2,004 research outputs found
On the Incidence of C IV Absorbers Along the Sightlines to Gamma-Ray Bursts
We report on the statistics of strong (W_r > 0.15 A) C IV absorbers at
z=1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a
recent survey for strong Mg II absorption systems at z < 2, we find that the
number of C IV absorbers per unit redshift dN/dz does not show a significant
deviation from previous surveys using quasi-stellar objects (QSOs) as
background sources. We find that the number density of C IV toward GRBs is
dN/dz(z~1.5)= 2.2 +2.8/-1.4, dN/dz(z~2.5)= 2.3 +1.8/-1.1 and dN/dz(z~3.5)= 1.1
+2.6/-0.9. These numbers are consistent with previous C IV surveys using QSO
spectra. Binning the entire dataset, we set a 95% c.l. upper limit to the
excess of C IV absorbers along GRB sightlines at twice the incidence observed
along QSO sightlines. Furthermore, the distribution of equivalent widths of the
GRB and QSO samples are consistent with being drawn from the same parent
population. Although the results for Mg II and C IV absorbers along GRB
sightlines appear to contradict one another, we note that the surveys are
nearly disjoint: the C IV survey corresponds to higher redshift and more highly
ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical
samples may constrain properties of the galaxies hosting these metals (e.g.
mass, dust content) and/or the coherence-length of the gas giving rise to the
metal-line absorption.Comment: Accepted version (for publication in ApJ), results unchanged, 18
pages, 3 tables, 5 figure
Probing the ISM Near Star Forming Regions with GRB Afterglow Spectroscopy: Gas, Metals, and Dust
We study the chemical abundances of the interstellar medium surrounding high
z gamma-ray bursts (GRBs) through analysis of the damped Lya systems (DLAs)
identified in afterglow spectra. These GRB-DLAs are characterized by large HI
column densities N(HI) and metallicities [M/H] spanning 1/100 to nearly solar,
with median [M/H]>-1. The majority of GRB-DLAs have [M/H] values exceeding the
cosmic mean metallicity of atomic gas at z>2, i.e. if anything, the GRB-DLAs
are biased to larger metallicity. We also observe (i) large [Zn/Fe] values
(>+0.6) and sub-solar Ti/Fe ratios which imply substantial differential
depletion, (ii) large a/Fe ratios suggesting nucleosynthetic enrichment by
massive stars, and (iii) low C^0/C^+ ratios (<10^{-4}). Quantitatively, the
observed depletion levels and C^0/C^+ ratios of the gas are not characteristic
of cold, dense HI clouds in the Galactic ISM. We argue that the GRB-DLAs
represent the ISM near the GRB but not gas directly local to the GRB (e.g. its
molecular cloud or circumstellar material). We compare these observations with
DLAs intervening background quasars (QSO-DLAs). The GRB-DLAs exhibit larger
N(HI) values, higher a/Fe and Zn/Fe ratios, and have higher metallicity than
the QSO-DLAs. We argue that the differences primarily result from
galactocentric radius-dependent differences in the ISM: GRB-DLAs preferentially
probe denser, more depleted, higher metallicity gaslocated in the inner few kpc
whereas QSO-DLAs are more likely to intersect the less dense, less enriched,
outer regions of the galaxy. Finally, we investigate whether dust obscuration
may exclude GRB-DLA sightlines from QSO-DLA samples; we find that the majority
of GRB-DLAs would be recovered which implies little observational bias against
large N(HI) systems.Comment: 16 pages, 9 figures. Submitted to Ap
Carbon dioxide in the atmosphere: Isotopic exchange with ozone and its use as a tracer in the middle atmosphere
Atmospheric heavy ozone is enriched in the isotopes ^(18)O and ^(17)O. The magnitude of this enhancement, of the order of 100â°, is very large compared with that commonly known in atmospheric chemistry and geochemistry. The heavy oxygen atom in heavy ozone is therefore useful as a tracer of chemical species and pathways that involve ozone or its derived products. As a test of the isotopic exchange reactions, we successfully carry out a series of numerical experiments to simulate the results of the laboratory experiments performed by Wen and Thiemens [1993] on ozone and CO_2. A small discrepancy between the experimental and the model values for ^(17)O exchange is also revealed. The results are used to compute the magnitude of isotopic exchange between ozone and carbon dioxide via the excited atom O(^1D) in the middle atmosphere. The model for ^(18)O is in good agreement with the observed values
A human antibody against Zika virus crosslinks the E protein to prevent infection
The recent Zika virus (ZIKV) epidemic has been linked to unusual and severe clinical manifestations including microcephaly in fetuses of infected pregnant women and Guillian-BarrĂ© syndrome in adults. Neutralizing antibodies present a possible therapeutic approach to prevent and control ZIKV infection. Here we present a 6.2âĂ
resolution three-dimensional cryo-electron microscopy (cryoEM) structure of an infectious ZIKV (strain H/PF/2013, French Polynesia) in complex with the Fab fragment of a highly therapeutic and neutralizing human monoclonal antibody, ZIKV-117. The antibody had been shown to prevent fetal infection and demise in mice. The structure shows that ZIKV-117 Fabs cross-link the monomers within the surface E glycoprotein dimers as well as between neighbouring dimers, thus preventing the reorganization of E protein monomers into fusogenic trimers in the acidic environment of endosomes
A New Constraint on the Escape Fraction in Distant Galaxies Using Gamma-ray Burst Afterglow Spectroscopy
We describe a new method to measure the escape fraction fesc of ionizing
radiation from distant star-forming galaxies using the afterglow spectra of
long-duration gamma-ray bursts (GRBs). Optical spectra of GRB afterglows allow
us to evaluate the optical depth of the host ISM, according to the neutral
hydrogen column density N(HI) observed along the sightlines toward the
star-forming regions where the GRBs are found. Different from previous effort
in searching for faint, transmitted Lyman continuum photons, our method is not
subject to background subtraction uncertainties and does not require prior
knowledge of either the spectral shape of the host galaxy population or the IGM
Lya forest absorption along these GRB sightlines. Because most GRBs occur in
sub-L_* galaxies, our study also offers the first constraint on fesc for
distant low-mass galaxies that dominate the cosmic luminosity density. We have
compiled a sample of 27 GRBs at redshift z>2 for which the underlying N(HI) in
the host ISM are known. These GRBs together offer a statistical sampling of the
integrated optical depth to ionizing photons along random sightlines from
star-forming regions in the host galaxies, and allow us to estimate the mean
escape fraction averaged over different viewing angles. We find
=0.02\pm 0.02 and place a 95% c.l. upper limit <= 0.075 for these
hosts. We discuss possible biases of our approach and implications of the
result. Finally, we propose to extend this technique for measuring at
z~0.2 using spectra of core-collapse supernovae.Comment: Five journal pages, including one figure; ApJL in pres
The Host Galaxy of GRB 031203: Implications of its low metallicity, low redshift, and starburst nature
We present Keck/NIRSPEC near-IR images and Magellan/IMACS optical
spectroscopy of the host galaxy of GRB 031203. The host is an actively
star-forming galaxy at z=0.1055 +/- 0.0001. This is the lowest redshift GRB
to-date, aside from GRB 980425. From the hydrogen Balmer lines, we infer an
extinction of A_V = 3.62 +/- 0.25 or a total reddening E_T(B-V) = 1.17 +/- 0.1
toward the sightline to the nebular regions. After correcting for reddening, we
perform an emission-line analysis and derive an ISM temperature of
T=13400+/-2000K and electron density of n_e = 300 cm^(-3). These imply a
metallicity [O/H]=-0.72+/-0.15 dex and a roughly solar abundance pattern for N,
Ne, S, and Ar. Integrating Ha, we infer a dust-corrected star formation rate
(SFR) of > 11 Msol/yr. These observations have the following implications: (1)
the galaxy has a low K'-band luminosity L ~ L^*/5, typical of GRB host
galaxies; (2) the low redshift indicates GRB 031203 had an isotropic-equivalent
gamma-ray energy release smaller than all previous confirmed GRB events. The
burst discovery raises the likelihood of identifying many additional low z, low
flux events with Swift; (3) the large SFR, low metallicity, and the inferred
hard radiation field is suggestive of massive star formation, supporting the
collapsar model; (4) several lines of evidence argue against the identification
of GRB 031203 as an X-ray flash event.Comment: 8 pages, 3 figures. Uses emulateapj5.sty. Accepted to the
Astrophysical Journal on April 20, 200
Sequential addition of neuronal stem cell temporal cohorts generates a feed-forward circuit in the Drosophila larval nerve cord
How circuits self-assemble starting from neuronal stem cells is a fundamental question in developmental neurobiology. Here, we addressed how neurons from different stem cell lineages wire with each other to form a specific circuit motif. In Drosophila larvae, we combined developmental genetics (Twin spot MARCM, Multi-color Flip Out, permanent labeling) with circuit analysis (calcium imaging, connectomics, network science). For many lineages, neuronal progeny are organized into subunits called temporal cohorts. Temporal cohorts are subsets of neurons born within a tight time window that have shared circuit level function. We find sharp transitions in patterns of input connectivity at temporal cohort boundaries. In addition, we identify a feed-forward circuit that encodes the onset of vibration stimuli. This feed-forward circuit is assembled by preferential connectivity between temporal cohorts from different lineages. Connectivity does not follow the often-cited early-to-early, late-to-late model. Instead, the circuit is formed by sequential addition of temporal cohorts from different lineages, with circuit output neurons born before circuit input neurons. Further, we generate new tools for the fly community. Our data raise the possibility that sequential addition of neurons (with outputs oldest and inputs youngest) could be one fundamental strategy for assembling feed-forward circuits
An experimental study on thermal runaway characteristics of lithium-ion batteries with high specific energy and prediction of heat release rate
Understanding the potential thermal hazards of lithium-ion batteries (LIBs) during thermal runaway (TR) is helpful to assess the safety of LIB during storage, transport and use. This paper presents a comprehensive analysis of the thermal runaway (TR) characteristics of type 21700 cylindrical LIBs with a specific energy of 266 Wâh/kg. The batteries with both 30% state of charge (SOC) and 100% SOC were triggered to TR by uniform heating using a flexible heater in a laboratory environment. Three high definition cameras and one high-speed camera were placed to capture TR behavior and flame evolution from different viewpoints. Correlation between the heat release rate (HRR) and the mean flame height of turbulent jet diffusion flame were used to estimate the HRRs of LIBs. Additional characteristics of cell failure (for cells with 100% and 30% SOC) were also noted for comparison, including: number of objects ejected from the cell; sparks and subsequent jet fires. An approach has been developed to estimate the HRRs from TR triggered fires and results compared with previous HRR measurements for type 18650 cylindrical cells with a similar cathode composition
Should physical activity recommendations be ethnicity-specific? Evidence from a cross-sectional study of south Asian and European men
Background
Expert bodies and health organisations recommend that adults undertake at least 150 min.weekâ1 of moderate-intensity physical activity (MPA). However, the underpinning data largely emanate from studies of populations of European descent. It is unclear whether this level of activity is appropriate for other ethnic groups, particularly South Asians, who have increased cardio-metabolic disease risk compared to Europeans. The aim of this study was to explore the level of MPA required in South Asians to confer a similar cardio-metabolic risk profile to that observed in Europeans undertaking the currently recommended MPA level of 150 min.weekâ1.<p></p>
Methods
Seventy-five South Asian and 83 European men, aged 40â70, without cardiovascular disease or diabetes had fasted blood taken, blood pressure measured, physical activity assessed objectively (using accelerometry), and anthropometric measures made. Factor analysis was used to summarise measured risk biomarkers into underlying latent âfactorsâ for glycaemia, insulin resistance, lipid metabolism, blood pressure, and overall cardio-metabolic risk. Age-adjusted regression models were used to determine the equivalent level of MPA (in bouts of â„10 minutes) in South Asians needed to elicit the same value in each factor as Europeans undertaking 150 min.weekâ1 MPA.<p></p>
Findings
For all factors, except blood pressure, equivalent MPA values in South Asians were significantly higher than 150 min.weekâ1; the equivalent MPA value for the overall cardio-metabolic risk factor was 266 (95% CI 185-347) min.weekâ1.<p></p>
Conclusions
South Asian men may need to undertake greater levels of MPA than Europeans to exhibit a similar cardio-metabolic risk profile, suggesting that a conceptual case can be made for ethnicity-specific physical activity guidance. Further study is needed to extend these findings to women and to replicate them prospectively in a larger cohort.<p></p>
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