11,767 research outputs found
The Influence of Nuclear Composition on the Electron Fraction in the Post-Core-Bounce Supernova Environment
We study the early evolution of the electron fraction (or, alternatively, the
neutron-to-proton ratio) in the region above the hot proto-neutron star formed
after a supernova explosion. We study the way in which the electron fraction in
this environment is set by a competition between lepton (electron, positron,
neutrino, and antineutrino) capture processes on free neutrons and protons and
nuclei. Our calculations take explicit account of the effect of nuclear
composition changes, such as formation of alpha particles (the alpha effect)
and the shifting of nuclear abundances in nuclear statistical equilibrium
associated with cooling in near-adiabatic outflow. We take detailed account of
the process of weak interaction freeze-out in conjunction with these nuclear
composition changes. Our detailed treatment shows that the alpha effect can
cause significant increases in the electron fraction, while neutrino and
antineutrino capture on heavy nuclei tends to have a buffering effect on this
quantity. We also examine the effect on weak rates and the electron fraction of
fluctuations in time in the neutrino and antineutrino energy spectra arising
from hydrodynamic waves. Our analysis is guided by the Mayle & Wilson supernova
code numerical results for the neutrino energy spectra and density and velocity
profiles.Comment: 38 pages, AAS LaTeX, 8 figure
Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
The massive and luminous star-forming region W49A is a well known Galactic
candidate to probe the physical conditions and chemistry similar to those
expected in external starburst galaxies. We aim to probe the physical and
chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT
Spectral Legacy Survey, which covers the frequency range between 330 and 373
GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP
instrument on JCMT provides information on the spatial structure and kinematics
of the cloud. For species where multiple transitions are available, we estimate
excitation temperatures and column densities. We detected 255 transitions
corresponding to 60 species in the 330-373 GHz range at the center position of
W49A. Excitation conditions can be probed for 16 molecules. The chemical
composition suggests the importance of shock-, PDR-, and hot core chemistry.
Many molecular lines show a significant spatial extent across the maps
including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of
UV-irradiation (e.g. CN and C2H). Large variations are seen between the
sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly
red-shifted emission toward the Northern clump, and emission peaking around the
expected source velocity toward the South-west clump. A comparison of column
density ratios of characteristic species observed toward W49A to Galactic PDRs
suggests that while the chemistry toward the W49A center is driven by a
combination of UV-irradiation and shocks, UV-irradiation dominates for the
Northern Clump, Eastern tail, and South-west clump regions. A comparison to a
starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with
respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A
and the >1 kpc regions in external galaxies with global star-formation.Comment: Proposed for acceptance in A&A, abstract abridge
The measurement of velocity gradients in laminar flow by homodyne light-scattering spectroscopy
A technique for measuring velocity gradients in laminar flows by homodyne light
scattering is presented. A theory which describes the light-scattering spectrum is
derived that includes the effects of different types of linear flow fields, particle diffusion
and the intensity profile in the scattering volume. The conditions which must be
satisfied in order that the theory describe the experimental situation are outlined and
complementary experiments are performed which both verify the theory and apply
the technique. Verification is provided using the flow in a Couette device, and the flow
due to single rotating cylinder in a large bath of fluid. The technique is then applied
to measure the spatial variation of the shear rate in a four-roll mill
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Deer reduce habitat quality for a woodland songbird: evidence from settlement patterns, demographic parameters, and body condition.
Understanding avian responses to ungulate-induced habitat modification is important because deer populations are increasing across much of temperate Europe and North America. Our experimental study examined whether habitat quality for Blackcaps (Sylvia atricapilla) in young woodland in eastern England was affected by deer, by comparing Blackcap behavior, abundance, and condition between paired plots (half of each pair protected from deer). The vegetation in each pair of plots was the same age. The Blackcap is an ideal model species for testing effects of deer on avian habitat quality because it is dependent on dense understory vegetation and is abundant throughout much of Europe. We compared timing of settlement, abundance, age structure (second-year vs. after-second-year), and phenotypic quality (measured as a body condition index, body mass divided by tarsus length) between experimental and control plots. We used point counts to examine Blackcap distribution, and standardized mist netting to collect demographic and biometric data. Incidence of singing Blackcaps was higher in nonbrowsed than in browsed plots, and singing males were recorded in nonbrowsed plots earlier in the season, indicating earlier and preferential territory establishment. Most Blackcaps, both males and females, were captured in vegetation prior to canopy closure (2–4 years of regrowth). Body condition was superior for male Blackcaps captured in nonbrowsed plots; for second-year males this was most marked in vegetation prior to canopy closure. We conclude that deer browsing in young woodland can alter habitat quality for understory-dependent species, with potential consequences for individual fitness and population productivity beyond the more obvious effects on population density
Real-time depth sectioning: Isolating the effect of stress on structure development in pressure-driven flow
Transient structure development at a specific distance from the channel wall in a pressure-driven flow is obtained from a set of real-time measurements that integrate contributions throughout the thickness of a rectangular channel. This “depth sectioning method” retains the advantages of pressure-driven flow while revealing flow-induced structures as a function of stress. The method is illustrated by applying it to isothermal shear-induced crystallization of an isotactic polypropylene using both synchrotron x-ray scattering and optical retardance. Real-time, depth-resolved information about the development of oriented precursors reveals features that cannot be extracted from ex-situ observation of the final morphology and that are obscured in the depth-averaged in-situ measurements. For example, at 137 °C and at the highest shear stress examined (65 kPa), oriented thread-like nuclei formed rapidly, saturated within the first 7 s of flow, developed significant crystalline overgrowth during flow and did not relax after cessation of shear. At lower stresses, threads formed later and increased at a slower rate. The depth sectioning method can be applied to the flow-induced structure development in diverse complex fluids, including block copolymers, colloidal systems, and liquid-crystalline polymers
The art of being human : a project for general philosophy of science
Throughout the medieval and modern periods, in various sacred and secular guises, the unification of all forms of knowledge under the rubric of ‘science’ has been taken as the prerogative of humanity as a species. However, as our sense of species privilege has been called increasingly into question, so too has the very salience of ‘humanity’ and ‘science’ as general categories, let alone ones that might bear some essential relationship to each other. After showing how the ascendant Stanford School in the philosophy of science has contributed to this joint demystification of ‘humanity’ and ‘science’, I proceed on a more positive note to a conceptual framework for making sense of science as the art of being human. My understanding of ‘science’ is indebted to the red thread that runs from Christian theology through the Scientific Revolution and Enlightenment to the Humboldtian revival of the university as the site for the synthesis of knowledge as the culmination of self-development. Especially salient to this idea is science‘s epistemic capacity to manage modality (i.e. to determine the conditions under which possibilities can be actualised) and its political capacity to organize humanity into projects of universal concern. However, the challenge facing such an ideal in the twentyfirst century is that the predicate ‘human’ may be projected in three quite distinct ways, governed by what I call ‘ecological’, ‘biomedical’ and ‘cybernetic’ interests. Which one of these future humanities would claim today’s humans as proper ancestors and could these futures co-habit the same world thus become two important questions that general philosophy of science will need to address in the coming years
The Masers Towards IRAS 20126+4104
We present MERLIN observations of OH, water and methanol masers towards the
young high mass stellar object IRAS 20126+4104. Emission from the 1665-MHz OH,
22-GHz H_2O and 6.7-GHz CH_3OH masers is detected and all originates very close
to the central source. The OH and methanol masers appear to trace part of the
circumstellar disk around the central source. The positions and velocities of
the OH and methanol masers are consistent with Keplerian rotation around a
central mass of ~ 5 Msun. The water masers are offset from the OH and methanol
masers and have significantly changed since they were last observed, but still
appear to be associated to the outflow from the source. All the OH masers
components are circularly polarised, in some cases reaching 100 percent while
some OH components also have linear polarisation. We identify one Zeeman pair
of OH masers and the splitting of this pair indicates a magnetic field of
strength ~ 11 mG within ~ 0.5" (850 AU) of the central source. The OH and
methanol maser emission suggest that the disk material is dense, n > 10^6
cm^-3, and warm, T > 125 K and the high abundance of methanol required by the
maser emission is consistent with the evaporation of the mantles on dust grains
in the disk as a result of heating or shocking of the disk materialComment: 9 pages, 7 figures and 6 table
Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition
We report surface shear rheological measurements on dense insoluble
monolayers of micron sized colloidal spheres at the oil/water interface and of
the protein -lactoglobulin at the air/water surface. As expected, the
elastic modulus shows a changing character in the response, from a viscous
liquid towards an elastic solid as the concentration is increased, and a change
from elastic to viscous as the shear frequency is increased. Surprisingly,
above a critical packing fraction, the complex elastic modulus curves measured
at different concentrations can be superposed to form a master curve, by
rescaling the frequency and the magnitude of the modulus. This provides a
powerful tool for the extrapolation of the material response function outside
the experimentally accessible frequency range. The results are discussed in
relation to recent experiments on bulk systems, and indicate that these two
dimensional monolayers should be regarded as being close to a soft glass state.Comment: to appear in PR
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