85 research outputs found
Constructing Dirac linear fermions in terms of non-linear Heisenberg spinors
We show that the massive (or massless) neutrinos can be described as special
states of Heisenberg nonlinear spinors. As a by-product of this decomposition a
particularly attractive consequence appears: the possibility of relating the
existence of only three species of mass-less neutrinos to such internal
non-linear structure. At the same time it allows the possibility that neutrino
oscillation can occurs even for massless neutrinos
Fundamental Strings, Holography, and Nonlinear Superconformal Algebras
We discuss aspects of holography in the AdS_3 \times S^p near string geometry
of a collection of straight fundamental heterotic strings. We use anomalies and
symmetries to determine general features of the dual CFT. The symmetries
suggest the appearance of nonlinear superconformal algebras, and we show how
these arise in the framework of holographic renormalization methods. The
nonlinear algebras imply intricate formulas for the central charge, and we show
that in the bulk these correspond to an infinite series of quantum gravity
corrections. We also makes some comments on the worldsheet sigma-model for
strings on AdS_3\times S^2, which is the holographic dual geometry of parallel
heterotic strings in five dimensions.Comment: 25 page
Alcohol Schema Acquisition in Preschoolers: Differences Between Children of Alcoholics and Children of Nonalcoholics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65209/1/j.1530-0277.1995.tb00982.x.pd
The origin of dust in galaxies revisited: the mechanism determining dust content
The origin of cosmic dust is a fundamental issue in planetary science. This
paper revisits the origin of dust in galaxies, in particular, in the Milky Way,
by using a chemical evolution model of a galaxy composed of stars, interstellar
medium, metals (elements heavier than helium), and dust. We start from a review
of time-evolutionary equations of the four components, and then, we present
simple recipes for the stellar remnant mass and yields of metal and dust based
on models of stellar nucleosynthesis and dust formation. After calibrating some
model parameters with the data from the solar neighborhood, we have confirmed a
shortage of the stellar dust production rate relative to the dust destruction
rate by supernovae if the destruction efficiency suggested by theoretical works
is correct. If the dust mass growth by material accretion in molecular clouds
is active, the observed dust amount in the solar neighborhood is reproduced. We
present a clear analytic explanation of the mechanism for determining dust
content in galaxies after the activation of accretion growth: a balance between
accretion growth and supernova destruction. Thus, the dust content is
independent of the uncertainty of the stellar dust yield after the growth
activation. The timing of the activation is determined by a critical metal mass
fraction which depends on the growth and destruction efficiencies. The solar
system formation seems to have occurred well after the activation and plenty of
dust would have existed in the proto-solar nebula.Comment: 12 pages, 11 figure
Cold gas accretion in galaxies
Evidence for the accretion of cold gas in galaxies has been rapidly
accumulating in the past years. HI observations of galaxies and their
environment have brought to light new facts and phenomena which are evidence of
ongoing or recent accretion:
1) A large number of galaxies are accompanied by gas-rich dwarfs or are
surrounded by HI cloud complexes, tails and filaments. It may be regarded as
direct evidence of cold gas accretion in the local universe. It is probably the
same kind of phenomenon of material infall as the stellar streams observed in
the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI
have been found in nearby spiral galaxies. While a large fraction of this gas
is produced by galactic fountains, it is likely that a part of it is of
extragalactic origin. 3) Spirals are known to have extended and warped outer
layers of HI. It is not clear how these have formed, and how and for how long
the warps can be sustained. Gas infall has been proposed as the origin. 4) The
majority of galactic disks are lopsided in their morphology as well as in their
kinematics. Also here recent accretion has been advocated as a possible cause.
In our view, accretion takes place both through the arrival and merging of
gas-rich satellites and through gas infall from the intergalactic medium (IGM).
The infall may have observable effects on the disk such as bursts of star
formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold
gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates
needed to sustain the observed star formation (~1 Msol/yr), additional infall
of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages.
Full-resolution version available at
http://www.astron.nl/~oosterlo/accretionRevie
The New Galaxy: Signatures of its Formation
The formation and evolution of galaxies is one of the great outstanding
problems of astrophysics. Within the broad context of hierachical structure
formation, we have only a crude picture of how galaxies like our own came into
existence. A detailed physical picture where individual stellar populations can
be associated with (tagged to) elements of the protocloud is far beyond our
current understanding. Important clues have begun to emerge from both the
Galaxy (near-field cosmology) and the high redshift universe (far-field
cosmology). Here we focus on the fossil evidence provided by the Galaxy.
Detailed studies of the Galaxy lie at the core of understanding the complex
processes involved in baryon dissipation. This is a necessary first step
towards achieving a successful theory of galaxy formation.Comment: 51 pages (with figs embedded) + 4 colour plates. The interested
reader is strongly encouraged to ignore the latex version and low res figures
within; instead, download the properly typeset paper (6 Mby) and colour
plates (3 Mby) from ftp://www.aao.gov.au/pub/local/jbh/araa/Galley
Gas flows, star formation and galaxy evolution
In the first part of this article we show how observations of the chemical
evolution of the Galaxy: G- and K-dwarf numbers as functions of metallicity,
and abundances of the light elements, D, Li, Be and B, in both stars and the
interstellar medium (ISM), lead to the conclusion that metal poor HI gas has
been accreting to the Galactic disc during the whole of its lifetime, and is
accreting today at a measurable rate, ~2 Msun per year across the full disc.
Estimates of the local star formation rate (SFR) using methods based on stellar
activity, support this picture. The best fits to all these data are for models
where the accretion rate is constant, or slowly rising with epoch. We explain
here how this conclusion, for a galaxy in a small bound group, is not in
conflict with graphs such as the Madau plot, which show that the universal SFR
has declined steadily from z=1 to the present day. We also show that a model in
which disc galaxies in general evolve by accreting major clouds of low
metallicity gas from their surroundings can explain many observations, notably
that the SFR for whole galaxies tends to show obvious variability, and
fractionally more for early than for late types, and yields lower dark to
baryonic matter ratios for large disc galaxies than for dwarfs. In the second
part of the article we use NGC 1530 as a template object, showing from
Fabry-Perot observations of its Halpha emission how strong shear in this
strongly barred galaxy acts to inhibit star formation, while compression acts
to stimulate it.Comment: 20 pages, 10 figures, to be presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, Eds. D.L. Block, K.C. Freeman, I. Puerari, & R. Groes
A novel multiplex assay for simultaneous quantification of total and S129 phosphorylated human alpha-synuclein
Host Phylogeny Determines Viral Persistence and Replication in Novel Hosts
Pathogens switching to new hosts can result in the emergence of new infectious diseases, and determining which species are likely to be sources of such host shifts is essential to understanding disease threats to both humans and wildlife. However, the factors that determine whether a pathogen can infect a novel host are poorly understood. We have examined the ability of three host-specific RNA-viruses (Drosophila sigma viruses from the family Rhabdoviridae) to persist and replicate in 51 different species of Drosophilidae. Using a novel analytical approach we found that the host phylogeny could explain most of the variation in viral replication and persistence between different host species. This effect is partly driven by viruses reaching a higher titre in those novel hosts most closely related to the original host. However, there is also a strong effect of host phylogeny that is independent of the distance from the original host, with viral titres being similar in groups of related hosts. Most of this effect could be explained by variation in general susceptibility to all three sigma viruses, as there is a strong phylogenetic correlation in the titres of the three viruses. These results suggest that the source of new emerging diseases may often be predictable from the host phylogeny, but that the effect may be more complex than simply causing most host shifts to occur between closely related hosts
Arginine Metabolism by Macrophages Promotes Cardiac and Muscle Fibrosis in mdx Muscular Dystrophy
Duchenne muscular dystrophy (DMD) is the most common, lethal disease of childhood. One of 3500 new-born males suffers from this universally-lethal disease. Other than the use of corticosteroids, little is available to affect the relentless progress of the disease, leading many families to use dietary supplements in hopes of reducing the progression or severity of muscle wasting. Arginine is commonly used as a dietary supplement and its use has been reported to have beneficial effects following short-term administration to mdx mice, a genetic model of DMD. However, the long-term effects of arginine supplementation are unknown. This lack of knowledge about the long-term effects of increased arginine metabolism is important because elevated arginine metabolism can increase tissue fibrosis, and increased fibrosis of skeletal muscles and the heart is an important and potentially life-threatening feature of DMD.We use both genetic and nutritional manipulations to test whether changes in arginase metabolism promote fibrosis and increase pathology in mdx mice. Our findings show that fibrotic lesions in mdx muscle are enriched with arginase-2-expressing macrophages and that muscle macrophages stimulated with cytokines that activate the M2 phenotype show elevated arginase activity and expression. We generated a line of arginase-2-null mutant mdx mice and found that the mutation reduced fibrosis in muscles of 18-month-old mdx mice, and reduced kyphosis that is attributable to muscle fibrosis. We also observed that dietary supplementation with arginine for 17-months increased mdx muscle fibrosis. In contrast, arginine-2 mutation did not reduce cardiac fibrosis or affect cardiac function assessed by echocardiography, although 17-months of dietary supplementation with arginine increased cardiac fibrosis. Long-term arginine treatments did not decrease matrix metalloproteinase-2 or -9 or increase the expression of utrophin, which have been reported as beneficial effects of short-term treatments.Our findings demonstrate that arginine metabolism by arginase promotes fibrosis of muscle in muscular dystrophy and contributes to kyphosis. Our findings also show that long-term, dietary supplementation with arginine exacerbates fibrosis of dystrophic heart and muscles. Thus, commonly-practiced dietary supplementation with arginine by DMD patients has potential risk for increasing pathology when performed for long periods, despite reports of benefits acquired with short-term supplementation
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