7,505 research outputs found
Study of the technique of stellar occultation
The results are reported of a study of the stellar occultation technique for measuring the composition of the atmosphere. The intensity of starlight was monitored during the occultation using the Wisconsin stellar ultraviolet photometers aboard the Orbiting Astronomical Observatory (OAO-A2). A schematic diagram of an occultation is shown where the change in intensity at a given wavelength is illustrated. The vertical projection of the attenuation region is typically 60 km deep for molecular oxygen and 30 km deep for ozone. Intensity profiles obtained during various occultations were analyzed by first determining the tangential columm density of the absorbing gases, and then Abel inverting the column densities to obtain the number density profile. Errors are associated with each step in the inversion scheme and have been considered as an integral part of this study
Inferring Species Trees Directly from Biallelic Genetic Markers: Bypassing Gene Trees in a Full Coalescent Analysis
The multi-species coalescent provides an elegant theoretical framework for
estimating species trees and species demographics from genetic markers.
Practical applications of the multi-species coalescent model are, however,
limited by the need to integrate or sample over all gene trees possible for
each genetic marker. Here we describe a polynomial-time algorithm that computes
the likelihood of a species tree directly from the markers under a finite-sites
model of mutation, effectively integrating over all possible gene trees. The
method applies to independent (unlinked) biallelic markers such as well-spaced
single nucleotide polymorphisms (SNPs), and we have implemented it in SNAPP, a
Markov chain Monte-Carlo sampler for inferring species trees, divergence dates,
and population sizes. We report results from simulation experiments and from an
analysis of 1997 amplified fragment length polymorphism (AFLP) loci in 69
individuals sampled from six species of {\em Ourisia} (New Zealand native
foxglove)
Dissecting the Red Sequence--II. Star Formation Histories of Early-Type Galaxies Throughout the Fundamental Plane
This analysis uses spectra of ~16,000 nearby SDSS quiescent galaxies to track
variations in galaxy star formation histories along and perpendicular to the
Fundamental Plane (FP). We sort galaxies by their FP properties (sigma, R_e,
and I_e) and construct high S/N mean galaxy spectra that span the breadth and
thickness of the FP. From these spectra, we determine mean luminosity-weighted
ages, [Fe/H], [Mg/H], and [Mg/Fe] based on single stellar population models
using the method described in Graves & Schiavon (2008). In agreement with
previous work, the star formation histories of early-type galaxies are found to
form a two-parameter family. The major trend is that mean age, [Fe/H], [Mg/H],
and [Mg/Fe] all increase with sigma. However, no stellar population property
shows any dependence on R_e at fixed sigma, suggesting that sigma and not
dynamical mass (M_dyn ~ sigma^2 R_e) is the better predictor of past star
formation history. In addition to the main trend with sigma, galaxies also show
a range of population properties at fixed sigma that are strongly correlated
with surface brightness residuals from the FP, such that higher surface
brightness galaxies have younger mean ages, higher [Fe/H], higher [Mg/H], and
lower [Mg/Fe] than lower-surface brightness galaxies. These latter trends are a
major new constraint on star-formation histories.Comment: 23 pages, 14 figures. Accepted to Ap
Sub-picosecond compression by velocity bunching in a photo-injector
We present an experimental evidence of a bunch compression scheme that uses a
traveling wave accelerating structure as a compressor. The bunch length issued
from a laser-driven radio-frequency electron source was compressed by a factor
>3 using an S-band traveling wave structure located immediately downstream from
the electron source. Experimental data are found to be in good agreement with
particle tracking simulations.Comment: 19 pages, 9 figures, submitted to Phys. Rev. Spec. Topics A&
Dissecting the Red Sequence. IV. The Role of Truncation in the Two-Dimensional Family of Early-Type Galaxy Star Formation Histories
In the three-dimensional parameter space defined by velocity dispersion,
effective radius (R_e), and effective surface brightness (I_e), early-type
galaxies are observed to populate a two-dimensional fundamental plane (FP) with
finite thickness. In Paper III of this series, we showed that the thickness of
the FP is predominantly due to variations in the stellar mass surface density
(Sigma_*) inside the effective radius R_e. These variations represent
differences in the dark matter fraction inside R_e (or possibly differences in
the initial mass function) from galaxy to galaxy. This means that galaxies do
not wind up below the FP at lower surface brightness due to the passive fading
of their stellar populations; they are structurally different. Here, we show
that these variations in Sigma_* at fixed dynamical mass (M_dyn) are linked to
differences in the galaxy stellar populations, and therefore to differences in
their star formation histories. We demonstrate that the ensemble of stellar
population and Sigma_* variations through the FP thickness can be explained by
a model in which early-type galaxies at fixed M_dyn have their star formation
truncated at different times. The thickness of the FP can therefore be
interpreted as a sequence of truncation times. Galaxies below the FP have
earlier truncation times for a given M_dyn, resulting in lower Sigma_*, older
ages, lower metallicities in both [Fe/H] and [Mg/H], and higher [Mg/Fe]. We
show that this model is quantitatively consistent with simple expectations for
chemical enrichment in galaxies. We also present fitting functions for
luminosity-weighted age, [Fe/H], [Mg/H], and [Mg/Fe] as functions of the FP
parameters velocity dispersion, R_e, and I_e. These provide a new tool for
estimating the stellar population properties of quiescent early-type galaxies
for which high-quality spectra are not available.Comment: 21 pages, 9 figures. Accepted to Ap
Millisecond accuracy video display using OpenGL under Linux
To measure people’s reaction times to the nearest millisecond, it is necessary to know exactly when
a stimulus is displayed. This article describes how to display stimuli with millisecond accuracy on a
normal CRT monitor, using a PC running Linux. A simple C program is presented to illustrate how this
may be done within X Windows using the OpenGL rendering system. A test of this system is reported
that demonstrates that stimuli may be consistently displayed with millisecond accuracy. An algorithm
is presented that allows the exact time of stimulus presentation to be deduced, even if there are relatively
large errors in measuring the display time
Synthesis And Characterization Of (pyNO−)2GaCl: A Redox-Active Gallium Complex
We report the synthesis of a gallium complex incorporating redox-active pyridyl nitroxide ligands. The (pyNO−)2GaCl complex was prepared in 85% yield via a salt metathesis route and was characterized by 1H and 13C NMR spectroscopies, X-ray diffraction, and theory. UV–Vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties of the complex, respectively. Our discussion focuses primarily on a comparison of the gallium complex to the corresponding aluminum derivative and shows that although the complexes are very similar, small differences in the electronic structure of the complexes can be correlated to the identity of the metal
Ages and Abundances of Red Sequence Galaxies as a Function of LINER Emission Line Strength
Although the spectrum of a prototypical early-type galaxy is assumed to lack
emission lines, a substantial fraction (likely as high as 30%) of nearby red
sequence galaxy spectra contain emission lines with line ratios characteristic
of low ionization nuclear emission-line regions (LINERs). We use spectra of
~6000 galaxies from the Sloan Digital Sky Survey (SDSS) in a narrow redshift
slice (0.06 < z < 0.08) to compare the stellar populations of red sequence
galaxies with and without LINER-like emission. The spectra are binned by
internal velocity dispersion and by emission properties to produce high S/N
stacked spectra. The recent stellar population models of R. Schiavon (2007)
make it possible to measure ages, [Fe/H], and individual elemental abundance
ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We
find that red sequence galaxies with strong LINER-like emission are
systematically 2-3.5 Gyr (10-40%) younger than their emission-free counterparts
at the same velocity dispersion. This suggests a connection between the
mechanism powering the emission (whether AGN, post-AGB stars, shocks, or
cooling flows) and more recent star formation in the galaxy. We find that mean
stellar age and [Fe/H] increase with velocity dispersion for all galaxies.
Elemental abundance [Mg/Fe] increases modestly with velocity dispersion in
agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly
with velocity dispersion than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar
for all galaxies. At fixed velocity dispersion, galaxies with fainter r-band
luminosities have lower [Fe/H] and older ages but similar abundance ratios
compared to brighter galaxies.Comment: 25 pages, 17 figures, Accepted for publication in ApJ as of 16 July
2007; acceptance status updated, paper unchange
An Initial Assessment of the Potential of Genomic Analysis to Help Inform Bighorn Sheep Management
Genetic research may be a useful approach for understanding factors that could impact productivity and restoration of bighorn sheep (Ovis canadensis) herds. For example, genetic consequences of inbreeding in small populations can impact recruitment and local adaptations can influence translocation success. This modest pilot study quantified genetic attributes of bighorn sheep populations with a range of different herd histories in Montana and Wyoming to investigate genetic similarity and differences, genetic heterogeneity and genetic distance. Employing an Ovine array containing about 700,000 single nucleotide polymorphisms (SNPs) with approximately 24,000 markers that are informative for Rocky Mountain bighorn sheep, we used whole genome genotyping to analyze genetic material. This technique represents a significant advancement in genetic analysis of bighorn sheep, as most previous studies have used microsatellites and less than 200 genetic markers. We analyzed approximately fifteen individuals from each of four different populations that we predicted would differ in genetic characteristics, due to population dissimilarities that potentially impacted their genetics, including origin (native/reintroduced), population size, bottleneck history, degree of connectivity, and augmentation history. We selected four populations that provided a spectrum of these herd attributes, including the Tendoys, Stillwater and Glacier National Park in Montana and the northeastern Greater Yellowstone Area in Wyoming. We present the results of this effort and examine expected and observed heterogeneity and genetic distance estimates to evaluate the potential for links between genetics and herd demography. We discuss the utility of genetic analyses in improving knowledge of bighorn sheep populations and potential implications for bighorn sheep management
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