438 research outputs found
Nonlinear Elasticity in Biological Gels
Unlike most synthetic materials, biological materials often stiffen as they
are deformed. This nonlinear elastic response, critical for the physiological
function of some tissues, has been documented since at least the 19th century,
but the molecular structure and the design principles responsible for it are
unknown. Current models for this response require geometrically complex ordered
structures unique to each material. In this Article we show that a much simpler
molecular theory accounts for strain stiffening in a wide range of molecularly
distinct biopolymer gels formed from purified cytoskeletal and extracellular
proteins. This theory shows that systems of semi-flexible chains such as
filamentous proteins arranged in an open crosslinked meshwork invariably
stiffen at low strains without the need for a specific architecture or multiple
elements with different intrinsic stiffnesses.Comment: 23 pages, 5 figures, submitted to Natur
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey
(Abridged) We perform an analysis of spectra and photometry for 22,770 stars
included in the third data release (DR3) of the SDSS. We measure radial
velocities and, based on a model-atmosphere analysis, derive estimates ofthe
atmospheric parameters (effective temperature, surface gravity, and [Fe/H]) for
each star. Stellar evolution models are then used to estimate distances. The
SDSS sample covers a range in stellar brightness of 14 < V < 22, and comprises
large numbers of F- and G-type stars from the thick-disk and halo populations
(up to 100 kpc from the galactic plane), therefore including some of the oldest
stars in the Milky Way. In agreement with previous results from the literature,
we find that halo stars exhibit a broad range of iron abundances, with a peak
at [Fe/H] ~ -1.4. This population exhibits essentially no galactic rotation.
Thick-disk G-dwarf stars at distances from the galactic plane in the range
1<|z|<3 kpc show a much more compact metallicity distribution, with a maximum
at [Fe/H] ~ -0.7, and a median galactic rotation velocity at that metallicity
of 157 +/- 4 km/s (a lag relative to the thin disk of 63 km/s). A comparison of
color indices and metal abundances with isochrones indicates that no
significant star formation has taken place in the halo in the last ~ 11 Gyr,
but there are thick-disk stars which are at least 2 Gyr younger. We find the
metallicities of thick-disk stars to be nearly independent of galactocentric
distance between 5 and 14 kpc, in contrast with the marked gradients found in
the literature for the thin disk. No vertical metallicity gradient is apparent
for the thick disk, but we detect a gradient inits rotational velocity of -16
+/- 4 km/s/kpc between 1 and 3 kpc from the plane.Comment: 18 pages, 16 figures; accepted for publication in the ApJ; also
available from http://hebe.as.utexas.edu
The Orbit of the Orphan Stream
We use recent SEGUE spectroscopy and SDSS and SEGUE imaging data to measure
the sky position, distance, and radial velocities of stars in the tidal debris
stream that is commonly referred to as the "Orphan Stream." We fit orbital
parameters to the data, and find a prograde orbit with an apogalacticon,
perigalacticon, and eccentricity of 90 kpc, 16.4 kpc and 0.7, respectively.
Neither the dwarf galaxy UMa II nor the Complex A gas cloud have velocities
consistent with a kinematic association with the Orphan Stream. It is possible
that Segue-1 is associated with the Orphan Stream, but no other known Galactic
clusters or dwarf galaxies in the Milky Way lie along its orbit. The detected
portion of the stream ranges from 19 to 47 kpc from the Sun and is an indicator
of the mass interior to these distances. There is a marked increase in the
density of Orphan Stream stars near (l,b)=(253,49) deg., which could indicate
the presence of the progenitor at the edge of the SDSS data. If this is the
progenitor, then the detected portion of the Orphan Stream is a leading tidal
tail. We find blue horizontal branch (BHB) stars and F turnoff stars associated
with the Orphan Stream. The turnoff color is (g-r)_0=0.22. The BHB stars have a
low metallicity of [Fe/H]=-2.1. The orbit is best fit to a halo potential with
a halo plus disk mass of about 2.6x10^11 Solar masses, integrated to 60 kpc
from the Galactic center. Our best fit is found with a logarithmic halo speed
of v_halo=73+/-24 km/s, a disk+bulge mass of M(R< 60 kpc) = 1.3x10^11 Solar
masses, and a halo mass of M(R< 60 kpc) = 1.4x10^11 Solar masses. The Orphan
Stream is projected to extend to 90 kpc from the Galactic center, and
measurements of these distant parts of the stream would be a powerful probe of
the mass of the Milky Way (truncated).Comment: 17 Figures, ApJ accepte
Tan relations in one dimension
We derive exact relations that connect the universal -decay of the
momentum distribution at large with both thermodynamic properties and
correlation functions of two-component Fermi gases in one dimension with
contact interactions. The relations are analogous to those obtained by Tan in
the three-dimensional case and are derived from an operator product expansion
of the one- and two-particle density matrix. They extend earlier results by
Olshanii and Dunjko [Phys. Rev. Lett. 91, 090401 (2003)] for the bosonic
Lieb-Liniger gas. As an application, we calculate the pair distribution
function at short distances and the dimensionless contact in the limit of
infinite repulsion. The ground state energy approaches a universal constant in
this limit, a behavior that also holds in the three-dimensional case. In both
one and three dimensions, a Stoner instability to a saturated ferromagnet for
repulsive fermions with zero range interactions is ruled out at any finite
coupling.Comment: 8 figures, 27 pages - Updated to status of published versio
The Metallicity Distribution Function of the Halo of the Milky Way
We report on the distribution of metallicities, [Fe/H], for very metal-poor
stars in the halo of the Galaxy. Although the primary information on the nature
of the Metallicity Distribution Function (MDF) is obtained from the two major
recent surveys for metal-poor stars, the HK survey of Beers and collaborators,
and the Hamburg/ESO Survey of Christlieb and collaborators, we also discuss the
MDF derived from the publicly available database of stellar spectra and
photometry contained in the third data release of the Sloan Digital Sky Survey
(SDSS DR-3). Even though the SDSS was not originally planned as a stellar
survey, significant numbers of stars have been observed to date -- DR-3
contains spectroscopy for over 70,000 stars, at least half of which are
suitable for abundance determinations. There are as many very metal-poor
([Fe/H] < -2.0) stars in DR-3 as have been obtained from all previous survey
efforts combined. We also discuss prospects for significant expansion of the
list of metal-poor stars to be obtained from the recently funded extension of
the SDSS, which includes the project SEGUE: Sloan Extension for Galactic
Understanding and Exploration.Comment: To appear in: From Lithium to Uranium: Elemental Tracers of Early
Cosmic Evolution, IAU Symposium 228, V. Hill, P. Francois & F. Primas, ed
Tracing Sagittarius Structure with SDSS and SEGUE Imaging and Spectroscopy
We show that the Sagittarius dwarf tidal stream can be traced with very red
K/M-giant stars selected from SDSS photometry. A subset of these stars are
spectroscopically confirmed with SEGUE and SDSS spectra, and the distance scale
of 2MASS and SDSS M giants is calibrated to the RR Lyrae distance scale. The
absolute g band magnitude of the K/M-giant stars at the tip of the giant branch
is M_g=-1.0. The line-of-sight velocities of the M giant and BHB stars that are
spatially coincident with the Sgr dwarf tidal stream are consistent with those
of previous authors, reinforcing the need for new models that can explain all
of the Sgr tidal debris stream observations. We estimate stellar densities
along the tidal tails that can be used to help constrain future models. The
K/M-giant, BHB, and F-turnoff stars in the lower surface brightness tidal
stream that is adjacent to the main leading Sgr dwarf tidal tail have
velocities and metallicities that are similar to those of the stars in the
leading tidal tail. The ratio of K/M giants to BHBs and BHBs to F-turnoff stars
are also similar for both branches of the leading tidal tail. We show that
there is an additional low-metallicity tidal stream near the Sgr trailing tidal
tail.Comment: 19 figures, accepted for publication in ApJ, references update
A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions
Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact
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