1,852 research outputs found
Three-dimensional flow structure and bed morphology in large elongate meander loops with different outer bank roughness characteristics
© 2016. American Geophysical Union. All Rights Reserved. Few studies have examined the three-dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three-dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near-bank flow characteristics. Three-dimensional velocities were measured during two different eventsâa near-bankfull flow and an overbank event. Detailed data on channel bathymetry and bed form geometry were obtained during a near-bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near-bank velocities during the overbank event are less than those for the near-bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at subbankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near-bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops
GRB Energetics and the GRB Hubble Diagram: Promises and Limitations
We present a complete sample of 29 GRBs for which it has been possible to
determine temporal breaks (or limits) from their afterglow light curves. We
interpret these breaks within the framework of the uniform conical jet model,
incorporating realistic estimates of the ambient density and propagating error
estimates on the measured quantities. In agreement with our previous analysis
of a smaller sample, the derived jet opening angles of those 16 bursts with
redshifts result in a narrow clustering of geometrically-corrected gamma-ray
energies about E_gamma = 1.33e51 erg; the burst-to-burst variance about this
value is a factor of 2.2. Despite this rather small scatter, we demonstrate in
a series of GRB Hubble diagrams, that the current sample cannot place
meaningful constraints upon the fundamental parameters of the Universe. Indeed
for GRBs to ever be useful in cosmographic measurements we argue the necessity
of two directions. First, GRB Hubble diagrams should be based upon fundamental
physical quantities such as energy, rather than empirically-derived and
physically ill-understood distance indicators. Second, a more homogeneous set
should be constructed by culling sub-classes from the larger sample. These
sub-classes, though now first recognizable by deviant energies, ultimately must
be identifiable by properties other than those directly related to energy. We
identify a new sub-class of GRBs (``f-GRBs'') which appear both underluminous
by factors of at least 10 and exhibit a rapid fading at early times. About
10-20% of observed long-duration bursts appear to be f-GRBs.Comment: Accepted to the Astrophysical Journal (20 May 2003). 19 pages, 3
Postscript figure
Effects of Dust Geometry in Lyman Alpha Galaxies at z = 4.4
Equivalent widths (EWs) observed in high-redshift Lyman alpha galaxies could
be stronger than the EW intrinsic to the stellar population if dust is present
residing in clumps in the inter-stellar medium (ISM). In this scenario,
continuum photons could be extinguished while the Lyman alpha photons would be
resonantly scattered by the clumps, eventually escaping the galaxy. We
investigate this radiative transfer scenario with a new sample of six Lyman
alpha galaxy candidates in the GOODS CDF-S, selected at z = 4.4 with
ground-based narrow-band imaging obtained at CTIO. Grism spectra from the HST
PEARS survey confirm that three objects are at z = 4.4, and that another object
contains an active galactic nuclei (AGN). If we assume the other five (non-AGN)
objects are at z = 4.4, they have rest-frame EWs from 47 -- 190 A. We present
results of stellar population studies of these objects, constraining their
rest-frame UV with HST and their rest-frame optical with Spitzer. Out of the
four objects which we analyzed, three objects were best-fit to contain stellar
populations with ages on the order of 1 Myr and stellar masses from 3 - 10 x
10^8 solar masses, with dust in the amount of A_1200 = 0.9 - 1.8 residing in a
quasi-homogeneous distribution. However, one object (with a rest EW ~ 150 A)
was best fit by an 800 Myr, 6.6 x 10^9 solar mass stellar population with a
smaller amount of dust (A_1200 = 0.4) attenuating the continuum only. In this
object, the EW was enhanced ~ 50% due to this dust. This suggests that large EW
Lyman alpha galaxies are a diverse population. Preferential extinction of the
continuum in a clumpy ISM deserves further investigation as a possible cause of
the overabundance of large-EW objects that have been seen in narrow-band
surveys in recent years.Comment: Submitted to the Astrophysical Journal. 35 pages, 7 figures and 4
table
Constraining the Lyα escape fraction with far-infrared observations of Lyα emitters
We study the far-infrared properties of 498 Lyα emitters (LAEs) at z = 2.8, 3.1, and 4.5 in the Extended Chandra Deep Field-South, using 250, 350, and 500ÎŒm data from the Herschel Multi-tiered Extragalactic Survey and 870ÎŒm data from the LABOCA ECDFS Submillimeter Survey. None of the 126, 280, or 92 LAEs at z = 2.8, 3.1, and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching 1Ï depths of âŒ0.1 to 0.4 mJy. The LAEs are also undetected at ?3Ï in the stacks, although a 2.5Ï signal is observed at 870ÎŒm for the z = 2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including an M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star formation rates of the LAEs. These star formation rates are then combined with those inferred from the Lyα and UV emission to determine lower limits on the LAEsâ Lyα escape fraction (f esc (Lyα)). For the Sd SED template, the inferred LAEs f esc (Lyα) are ?30% (1Ï) at z = 2.8, 3.1, and 4.5, which are all significantly higher than the global f esc (Lyα) at these redshifts. Thus, if the LAEs f esc (Lyα) follows the global evolution, then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE f esc (Lyα) of âŒ10%â20% (1Ï), all of which are slightly higher than the global evolution of f esc (Lyα), but consistent with it at the 2Ïâ3Ï level
Survey of bacterial diversity in chronic wounds using Pyrosequencing, DGGE, and full ribosome shotgun sequencing
<p>Abstract</p> <p>Background</p> <p>Chronic wound pathogenic biofilms are host-pathogen environments that colonize and exist as a cohabitation of many bacterial species. These bacterial populations cooperate to promote their own survival and the chronic nature of the infection. Few studies have performed extensive surveys of the bacterial populations that occur within different types of chronic wound biofilms. The use of 3 separate16S-based molecular amplifications followed by pyrosequencing, shotgun Sanger sequencing, and denaturing gradient gel electrophoresis were utilized to survey the major populations of bacteria that occur in the pathogenic biofilms of three types of chronic wound types: diabetic foot ulcers (D), venous leg ulcers (V), and pressure ulcers (P).</p> <p>Results</p> <p>There are specific major populations of bacteria that were evident in the biofilms of all chronic wound types, including <it>Staphylococcus, Pseudomonas, Peptoniphilus, Enterobacter, Stenotrophomonas, Finegoldia</it>, and <it>Serratia </it>spp. Each of the wound types reveals marked differences in bacterial populations, such as pressure ulcers in which 62% of the populations were identified as obligate anaerobes. There were also populations of bacteria that were identified but not recognized as wound pathogens, such as <it>Abiotrophia para-adiacens </it>and <it>Rhodopseudomonas </it>spp. Results of molecular analyses were also compared to those obtained using traditional culture-based diagnostics. Only in one wound type did culture methods correctly identify the primary bacterial population indicating the need for improved diagnostic methods.</p> <p>Conclusion</p> <p>If clinicians can gain a better understanding of the wound's microbiota, it will give them a greater understanding of the wound's ecology and will allow them to better manage healing of the wound improving the prognosis of patients. This research highlights the necessity to begin evaluating, studying, and treating chronic wound pathogenic biofilms as multi-species entities in order to improve the outcomes of patients. This survey will also foster the pioneering and development of new molecular diagnostic tools, which can be used to identify the community compositions of chronic wound pathogenic biofilms and other medical biofilm infections.</p
How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts
If gamma ray bursts are highly collimated, the energy requirements of each
event may be reduced by several (~ 4-6) orders of magnitude, and the event rate
increased correspondingly. Extreme conditions in gamma ray bursters lead to
highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in
strong forward beaming of the emitted radiation in the observer's rest frame.
Thus, all information on gamma ray bursts comes from those ejecta emitted in a
narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at
present ignorant of whether there are ejecta outside that cone or not.
The recent detection of longer wavelength transients following gamma ray
bursts allows an empirical test of whether gamma ray bursts are collimated jets
or spherical fireballs. The bulk Lorentz factor of the burst ejecta will
decrease with time after the event, as the ejecta sweep up the surrounding
medium. Thus, radiation from the ejecta is beamed into an ever increasing solid
angle as the burst remnant evolves. It follows that if gamma ray bursts are
highly collimated, many more optical and radio transients should be observed
without associated gamma rays than with them. Published supernova searches may
contain enough data to test the most extreme models of gamma ray beaming. We
close with a brief discussion of other possible consequences of beaming,
including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses
AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal
Letters, vol. 487, p. L1 (20 September 1997
Prompt optical observations of GRB050319 with the Swift UVOT
The UVOT telescope on the Swift observatory has detected optical afterglow
emission from GRB 050319. The flux declines with a power law slope of alpha =
-0.57 between the start of observations some 230 seconds after the burst onset
(90s after the burst trigger) until it faded below the sensitivity threshold of
the instrument after ~5 x 10^4s. There is no evidence for the rapidly declining
component in the early light curve that is seen at the same time in the X-ray
band. The afterglow is not detected in UVOT shortward of the B-band, suggesting
a redshift of about 3.5. The optical V-band emission lies on the extension of
the X-ray spectrum, with an optical to X-ray slope of beta = -0.8. The
relatively flat decay rate of the burst suggests that the central engine
continues to inject energy into the fireball for as long as a few x 10^4s after
the burst.Comment: 16 pages, 4 figures, accepted by Ap
Are some breaks in GRB afterglows caused by their spectra?
Sharp breaks have been observed in the afterglow light curves of several
GRBs; this is generally explained by the jet model. However, there are still
some uncertainties concerning this interpretation due to the unclear
hydrodynamics of jet sideways expansion. Here we propose an alternative
explanation to these observed breaks. If we assume that the multiwavelength
spectra of GRB afterglows are not made of exact power law segments but their
slope changes smoothly, i.e. , where is the spectral
index, we find that this fact can very nicely explain the afterglow light
curves showing breaks. Therefore we suggest that some breaks in the afterglow
light curves may be caused by their curved spectra. The main feature of this
interpretation is that the break time is dependent on the observed frequency,
while the jet model produces achromatic breaks in the light curves. In
addition, it is very important to know the position of the characteristic
frequency in the multiwavelength spectrum at the time of the break,
since it is a further discriminant between our model and the jet model. We find
that although the optical light curves of seven GRB afterglows can be well
fitted by the model we propose, in fact only one of them (i.e. GRB000926) can
be explained in this framework, since for other ones the characteristic
frequency is either above the optical after the break or below the
optical before the break.Comment: Latex, revised version accepted by A&
The X-ray emission lines in GRB afterglows: the evidence for the two-component jet model
Recently, X-ray emission lines have been observed in X-ray afterglows of
several -ray bursts. It is a major breakthrough for understanding the
nature of the progenitors. It is proposed that the X-ray emission lines can be
well explained by the Geometry-Dominated models, but in these models the
illuminating angle is much larger than that of the collimated jet of the
-ray bursts(GRBs). For GRB 011211, we obtain the illuminating angle is
about , while the angle of GRB jet is only ,
so we propose that the outflow of the GRBs with emission lines should have two
distinct components. The wide component illuminates the reprocessing material,
and produces the emission lines, while the narrow one produces the -ray
bursts. The observations show that the energy for producing the emission lines
is higher than that of the GRBs. In this case, when the wide component
dominates the afterglows, a bump will appear in the GRBs afterglows. For GRB
011211, the emergence time of the bump is less than 0.05 days after the GRB, it
is obviously too early for the observation to catch it. With the presence of
the X-ray emission lines there should also be a bright emission component
between the UV and the soft X-rays. These features can be tested by the
satellite in the near future.Comment: 10 pags, 1 figure, ChJAA in pres
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