3,189 research outputs found
How does ecological disturbance influence genetic diversity?
Environmental disturbance underpins the dynamics and diversity of many of the ecosystems of the world, yet its influence on the patterns and distribution of genetic diversity is poorly appreciated. We argue here that disturbance history may be the major
Stress Tensors for Instantaneous Vacua in 1+1 Dimensions
The regularized expectation value of the stress-energy tensor for a massless
bosonic or fermionic field in 1+1 dimensions is calculated explicitly for the
instantaneous vacuum relative to any Cauchy surface (here a spacelike curve) in
terms of the length L of the curve (if closed), the local extrinsic curvature K
of the curve, its derivative K' with respect to proper distance x along the
curve, and the scalar curvature R of the spacetime: T_{00} = - epsilon
pi/(6L^2) - K^2/(24 pi), T_{01} = - K'/(12 pi), T_{11} = - epsilon pi/(6L^2) -
K^2/(24 pi) + R/(24 pi), in an orthonormal frame with the spatial vector
parallel to the curve. Here epsilon = 1 for an untwisted (i.e., periodic in x)
one-component massless bosonic field or for a twisted (i.e., antiperiodic in x)
two-component massless fermionic field, epsilon = -1/2 for a twisted
one-component massless bosonic field, and epsilon = - 2 for an untwisted
two-component massless fermionic field. The calculation uses merely the
energy-momentum conservation law and the trace anomaly (for which a very simple
derivation is also given herein, as well as the expression for the Casimir
energy of bosonic and fermionic fields twisted by an arbitrary amount in
R^{D-1} x S^1). The two coordinate and conformal invariants of a quantum state
that are (nonlocally) determined by the stress-energy tensor are given.
Applications to topologically modified deSitter spacetimes, to a flat cylinder,
and to Minkowski spacetime are discussed.Comment: LaTeX, 28 pages, last term of Eq. (79) correcte
A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
We report on the unprecedented Red Supergiant (RSG) population of a massive
young cluster, located at the base of the Scutum-Crux Galactic arm. We identify
candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution
spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead
equivalent width and high-precision radial velocity measurements to identify a
core grouping of 26 physically-associated RSGs -- the largest such cluster
known to-date. Using the stars' velocity dispersion, and their inferred
luminosities in conjuction with evolutionary models, we argue that the cluster
has an initial mass of 40,000\msun, and is therefore among the most
massive in the galaxy. Further, the cluster is only a few hundred parsecs away
from the cluster of 14 RSGs recently reported by Figer et al (2006). These two
RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the
unique opportunity to study the pre-supernova evolution of massive stars, and
the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE,
MIPSGAL and MAGPIS survey data to identify several objects in the field of the
larger cluster which seem to be indicative of recent region-wide starburst
activity at the point where the Scutum-Crux arm intercepts the Galactic bulge.
Future abundance studies of these clusters will therefore permit the study of
the chemical evolution and metallicity gradient of the Galaxy in the region
where the disk meets the bulge.Comment: 49 pages, 22 figures. Accepted for publication in ApJ. Version with
hi-res figures can be found at http://www.cis.rit.edu/~bxdpci/RSGC2.pd
FusC, a member of the M16 protease family acquired by bacteria for iron piracy against plants.
Iron is essential for life. Accessing iron from the environment can be a limiting factor that determines success in a given environmental niche. For bacteria, access of chelated iron from the environment is often mediated by TonB-dependent transporters (TBDTs), which are β-barrel proteins that form sophisticated channels in the outer membrane. Reports of iron-bearing proteins being used as a source of iron indicate specific protein import reactions across the bacterial outer membrane. The molecular mechanism by which a folded protein can be imported in this way had remained mysterious, as did the evolutionary process that could lead to such a protein import pathway. How does the bacterium evolve the specificity factors that would be required to select and import a protein encoded on another organism's genome? We describe here a model whereby the plant iron-bearing protein ferredoxin can be imported across the outer membrane of the plant pathogen Pectobacterium by means of a Brownian ratchet mechanism, thereby liberating iron into the bacterium to enable its growth in plant tissues. This import pathway is facilitated by FusC, a member of the same protein family as the mitochondrial processing peptidase (MPP). The Brownian ratchet depends on binding sites discovered in crystal structures of FusC that engage a linear segment of the plant protein ferredoxin. Sequence relationships suggest that the bacterial gene encoding FusC has previously unappreciated homologues in plants and that the protein import mechanism employed by the bacterium is an evolutionary echo of the protein import pathway in plant mitochondria and plastids
Entropy and Uncertainty of Squeezed Quantum Open Systems
We define the entropy S and uncertainty function of a squeezed system
interacting with a thermal bath, and study how they change in time by following
the evolution of the reduced density matrix in the influence functional
formalism. As examples, we calculate the entropy of two exactly solvable
squeezed systems: an inverted harmonic oscillator and a scalar field mode
evolving in an inflationary universe. For the inverted oscillator with weak
coupling to the bath, at both high and low temperatures, , where r is
the squeeze parameter. In the de Sitter case, at high temperatures, where , being the coupling to the bath and H
the Hubble constant. These three cases confirm previous results based on more
ad hoc prescriptions for calculating entropy. But at low temperatures, the de
Sitter entropy is noticeably different. This result, obtained
from a more rigorous approach, shows that factors usually ignored by the
conventional approaches, i.e., the nature of the environment and the coupling
strength betwen the system and the environment, are important.Comment: 36 pages, epsfig, 2 in-text figures include
Susskind's Challenge to the Hartle-Hawking No-Boundary Proposal and Possible Resolutions
Given the observed cosmic acceleration, Leonard Susskind has presented the
following argument against the Hartle-Hawking no-boundary proposal for the
quantum state of the universe: It should most likely lead to a nearly empty
large de Sitter universe, rather than to early rapid inflation. Even if one
adds the condition of observers, they are most likely to form by quantum
fluctuations in de Sitter and therefore not see the structure that we observe.
Here I present my own amplified version of this argument and consider possible
resolutions, one of which seems to imply that inflation expands the universe to
be larger than 10^{10^{10^{122}}} Mpc.Comment: 24 pages, LaTeX, 8 references added and a distinction between Linde's
and Vilenkin's tunneling proposal
No-Bang Quantum State of the Cosmos
A quantum state of the entire cosmos (universe or multiverse) is proposed
which is the equal mixture of the Giddings-Marolf states that are
asymptotically single de Sitter spacetimes in both past and future and are
regular on the throat or neck of minimal three-volume. That is, states are
excluded that have a big bang or big crunch or which split into multiple
asymptotic de Sitter spacetimes. (For simplicity, transitions between different
values of the cosmological constant are assumed not to occur, though different
positive values are allowed.) The entropy of this mixed state appears to be of
the order of the three-fourths power of the Bekenstein-Hawking A/4 entropy of
de Sitter spacetime. Most of the component pure states do not have rapid
inflation, but when an inflaton is present and the states are weighted by the
volume at the end of inflation, a much smaller number of states may dominate
and give a large amount of inflation and hence may agree with observations.Comment: 18 pages, LaTeX, updated with a few new qualifications and reference
Diagnosis of Cystic Fibrosis in Screened Populations
Objective
Cystic fibrosis (CF) can be difficult to diagnose, even when newborn screening (NBS) tests yield positive results. This challenge is exacerbated by the multitude of NBS protocols, misunderstandings about screening vs diagnostic tests, and the lack of guidelines for presumptive diagnoses. There is also confusion regarding the designation of age at diagnosis.
Study design
To improve diagnosis and achieve standardization in definitions worldwide, the CF Foundation convened a committee of 32 experts with a mission to develop clear and actionable consensus guidelines on diagnosis of CF with an emphasis on screened populations, especially the newborn population. A comprehensive literature review was performed with emphasis on relevant articles published during the past decade.
Results
After reviewing the common screening protocols and outcome scenarios, 14 of 27 consensus statements were drafted that apply to screened populations. These were approved by 80% or more of the participants.
Conclusions
It is recommended that all diagnoses be established by demonstrating dysfunction of the CF transmembrane conductance regulator (CFTR) channel, initially with a sweat chloride test and, when needed, potentially with newer methods assessing membrane transport directly, such as intestinal current measurements. Even in babies with 2 CF-causing mutations detected via NBS, diagnosis must be confirmed by demonstrating CFTR dysfunction. The committee also recommends that the latest classifications identified in the Clinical and Functional Translation of CFTR project [http://www.cftr2.org/index.php] should be used to aid with CF diagnosis. Finally, to avoid delays in treatment, we provide guidelines for presumptive diagnoses and recommend how to determine the age of diagnosis
The cool supergiant population of the massive young star cluster RSGC1
We present new high-resolution near-IR spectroscopy and OH maser observations
to investigate the population of cool luminous stars of the young massive
Galactic cluster RSGC1. Using the 2.293\micron CO-bandhead feature, we make
high-precision radial velocity measurements of 16 of the 17 candidate Red
Supergiants (RSGs) identified by Figer et al. We show that F16 and F17 are
foreground stars, while we confirm that the rest are indeed
physically-associated RSGs. We determine that Star F15, also associated with
the cluster, is a Yellow Hypergiant based on its luminosity and spectroscopic
similarity to Cas. Using the cluster's radial velocity, we have derived
the kinematic distance to the cluster and revisited the stars' temperatures and
luminosities. We find a larger spread of luminosities than in the discovery
paper, consistent with a cluster age 30% older than previously thought
(122Myr), and a total initial mass of \msun. The
spatial coincidence of the OH maser with F13, combined with similar radial
velocities, is compelling evidence that the two are related. Combining our
results with recent SiO and HO maser observations, we find that those stars
with maser emission are the most luminous in the cluster. From this we suggest
that the maser-active phase is associated with the end of the RSG stage, when
the luminosity-mass ratios are at their highest.Comment: 31 pages, 11 figures. Accepted for publication in Ap
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
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