3,781 research outputs found
The Complicated Evolution of the ACIS Contamination Layer over the Mission Life of the Chandra X-ray Observatory
The Chandra X-ray Observatory was launched almost 19 years ago and has been
delivering spectacular science over the course of its mission. The Advanced CCD
Imager Spectrometer is the prime instrument on the satellite, conducting over
90% of the observations. The CCDs operate at a temperature of -120 C and the
optical blocking filter in front of the CCDs is at a temperature of
approximately -60C. The surface of the OBF has accumulated a layer of
contamination over the course of the mission. We have been characterizing the
thickness, chemical composition, and spatial distribution of the contamination
layer as a function of time over the mission. All three have exhibited
significant changes with time. There has been a dramatic decrease in the
accumulation rate of the contaminant starting in 2017. The lower accumulation
rate may be due to a decrease in the deposition rate or an increase in the
vaporization rate or a combination of the two. We show that the current
calibration file which models the additional absorption of the contamination
layer is significantly overestimating that additional absorption by using the
standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by
the International Astronomical Consortium for High Energy Calibration. In
addition, spectral data from the cluster of galaxies known as Abell 1795 and
the Blazar Markarian 421 are used to generate a model of the absorption
produced by the contamination layer. The Chandra X-ray Center calibration team
is preparing a revised calibration file that more accurately represents the
complex time dependence of the accumulation rate, the spatial dependence, and
the chemical composition of the contaminant. Given the rapid changes in the
contamination layer over the past year, future calibration observations at a
higher cadence will be necessary to more accurately monitor such changes.Comment: 15 pages, 10 figures, SPIE Astronomical Instruments and Telescopes
2018, Conference Series, 1069
Orbital magnetoelectric coupling at finite electric field
We extend the band theory of linear orbital magnetoelectric coupling to treat
crystals under finite electric fields. Previous work established that the
orbital magnetoelectric response of a generic insulator at zero field comprises
three contributions that were denoted as local circulation, itinerant
circulation, and Chern-Simons. We find that the expression for each of them is
modified by the presence of a dc electric field. Remarkably, the sum of the
three correction terms vanishes, so that the total coupling is still given by
the same formula as at zero field. This conclusion is confirmed by numerical
tests on a tight-binding model, for which we calculate the field-induced change
in the linear magnetoelectric coefficient.Comment: 4 pages, 2 figure
Thomas-Fermi versus one- and two-dimensional regimes of a trapped dipolar Bose-Einstein condensate
We derive the criteria for the Thomas-Fermi regime of a dipolar Bose-Einstein
condensate in cigar, pancake and spherical geometries. This also naturally
gives the criteria for the mean-field one- and two-dimensional regimes. Our
predictions, including the Thomas-Fermi density profiles, are shown to be in
excellent agreement with numerical solutions. Importantly, the anisotropy of
the interactions has a profound effect on the Thomas-Fermi/low-dimensional
criteria.Comment: 5 pages, 2 figure
Probiotic Therapy for the Prevention of Recurrent Spontaneous Preterm Birth
Preterm birth is the national leading cause of neonatal morbidity and mortality. Nearly 500,000 infants are born prematurely each year in the United States and the annual cost of prematurity averages over $26 billion dollars. Despite multiple interventions to reduce infection related prematurity many spontaneous preterm births remain attributed infection and inflammation. We hypothesize that prevention of infection and inflammation related spontaneous preterm birth is possible through enrichment, stabilization, and normalization of the vaginal microbiome. To test this hypothesis we will perform a randomized, double blind, placebo-controlled trial of intravaginal Lactobacillus probiotic in women at high-risk for spontaneous preterm birth. Primary outcome will be reduction frequency of spontaneous preterm birth prior to 37 weeks completed gestation. Results of this study will heighten understanding of the role of the normal vaginal flora in pregnancy and potentially uncover a new intervention capable of reducing preterm birth and the complications of prematurity
Chemical abundances in the protoplanetary disk LV2 (Orion): clues to the causes of the abundance anomaly in HII regions
Optical integral field spectroscopy of the archetype protoplanetary disk LV2
in the Orion Nebula is presented, taken with the VLT FLAMES/Argus fibre array.
The detection of recombination lines of CII and OII from this class of objects
is reported, and the lines are utilized as abundance diagnostics. The study is
complemented with the analysis of HST Faint Object Spectrograph ultraviolet and
optical spectra of the target contained within the Argus field of view. By
subtracting the local nebula background the intrinsic spectrum of the proplyd
is obtained and its elemental composition is derived for the first time. The
proplyd is found to be overabundant in carbon, oxygen and neon compared to the
Orion Nebula and the sun. The simultaneous coverage over LV2 of the CIII]
1908-A and [OIII] 5007-A collisionally excited lines (CELs) and CII and OII
recombination lines (RLs) has enabled us to measure the abundances of C++ and
O++ for LV2 with both sets of lines. The two methods yield consistent results
for the intrinsic proplyd spectrum, but not for the proplyd spectrum
contaminated by the generic nebula spectrum, thus providing one example where
the long-standing abundance anomaly plaguing metallicity studies of HII regions
has been resolved. These results would indicate that the standard
forbidden-line methods used in the derivation of light metal abundances in HII
regions in our own and other galaxies underestimate the true gas metallicity.Comment: Accepted by MNRAS November 8; 16 pages, 9 figs; typos corrected,
error in FWHMs in table 4 corrected in this versio
BOOK REVIEWS: Bumble Bees of North America. Paul H. Williams, Robbin W. Thorp, Leif L. Richardson, and Sheila R. Colla.
Bumblebee identification is generally considered straight- forward, yet mistakes often are made due to the degree of similarity between the color patterns of different species. Bumble Bees of North America aims to improve the accuracy of identifications by both casual observers and professionals through the use of intuitive diagrams, descriptions, and the more technical dichotomous keys. In addition to providing the first complete field guide to North American bumblebees, the authors make efficient use of the reader’s attention by summarizing taxonomic history, favored food plants, and environmental issues concerning bumblebees.
Bumble Bees of North America is organized into three distinct sections. The first section of the book details the collective knowledge regarding bumblebees, delving into issues such as taxonomy, conservation, parasites, and host plants. The authors begin by introducing readers to the cultural and economic value of bumblebees, and follow by informing readers of evidence for population decline and extinction in certain bumblebee species. This combination almost perfectly explains why a scientist might study bees, and subsequently why they need to be able to differentiate bumblebee species (i.e., in order to effectively study bees, scientists have to be able to tell them apart). Next, the authors supply a condensed summary of historical publications on North American bumblebees. Unfortunately, some of the historical sources cited in this summary are not included in the “Additional Resources” section at the end of the book, leaving the readers to intuit the titles of those publications if they are interested in reading these historical resources. Salient features of the introduction include eye-opening figures illustrating global and North American bumblebee diversity, the intensity of bumblebee collection on the continent, and finally, the life cycle of bumblebees
Spitzer reveals what's behind Orion's Bar
We present Spitzer Space Telescope observations of 11 regions SE of the
Bright Bar in the Orion Nebula, along a radial from the exciting star
theta1OriC, extending from 2.6 to 12.1'. Our Cycle 5 programme obtained deep
spectra with matching IRS short-high (SH) and long-high (LH) aperture grid
patterns. Most previous IR missions observed only the inner few arcmin. Orion
is the benchmark for studies of the ISM particularly for elemental abundances.
Spitzer observations provide a unique perspective on the Ne and S abundances by
virtue of observing the dominant ionization states of Ne (Ne+, Ne++) and S
(S++, S3+) in Orion and H II regions in general. The Ne/H abundance ratio is
especially well determined, with a value of (1.01+/-0.08)E-4. We obtained
corresponding new ground-based spectra at CTIO. These optical data are used to
estimate the electron temperature, electron density, optical extinction, and
the S+/S++ ratio at each of our Spitzer positions. That permits an adjustment
for the total gas-phase S abundance because no S+ line is observed by Spitzer.
The gas-phase S/H abundance ratio is (7.68+/-0.30)E-6. The Ne/S abundance ratio
may be determined even when the weaker hydrogen line, H(7-6) here, is not
measured. The mean value, adjusted for the optical S+/S++ ratio, is Ne/S =
13.0+/-0.6. We derive the electron density versus distance from theta1OriC for
[S III] and [S II]. Both distributions are for the most part decreasing with
increasing distance. A dramatic find is the presence of high-ionization Ne++
all the way to the outer optical boundary ~12' from theta1OriC. This IR result
is robust, whereas the optical evidence from observations of high-ionization
species (e.g. O++) at the outer optical boundary suffers uncertainty because of
scattering of emission from the much brighter inner Huygens Region.Comment: 60 pages, 16 figures, 10 tables. MNRAS accepte
Psycholinguistics in Fluency Disorders: Prearticulatory Speech Planning In Stuttering and Cluttering
The Covert Repair Hypothesis (CRH) is an account for speech errors in normally fluent speakers, and also hypothesizes errors in the phonological encoding stage in people who stutter (PWS). Previous research has shown that PWS exhibit poorer performance compared to typically fluent adults (TFA) on linguistic tasks designed to tap into the level of phonological encoding, such as phoneme monitoring. Stuttering and cluttering often co-occur, thus the field can benefit from extending this methodology to study people who clutter (PWC). Experiment 1 in Chapter 2 used phoneme monitoring to study phonological encoding in PWS and PWC, with three conclusions: (1) slower performance by PWS; (2) increased errors by PWS compared to TFA; and (3) similar performance by PWC compared to TFA, suggesting that PWC do not exhibit difficultly with phonological encoding at the single word level. One criticism of the CRH is that the cause of errors in the speech plan has not been accounted for. Chapter 3 proposed the Near Neighbor Interference Hypothesis (NNIH) as an account for errors in the speech plan in PWS, which hypothesizes that due to a lifetime of word-substitution behavior to avoid stuttering, semantic neighborhoods of PWS may be organized differently than TFA, with more neighbors and/or stronger connections between neighbors. Chapter 3 tested the NNIH by investigating the effects of the number of associates (NoA) and the degree of relatedness on performance during lexical decision. Previous research shows TFA respond faster to words with a high vs. low NoA, and words preceded by a picture with a high vs. a low degree of relatedness. Following from the NNIH, it was hypothesized that the magnitude of these effects would be greater in PWS. In Experiment 2, both groups responded faster to words with higher NoA, but PWS were slower to respond than TFA overall, regardless of NoA. In Experiment 3, PWS were not overall slower than TFA, and the effect of degree of relatedness was actually stronger for TFA than PWS. Together, these results suggest that rather than experiencing a benefit from more semantic neighbors, it appears PWS may experience interference from these additional neighbors. Overall, results suggest that PWS may have errors in their speech plan that originate prearticulatoraily, potentially at the lexical-semantic level, and are passed down to the phonological encoding level
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
Integral field spectroscopy of selected areas of the Bright Bar and Orion-S cloud in the Orion Nebula
We present integral field spectroscopy of two selected zones in the Orion
Nebula obtained with the Potsdam Multi-Aperture Spectrophotometer (PMAS),
covering the optical spectral range from 3500 to 7200 A and with a spatial
resolution of 1". The observed zones are located on the prominent Bright Bar
and on the brightest area at the northeast of the Orion South cloud, both
containing remarkable ionization fronts. We obtain maps of emission line fluxes
and ratios, electron density and temperatures, and chemical abundances. We
study the ionization structure and morphology of both fields, which ionization
fronts show different inclination angles with respect to the plane of the sky.
We find that the maps of electron density, O+/H+ and O/H ratios show a rather
similar structure. We interpret this as produced by the strong dependence on
density of the [OII] lines used to derive the O+ abundance, and that our
nominal values of electron density-derived from the [SII] line ratio-may be
slightly higher than the appropriate value for the O+ zone. We measure the
faint recombination lines of OII in the field at the northeast of the Orion
South cloud allowing us to explore the so-called abundance discrepancy problem.
We find a rather constant abundance discrepancy across the field and a mean
value similar to that determined in other areas of the Orion Nebula, indicating
that the particular physical conditions of this ionization front do not
contribute to this discrepancy.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
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