger than the size, and close to the power envelope.Comment: Published in at http://dx.doi.org/10.1214/13-AOS1100 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Beyond bathymetry: Mapping acoustic backscattering from the deep seafloor with Sea Beam
In its standard mode of operation, the multibeam echosounder Sea Beam produces high resolution bathymetric contour charts of the seafloor surveyed. However, additional information about the nature of the seafloor can be extracted from the structure of the echo signals received by the system. Such signals have been recorded digitally over a variety of seafloor environments for which independent observations from bottom photographs or sidescan sonars were available. An attempt is made to relate the statistical properties of the bottom‐backscattered sound field to the independently observed geologicalcharacteristics of the seafloor surveyed. Acoustic boundary mapping over flat areas is achieved by following trend changes in the acoustic data both along and across track. Such changes in the acoustics are found to correlate with changes in bottom type or roughness structure. The overall energy level of a partial angular‐dependence function of backscattering appears to depend strongly on bottom type, whereas the shape of the function does not. Clues to the roughness structure of the bottom are obtained by relating the shape of the probability density function of normal‐incidence echo envelopes to the degree of coherence in the backscattered acoustic field
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