141 research outputs found
Studies of Diffuse Interstellar Bands. V. Pairwise Correlations of Eight Strong DIBs and Neutral Hydrogen, Molecular Hydrogen, and Color Excess
We establish correlations between equivalent widths of eight diffuse
interstellar bands (DIBs), and examine their correlations with atomic hydrogen,
molecular hydrogen, and EB-V . The DIBs are centered at \lambda\lambda 5780.5,
6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order
of Pearson\^as correlation coefficient with N(H) (here defined as the column
density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent
width of \lambda 5780.5 is better correlated with column densities of H than
with E(B-V) or H2, confirming earlier results based on smaller datasets. We
show the same is true for six of the seven other DIBs presented here. Despite
this similarity, the eight strong DIBs chosen are not well enough correlated
with each other to suggest they come from the same carrier. We further conclude
that these eight DIBs are more likely to be associated with H than with H2, and
hence are not preferentially located in the densest, most UV shielded parts of
interstellar clouds. We suggest they arise from different molecules found in
diffuse H regions with very little H (molecular fraction f<0.01). Of the 133
stars with available data in our study, there are three with significantly
weaker \lambda 5780.5 than our mean H-5780.5 relationship, all of which are in
regions of high radiation fields, as previously noted by Herbig. The
correlations will be useful in deriving interstellar parameters when direct
methods are not available. For instance, with care, the value of N(H) can be
derived from W{\lambda}(5780.5).Comment: Accepted for publication in The Astrophysical Journal; 37 pages, 11
figures, 6 table
The GAMCIT gamma ray burst detector
The GAMCIT payload is a Get-Away-Special payload designed to search for high-energy gamma-ray bursts and any associated optical transients. This paper presents details on the design of the GAMCIT payload, in the areas of battery selection, power processing, electronics design, gamma-ray detection systems, and the optical imaging of the transients. The paper discusses the progress of the construction, testing, and specific design details of the payload. In addition, this paper discusses the unique challenges involved in bringing this payload to completion, as the project has been designed, constructed, and managed entirely by undergraduate students. Our experience will certainly be valuable to other student groups interested in taking on a challenging project such as a Get-Away-Special payload
Observational Constraints on Red and Blue Helium Burning Sequences
We derive the optical luminosity, colors, and ratios of the blue and red
helium burning (HeB) stellar populations from archival Hubble Space Telescope
observations of nineteen starburst dwarf galaxies and compare them with
theoretical isochrones from Padova stellar evolution models across
metallicities from Z=0.001 to 0.009. We find that the observational data and
the theoretical isochrones for both blue and red HeB populations overlap in
optical luminosities and colors and the observed and predicted blue to red HeB
ratios agree for stars older than 50 Myr over the time bins studied. These
findings confirm the usefulness of applying isochrones to interpret
observations of HeB populations. However, there are significant differences,
especially for the red HeB population. Specifically we find: (1) offsets in
color between the observations and theoretical isochrones of order 0.15 mag
(0.5 mag) for the blue (red) HeB populations brighter than M_V ~ -4 mag, which
cannot be solely due to differential extinction; (2) blue HeB stars fainter
than M_V ~ -3 mag are bluer than predicted; (3) the slope of the red HeB
sequence is shallower than predicted by a factor of ~3; and (4) the models
overpredict the ratio of the most luminous blue to red HeB stars corresponding
to ages <50 Myr. Additionally, we find that for the more metal-rich galaxies in
our sample (Z> 0.5 Zsolar) the red HeB stars overlap with the red giant branch
stars in the color magnitude diagrams, thus reducing their usefulness as
indicators of star formation for ages >100 Myr.Comment: 18 pages, 11 figures, 3 table
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Error, reproducibility and sensitivity : a pipeline for data processing of Agilent oligonucleotide expression arrays
Background
Expression microarrays are increasingly used to obtain large scale transcriptomic information on a wide range of biological samples. Nevertheless, there is still much debate on the best ways to process data, to design experiments and analyse the output. Furthermore, many of the more sophisticated mathematical approaches to data analysis in the literature remain inaccessible to much of the biological research community. In this study we examine ways of extracting and analysing a large data set obtained using the Agilent long oligonucleotide transcriptomics platform, applied to a set of human macrophage and dendritic cell samples.
Results
We describe and validate a series of data extraction, transformation and normalisation steps which are implemented via a new R function. Analysis of replicate normalised reference data demonstrate that intrarray variability is small (only around 2% of the mean log signal), while interarray variability from replicate array measurements has a standard deviation (SD) of around 0.5 log2 units ( 6% of mean). The common practise of working with ratios of Cy5/Cy3 signal offers little further improvement in terms of reducing error. Comparison to expression data obtained using Arabidopsis samples demonstrates that the large number of genes in each sample showing a low level of transcription reflect the real complexity of the cellular transcriptome. Multidimensional scaling is used to show that the processed data identifies an underlying structure which reflect some of the key biological variables which define the data set. This structure is robust, allowing reliable comparison of samples collected over a number of years and collected by a variety of operators.
Conclusions
This study outlines a robust and easily implemented pipeline for extracting, transforming normalising and visualising transcriptomic array data from Agilent expression platform. The analysis is used to obtain quantitative estimates of the SD arising from experimental (non biological) intra- and interarray variability, and for a lower threshold for determining whether an individual gene is expressed. The study provides a reliable basis for further more extensive studies of the systems biology of eukaryotic cells
Unusually Weak Diffuse Interstellar Bands toward HD 62542
As part of an extensive survey of diffuse interstellar bands (DIBs), we have
obtained optical spectra of the moderately reddened B5V star HD 62542, which is
known to have an unusual UV extinction curve of the type usually identified
with dark clouds. The typically strongest of the commonly catalogued DIBs
covered by the spectra -- those at 5780, 5797, 6270, 6284, and 6614 A -- are
essentially absent in this line of sight, in marked contrast with other lines
of sight of similar reddening. We compare the HD 62542 line of sight with
others exhibiting a range of extinction properties and molecular abundances and
interpret the weakness of the DIBs as an extreme case of deficient DIB
formation in a dense cloud whose more diffuse outer layers have been stripped
away. We comment on the challenges these observations pose for identifying the
carriers of the diffuse bands.Comment: 20 pages, 4 figures; aastex; accepted by Ap
Investigating the Cosmic-Ray Ionization Rate Near the Supernova Remnant IC 443 Through H3+ Observations
Observational and theoretical evidence suggests that high-energy Galactic
cosmic rays are primarily accelerated by supernova remnants. If also true for
low-energy cosmic rays, the ionization rate near a supernova remnant should be
higher than in the general Galactic interstellar medium (ISM). We have searched
for H3+ absorption features in 6 sight lines which pass through molecular
material near IC 443---a well-studied case of a supernova remnant interacting
with its surrounding molecular material---for the purpose of inferring the
cosmic-ray ionization rate in the region. In 2 of the sight lines (toward ALS
8828 and HD 254577) we find large H3+ column densities, N(H3+)~3*10^14 cm^-2,
and deduce ionization rates of zeta_2~2*10^-15 s^-1, about 5 times larger than
inferred toward average diffuse molecular cloud sight lines. However, the 3
sigma upper limits found for the other 4 sight lines are consistent with
typical Galactic values. This wide range of ionization rates is likely the
result of particle acceleration and propagation effects, which predict that the
cosmic-ray spectrum and thus ionization rate should vary in and around the
remnant. While we cannot determine if the H3+ absorption arises in post-shock
(interior) or pre-shock (exterior) gas, the large inferred ionization rates
suggest that IC 443 is in fact accelerating a large population of low-energy
cosmic rays. Still, it is unclear whether this population can propagate far
enough into the ISM to account for the ionization rate inferred in diffuse
Galactic sight lines.Comment: 14 pages, 3 figures, 4 table
The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds
Diffuse interstellar clouds show large abundances of H_3^+ which can be
maintained only by a high ionization rate of H_2. Cosmic rays are the dominant
ionization mechanism in this environment, so the large ionization rate implies
a high cosmic-ray flux, and a large amount of energy residing in cosmic rays.
In this paper we find that the standard propagated cosmic-ray spectrum predicts
an ionization rate much lower than that inferred from H_3^+. Low-energy (~10
MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be
directly detected; consequently, an otherwise unobservable enhancement of the
low-energy cosmic-ray flux offers a plausible explanation for the H_3^+
results. Beyond ionization, cosmic rays also interact with the interstellar
medium by spalling atomic nuclei and exciting atomic nuclear states. These
processes produce the light elements Li, Be, and B, as well as gamma-ray lines.
To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt
two physically-motivated cosmic-ray spectra which by construction reproduce the
ionization rate inferred in diffuse clouds, and investigate the implications of
these spectra on dense cloud ionization rates, light element abundances,
gamma-ray fluxes, and energetics. One spectrum proposed here provides an
explanation for the high ionization rate seen in diffuse clouds while still
appearing to be broadly consistent with other observables, but the shape of
this spectrum suggests that supernovae remnants may not be the predominant
accelerators of low-energy cosmic rays.Comment: 13 pages, 4 figures, 3 tables, accepted by Ap
Absorption Line Survey of H3+ toward the Galactic Center Sources II. Eight Infrared Sources within 30 pc of the Galactic Center
Infrared absorption lines of H3+, including the metastable R(3,3)l line, have
been observed toward eight bright infrared sources associated with hot and
massive stars located in and between the Galactic Center Cluster and the
Quintuplet Cluster 30 pc to the east. The absorption lines with high velocity
dispersion arise in the Galaxy's Central Molecular Zone (CMZ) as well as in
foreground spiral arms. The temperature and density of the gas in the CMZ, as
determined from the relative strengths of the H3+ lines, are T=200-300K and
n=50-200cm^-3. The detection of high column densities of H3+ toward all eight
stars implies that this warm and diffuse gaseous environment is widespread in
the CMZ. The products of the ionization rate and path length for these sight
lines are 1000 and 10 times higher than in dense and diffuse clouds in the
Galactic disk, respectively, indicating that the ionization rate, zeta, is not
less than 10^-15 s^-1 and that L is at least on the order of 50 pc. The warm
and diffuse gas is an important component of the CMZ, in addition to the three
previously known gaseous environments: (1) cold molecular clouds observed by
radio emission of CO and other molecules, (2) hot (T=10^4-10^6K) and highly
ionized diffuse gas (n_e=10-100cm^-3) seen in radio recombination lines, far
infrared atomic lines, and radio-wave scattering, and (3) ultra-hot
(T=10^7-10^8K) X-ray emitting plasma. Its prevalence significantly changes the
understanding of the environment of the CMZ. The sight line toward GC IRS 3 is
unique in showing an additional H3+ absorption component, which is interpreted
as due to either a cloud associated with circumnuclear disk or the "50 km s^-1
cloud" known from radio observations. An infrared pumping scheme is examined as
a mechanism to populate the (3,3) metastable level in this cloud.Comment: 15 pages, 5 figures, 4 tables Accepted for publication in the
Astrophysical Journa
Lithic technological responses to Late Pleistocene glacial cycling at Pinnacle Point Site 5-6, South Africa
There are multiple hypotheses for human responses to glacial cycling in the Late Pleistocene, including changes in population size, interconnectedness, and mobility. Lithic technological analysis informs us of human responses to environmental change because lithic assemblage characteristics are a reflection of raw material transport, reduction, and discard behaviors that depend on hunter-gatherer social and economic decisions. Pinnacle Point Site 5-6 (PP5-6), Western Cape, South Africa is an ideal locality for examining the influence of glacial cycling on early modern human behaviors because it preserves a long sequence spanning marine isotope stages (MIS) 5, 4, and 3 and is associated with robust records of paleoenvironmental change. The analysis presented here addresses the question, what, if any, lithic assemblage traits at PP5-6 represent changing behavioral responses to the MIS 5-4-3 interglacial-glacial cycle? It statistically evaluates changes in 93 traits with no a priori assumptions about which traits may significantly associate with MIS. In contrast to other studies that claim that there is little relationship between broad-scale patterns of climate change and lithic technology, we identified the following characteristics that are associated with MIS 4: increased use of quartz, increased evidence for outcrop sources of quartzite and silcrete, increased evidence for earlier stages of reduction in silcrete, evidence for increased flaking efficiency in all raw material types, and changes in tool types and function for silcrete. Based on these results, we suggest that foragers responded to MIS 4 glacial environmental conditions at PP5-6 with increased population or group sizes, 'place provisioning', longer and/or more intense site occupations, and decreased residential mobility. Several other traits, including silcrete frequency, do not exhibit an association with MIS. Backed pieces, once they appear in the PP5-6 record during MIS 4, persist through MIS 3. Changing paleoenvironments explain some, but not all temporal technological variability at PP5-6.Social Science and Humanities Research Council of Canada; NORAM; American-Scandinavian Foundation; Fundacao para a Ciencia e Tecnologia [SFRH/BPD/73598/2010]; IGERT [DGE 0801634]; Hyde Family Foundations; Institute of Human Origins; National Science Foundation [BCS-9912465, BCS-0130713, BCS-0524087, BCS-1138073]; John Templeton Foundation to the Institute of Human Origins at Arizona State Universit
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