274,218 research outputs found
Fourier Analysis of Gapped Time Series: Improved Estimates of Solar and Stellar Oscillation Parameters
Quantitative helio- and asteroseismology require very precise measurements of
the frequencies, amplitudes, and lifetimes of the global modes of stellar
oscillation. It is common knowledge that the precision of these measurements
depends on the total length (T), quality, and completeness of the observations.
Except in a few simple cases, the effect of gaps in the data on measurement
precision is poorly understood, in particular in Fourier space where the
convolution of the observable with the observation window introduces
correlations between different frequencies. Here we describe and implement a
rather general method to retrieve maximum likelihood estimates of the
oscillation parameters, taking into account the proper statistics of the
observations. Our fitting method applies in complex Fourier space and exploits
the phase information. We consider both solar-like stochastic oscillations and
long-lived harmonic oscillations, plus random noise. Using numerical
simulations, we demonstrate the existence of cases for which our improved
fitting method is less biased and has a greater precision than when the
frequency correlations are ignored. This is especially true of low
signal-to-noise solar-like oscillations. For example, we discuss a case where
the precision on the mode frequency estimate is increased by a factor of five,
for a duty cycle of 15%. In the case of long-lived sinusoidal oscillations, a
proper treatment of the frequency correlations does not provide any significant
improvement; nevertheless we confirm that the mode frequency can be measured
from gapped data at a much better precision than the 1/T Rayleigh resolution.Comment: Accepted for publication in Solar Physics Topical Issue
"Helioseismology, Asteroseismology, and MHD Connections
Spectral Energy Distribution Fitting: Application to Lyman Alpha-Emitting Galaxies
Spectral Energy Distribution (SED) fitting is a well-developed astrophysical
tool that has recently been applied to high-redshift Lyman Alpha-emitting
galaxies. If rest-frame ultraviolet through near-infrared photometry is
available, it allows the simultaneous determination of the star formation
history and dust extinction of a galaxy. Lyman Alpha-emitter SED fitting
results from the literature find star formation rates ~3 M_sun/yr, stellar
masses ~10^9 M_sun for the general population but ~10^10 M_sun for the subset
detected by IRAC, and very low dust extinction, A_V < 0.3, although a couple of
outlying analyses prefer significantly more dust and higher intrinsic star
formation rates. A checklist of 14 critical choices that must be made when
performing SED fitting is discussed.Comment: A review and discussion from the "Understanding Lyman-alpha Emitters"
meeting in Heidelberg, Oct. 2008, 10 pages, to be published in New Astronomy
Reviews. Full conference summary available as arXiv:0904.3335. Conference
home-page, with presentations, is
http://www.mpia.de/Public/Aktuelles/Tagungen/lae08/lae08.htm
A New Method for Protecting Interrelated Time Series with Bayesian Prior Distributions and Synthetic Data
Organizations disseminate statistical summaries of administrative data via the Web for unrestricted public use. They balance the trade-off between confidentiality protection and inference quality. Recent developments in disclosure avoidance techniques include the incorporation of synthetic data, which capture the essential features of underlying data by releasing altered data generated from a posterior predictive distribution. The United States Census Bureau collects millions of interrelated time series micro-data that are hierarchical and contain many zeros and suppressions. Rule-based disclosure avoidance techniques often require the suppression of count data for small magnitudes and the modification of data based on a small number of entities. Motivated by this problem, we use zero-inflated extensions of Bayesian Generalized Linear Mixed Models (BGLMM) with privacy-preserving prior distributions to develop methods for protecting and releasing synthetic data from time series about thousands of small groups of entities without suppression based on the of magnitudes or number of entities. We find that as the prior distributions of the variance components in the BGLMM become more precise toward zero, confidentiality protection increases and inference quality deteriorates. We evaluate our methodology using a strict privacy measure, empirical differential privacy, and a newly defined risk measure, Probability of Range Identification (PoRI), which directly measures attribute disclosure risk. We illustrate our results with the U.S. Census Bureau’s Quarterly Workforce Indicators
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Questionnaire study to gain an insight into the manufacturing and fitting process of artificial eyes in children: an ocularist perspective
Purpose
To gain an insight into the manufacturing and fitting of artificial eyes in children and potential improvements to the process.
Method
An online qualitative survey was distributed to 39 ocularists/prosthetists in Europe and Canada. Participants were recruited through purposive sampling, specifically maximum variation sampling from the researcher’s contacts and an online search.
Results
The findings highlighted the current impression technique as being the most difficult yet most important part of the current process for both the ocularist and child patient. Negatively affecting obtaining a good impression, the child patients distress can be reduced by their parents by providing encouragement, reassurance, practicing the insertion and removal of the artificial eye and being matter of fact. Whilst improvements to the current process provided mixed views, the incorporation of current technology was perceived as not being able to meet the requirements to produce aesthetically pleasing artificial eyes.
Conclusion
The current artificial eye process can be seen as an interaction with its success being dependent on the child patient’s acceptance and adjustment which is dependent on the factors associated to the process. Investigation into the needs of the patient and whether technology can improve the process are the next steps in its advancement
Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC
We present a detailed study of the viewing angles of the LMC disk plane. We
find that our viewing direction differs considerably from the commonly accepted
values, which has important implications for the structure of the LMC. The
discussion is based on an analysis of spatial variations in the apparent
magnitude of features in the near-IR color-magnitude diagrams extracted from
the DENIS and 2MASS surveys. Sinusoidal brightness variations with a
peak-to-peak amplitude of approximately 0.25 mag are detected as function of
position angle, for both AGB and RGB stars. This is naturally interpreted as
the result of distance variations, due to one side of the LMC plane being
closer to us than the opposite side. The best fitting geometric model of an
inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and
line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative
evidence that the LMC disk plane may be warped. Traditional methods to estimate
the position angle of the line of nodes have used either the major axis
position angle Theta_maj of the spatial distribution of tracers on the sky, or
the position angle Theta_max of the line of maximum gradient in the velocity
field, given that for a circular disk Theta_maj = Theta_max = Theta. The
present study does not rely on the assumption of circular symmetry, and is
considerably more accurate than previous studies of its kind. We find that the
actual position angle of the line of nodes differs considerably from both
Theta_maj and Theta_max, for which measurements have fallen in the range
140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not
circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm
On Physical Scales of Dark Matter Halos
It is common practice to describe formal size and mass scales of dark matter
halos as spherical overdensities with respect to an evolving density threshold.
Here, we critically investigate the evolutionary effects of several such
commonly used definitions and compare them to the halo evolution within fixed
physical scales as well as to the evolution of other intrinsic physical
properties of dark matter halos. It is shown that, in general, the traditional
way of characterizing sizes and masses of halos dramatically overpredicts the
degree of evolution in the last 10 Gyr, especially for low-mass halos. This
pseudo-evolution leads to the illusion of growth even though there are no major
changes within fixed physical scales. Such formal size definitions also serve
as proxies for the virialized region of a halo in the literature. In general,
those spherical overdensity scales do not coincide with the virialized region.
A physically more precise nomenclature would be to simply characterize them by
their very definition instead of calling such formal size and mass definitions
'virial'. In general, we find a discrepancy between the evolution of the
underlying physical structure of dark matter halos seen in cosmological
structure formation simulations and pseudo-evolving formal virial quantities.
We question the importance of the role of formal virial quantities currently
ubiquitously used in descriptions, models and relations that involve properties
of dark matter structures. Concepts and relations based on pseudo-evolving
formal virial quantities do not properly reflect the actual evolution of dark
matter halos and lead to an inaccurate picture of the physical evolution of our
universe.Comment: 17 pages, 14 figures, 1 table, ApJ accepte
The Megamaser Cosmology Project: IV. A Direct Measurement of the Hubble Constant from UGC 3789
In Papers I and II from the Megamaser Cosmology Project (MCP), we reported
initial observations of water masers in an accretion disk of a supermassive
black hole at the center of the galaxy UGC 3789, which gave an angular-diameter
distance to the galaxy and an estimate of Ho with 16% uncertainty. We have
since conducted more VLBI observations of the spatial-velocity structure of
these water masers, as well as continued monitoring of its spectrum to better
measure maser accelerations. These more extensive observations, combined with
improved modeling of the masers in the accretion disk of the central
supermassive black hole, confirm our previous results, but with signifcantly
improved accuracy. We find Ho = 68.9 +/- 7.1 km/s/Mpc; this estimate of Ho is
independent of other methods and is accurate to +/-10%, including sources of
systematic error. This places UGC 3789 at a distance of 49.6 +/- 5.1 Mpc, with
a central supermassive black hole of (1.16 +/- 0.12) x 10^7 Msun.Comment: to appear in Ap
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