4,182 research outputs found
The elusive old population of the dwarf spheroidal galaxy Leo I
We report the discovery of a significant old population in the dwarf
spheroidal (dSph) galaxy Leo I as a result of a wide-area search with the ESO
New Technology Telescope. Studies of the stellar content of Local Group dwarf
galaxies have shown the presence of an old stellar population in almost all of
the dwarf spheroidals. The only exception was Leo I, which alone appeared to
have delayed its initial star formation episode until just a few Gyr ago. The
color-magnitude diagram of Leo I now reveals an extended horizontal branch,
unambiguously indicating the presence of an old, metal-poor population in the
outer regions of this galaxy. Yet we find little evidence for a stellar
population gradient, at least outside R > 2' (0.16 kpc), since the old
horizontal branch stars of Leo I are radially distributed as their more
numerous intermediate-age helium-burning counterparts. The discovery of a
definitely old population in the predominantly young dwarf spheroidal galaxy
Leo I points to a sharply defined first epoch of star formation common to all
of the Local Group dSph's as well as to the halo of the Milky Way.Comment: 4 pages, 3 postscript figures, uses apjfonts.sty, emulateapj.sty.
Accepted for publication in ApJ Letter
Discrete-Lattice Model for Surface Bound States and Tunneling in d-Wave Superconductors
Surface bound states in a discrete-lattice model of a cuprate
superconductor are shown to be, in general, coherent superpositions of an
incoming excitation and more than one outgoing excitation, and a simple
graphical construction based on a surface Brillouin zone is developed to
describe their nature. In addition, a momentum-dependent lifetime contribution
to the width of these bound states as observed in tunneling experiments is
derived and elucidated in physical terms.Comment: 4 pages, 1 figure, revte
Sexual reproduction is the null hypothesis for life cycles of rust fungi
Sexual reproduction, mutation, and reassortment of nuclei increase genotypic diversity in rust fungi. Sexual reproduction is inherent to rust fungi, coupled with their coevolved plant hosts in native pathosystems. Rust fungi are hypothesised to exchange nuclei by somatic hybridisation with an outcome of increased genotypic diversity, independent of sexual reproduction. We provide criteria to demonstrate whether somatic exchange has occurred, including knowledge of parental haplotypes and rejection of fertilisation in normal rust life cycles
Entropy in the NUT-Kerr-Newman Black Holes Due to an Arbitrary Spin Field
Membrane method is used to compute the entropy of the NUT-Kerr-Newman black
holes. It is found that even though the Euler characteristic is greater than
two, the Bekenstein-Hawking area law is still satisfied. The formula relating the entropy and the Euler characteristic becomes inapplicable for
non-extreme four dimensional NUT-Kerr-Newman black holes
Realistic Surface Scattering and Surface Bound State Formation in the High T_c Superconductor YBa_2Cu_3O_{6+x}
Surface Umklapp scattering of quasiparticles, and surface roughness are shown
to play essential roles in the formation of the surface bound states in
realistic models for YBa_2Cu_3O_{6+x}. The results account for the shape, the
impurity dependence of the height, and for a proposed universal width of the
zero bias conductance peak.Comment: 4 pages, 1 figur
Au-Ag template stripped pattern for scanning probe investigations of DNA arrays produced by Dip Pen Nanolithography
We report on DNA arrays produced by Dip Pen Nanolithography (DPN) on a novel
Au-Ag micro patterned template stripped surface. DNA arrays have been
investigated by atomic force microscopy (AFM) and scanning tunnelling
microscopy (STM) showing that the patterned template stripped substrate enables
easy retrieval of the DPN-functionalized zone with a standard optical
microscope permitting a multi-instrument and multi-technique local detection
and analysis. Moreover the smooth surface of the Au squares (abput 5-10
angstrom roughness) allows to be sensitive to the hybridization of the
oligonucleotide array with label-free target DNA. Our Au-Ag substrates,
combining the retrieving capabilities of the patterned surface with the
smoothness of the template stripped technique, are candidates for the
investigation of DPN nanostructures and for the development of label free
detection methods for DNA nanoarrays based on the use of scanning probes.Comment: Langmuir (accepted
Quantum Gravity and Turning Points in the Semiclassical Approximation
The wavefunctional in quantum gravity gives an amplitude for 3-geometries and
matter fields. The four-space is usually recovered in a semiclassical
approximation where the gravity variables are taken to oscillate rapidly
compared to matter variables; this recovers the Schrodinger evolution for the
matter. We examine turning points in the gravity variables where this
approximation appears to be troublesome. We investigate the effect of such a
turning point on the matter wavefunction, in simple quantum mechanical models
and in a closed minisuperspace cosmology. We find that after evolving
sufficiently far from the turning point the matter wavefunction recovers to a
form close to that predicted by the semiclassical approximation, and we compute
the leading correction (from `backreaction') in a simple model. We also show
how turning points can appear in the gravitational sector in dilaton gravity.
We give some remarks on the behavior of the wavefunctional in the vicinity of
turning points in the context of dilaton gravity black holes.Comment: 32 pages, 3 Postscript figures, uses epsf.tex and fps.sty, some
discussion, references and Acknowledgements added, version to appear in Phys.
Rev.
The Physics of Maxwell's demon and information
Maxwell's demon was born in 1867 and still thrives in modern physics. He
plays important roles in clarifying the connections between two theories:
thermodynamics and information. Here, we present the history of the demon and a
variety of interesting consequences of the second law of thermodynamics, mainly
in quantum mechanics, but also in the theory of gravity. We also highlight some
of the recent work that explores the role of information, illuminated by
Maxwell's demon, in the arena of quantum information theory.Comment: 24 pages, 13 figures. v2: some refs added, figs improve
Structure Formation, Melting, and the Optical Properties of Gold/DNA Nanocomposites: Effects of Relaxation Time
We present a model for structure formation, melting, and optical properties
of gold/DNA nanocomposites. These composites consist of a collection of gold
nanoparticles (of radius 50 nm or less) which are bound together by links made
up of DNA strands. In our structural model, the nanocomposite forms from a
series of Monte Carlo steps, each involving reaction-limited cluster-cluster
aggregation (RLCA) followed by dehybridization of the DNA links. These links
form with a probability which depends on temperature and particle
radius . The final structure depends on the number of monomers (i. e. gold
nanoparticles) , , and the relaxation time. At low temperature, the
model results in an RLCA cluster. But after a long enough relaxation time, the
nanocomposite reduces to a compact, non-fractal cluster. We calculate the
optical properties of the resulting aggregates using the Discrete Dipole
Approximation. Despite the restructuring, the melting transition (as seen in
the extinction coefficient at wavelength 520 nm) remains sharp, and the melting
temperature increases with increasing as found in our previous
percolation model. However, restructuring increases the corresponding link
fraction at melting to a value well above the percolation threshold. Our
calculated extinction cross section agrees qualitatively with experiments on
gold/DNA composites. It also shows a characteristic ``rebound effect,''
resulting from incomplete relaxation, which has also been seen in some
experiments. We discuss briefly how our results relate to a possible sol-gel
transition in these aggregates.Comment: 12 pages, 10 figure
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