1,538 research outputs found
Neutrinos And Big Bang Nucleosynthesis
The early universe provides a unique laboratory for probing the frontiers of
particle physics in general and neutrino physics in particular. The primordial
abundances of the relic nuclei produced during the first few minutes of the
evolution of the Universe depend on the electron neutrinos through the
charged-current weak interactions among neutrons and protons (and electrons and
positrons and neutrinos), and on all flavors of neutrinos through their
contributions to the total energy density which regulates the universal
expansion rate. The latter contribution also plays a role in determining the
spectrum of the temperature fluctuations imprinted on the Cosmic Background
Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer
and helium-4 as a chronometer, the predictions of BBN and the CBR are compared
to observations. The successes of, as well as challenges to the standard models
of particle physics and cosmology are identified. While systematic
uncertainties may be the source of some of the current tensions, it could be
that the data are pointing the way to new physics. In particular, BBN and the
CBR are used to address the questions of whether or not the relic neutrinos
were fully populated in the early universe and, to limit the magnitude of any
lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129,
"Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O.
Botner, P. Carlson, P. O. Hulth, and T. Ohlsso
The DEEP2 Galaxy Redshift Survey: Discovery of Luminous, Metal-poor, Sta r-forming Galaxies at Redshifts z~0.7
We have discovered a sample of 17 metal-poor, yet luminous, star-forming
galaxies at redshifts z~0.7. They were selected from the initial phase of the
DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536
galaxies as those showing the temperature-sensitive [OIII]l4363 auroral line.
These rare galaxies have blue luminosities close to L*, high star formation
rates of 5 to 12 solar masses per year, and oxygen abundances of 1/3 to 1/10
solar. They thus lie significantly off the luminosity-metallicity relation
found previously for field galaxies with strong emission lines at redshifts
z~0.7. The prior surveys relied on indirect, empirical calibrations of the R23
diagnostic and the assumption that luminous galaxies are not metal-poor. Our
discovery suggests that this assumption is sometimes invalid. As a class, these
newly-discovered galaxies are: (1) more metal-poor than common classes of
bright emission-line galaxies at z~0.7 or at the present epoch; (2) comparable
in metallicity to z~3 Lyman Break Galaxies but less luminous; and (3)
comparable in metallicity to local metal-poor eXtreme Blue Compact Galaxies
(XBCGs), but more luminous. Together, the three samples suggest that the
most-luminous, metal-poor, compact galaxies become fainter over time.Comment: This is a .tgz file. It should create the following files: texto.tex,
tab1.tex, f1.eps and f2.eps. The LaTeX style used is emulateapj.cls, version
November 26, 2004. This submission is 5 pages long, one table and two
figures. To appear in ApJ
Chemical Evolution of Galaxies
Chemical evolution of galaxies brings together ideas on stellar evolution and
nucleosynthesis with theories of galaxy formation, star formation and galaxy
evolution, with all their associated uncertainties. In a new perspective
brought about by the Hubble Deep Field and follow-up investigations of global
star formation rates, diffuse background etc., it has become necessary to
consider the chemical composition of dark baryonic matter as well as that of
visible matter in galaxies.Comment: 6 pages, AAS LaTeX macros v5.0, Millennium Essay to appear in PASP,
Feb 200
Dissipative N - body code for galaxy evolution
The evolving galaxy is considered as a system of baryonic fragments embedded
into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to
gravitational and viscous interactions. Although the chemical evolution of each
separate fragment is treated in the frame of one -- zone close box model with
instantaneous recycling, its star formation (SF) activity is a function of mean
local gas density and, therefore, is strongly influenced by other interacting
fragments. In spite of its simplicity this model provides a realistic
description of the process of galaxy formation and evolution over the Hubble
timescale.Comment: 11 pages, LaTeX, 7 figures, using the article.sty, expected in
A&ApTr, 18, 83
Extent of pollution in planet-bearing stars
(abridged) Search for planets around main-sequence (MS) stars more massive
than the Sun is hindered by their hot and rapidly spinning atmospheres. This
obstacle has been sidestepped by radial-velocity surveys of those stars on
their post-MS evolutionary track (G sub-giant and giant stars). Preliminary
observational findings suggest a deficiency of short-period hot Jupiters around
the observed post MS stars, although the total fraction of them with known
planets appears to increase with their mass. Here we consider the possibility
that some very close- in gas giants or a population of rocky planets may have
either undergone orbital decay or been engulfed by the expanding envelope of
their intermediate-mass host stars. If such events occur during or shortly
after those stars' main sequence evolution when their convection zone remains
relatively shallow, their surface metallicity can be significantly enhanced by
the consumption of one or more gas giants. We show that stars with enriched
veneer and lower-metallicity interior follow slightly modified evolution tracks
as those with the same high surface and interior metallicity. As an example, we
consider HD149026, a marginal post MS 1.3 Msun star. We suggest that its
observed high (nearly twice solar) metallicity may be confined to the surface
layer as a consequence of pollution by the accretion of either a planet similar
to its known 2.7-day-period Saturn-mass planet, which has a 70 Mearth compact
core, or a population of smaller mass planets with a comparable total amount of
heavy elements. It is shown that an enhancement in surface metallicity leads to
a reduction in effective temperature, in increase in radius and a net decrease
in luminosity. The effects of such an enhancement are not negligible in the
determinations of the planet's radius based on the transit light curves.Comment: 25 pages, 8 figures, submitted to Ap
Ca II Triplet Spectroscopy of Giants in SMC Star Clusters: Abundances, Velocities and the Age-Metallicity Relation
We have obtained spectra at the Ca II triplet of individual red giants in
seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have
been used to determine mean abundances for six of the star clusters to a
typical precision of 0.12 dex. When combined with existing data for other
objects, the resulting SMC age-metallicity relation is generally consistent
with that for a simple model of chemical evolution, scaled to the present-day
SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and
NGC 339), however, have abundances that ~0.5 dex lower than that expected from
the mean age-metallicity relation. It is suggested that the formation of these
clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched
gas, perhaps from the Magellanic Stream. The spectra also yield radial
velocities for the seven clusters. The resulting velocity dispersion is 16 +/-
4 km/sec, consistent with those of the SMC planetary nebula and carbon star
populations.Comment: 28 pages including 4 figure
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Dynamic dark energy (DDE) models are often designed to solve the cosmic
coincidence (why, just now, is the dark energy density , the same
order of magnitude as the matter density ?) by guaranteeing for significant fractions of the age of the universe. This
typically entails ad-hoc tracking or oscillatory behaviour in the model.
However, such behaviour is neither sufficient nor necessary to solve the
coincidence problem. What must be shown is that a significant fraction of
observers see . Precisely when, and for how long, must a
DDE model have in order to solve the coincidence? We
explore the coincidence problem in dynamic dark energy models using the
temporal distribution of terrestrial-planet-bound observers. We find that any
dark energy model fitting current observational constraints on and
the equation of state parameters and , does have for a large fraction of observers in the universe. This demotivates DDE
models specifically designed to solve the coincidence using long or repeated
periods of .Comment: 16 pages, 8 figures, Submitted to Phys. Rev.
Remote participation during glycosylation reactions of galactose building blocks: Direct evidence from cryogenic vibrational spectroscopy
The stereoselective formation of 1,2âcisâglycosidic bonds is challenging. However, 1,2âcisâselectivity can be induced by remote participation of C4 or C6 ester groups. Reactions involving remote participation are believed to proceed via a key ionic intermediate, the glycosyl cation. Although mechanistic pathways were postulated many years ago, the structure of the reaction intermediates remained elusive owing to their shortâlived nature. Herein, we unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory. Acetyl groups at C4 ensure αâselective galactosylations by forming a covalent bond to the anomeric carbon in dioxoleniumâtype ions. Unexpectedly, also benzyl ether protecting groups can engage in remote participation and promote the stereoselective formation of 1,2âcisâglycosidic bonds
The Diffuse Gamma-Ray Background from Supernovae
The Cosmic Gamma-ray Background (CGB) in the MeV region is believed to be due
to photons from radioactivity produced in SNe throughout the history of
galaxies in the universe. In particular, gamma-ray line emission from the decay
chain 56Ni-> 56Co->56Fe provides the dominant photon source. Although iron
synthesis occurs in all types of SNe, the contribution to the CGB is dominated
by SNIa events due to their higher photon escape probabilities. Estimates of
the star formation history in the universe suggest a rapid increase by a factor
\~ 10 from the present to a redshift z_p ~ 1.5, beyond which it either remains
constant or decreases slowly. We integrate the observed star formation history
to determine the CGB from the corresponding SN rate history. In addition to
gamma-rays from short-lived radioactivity in SNIa and SNII/Ibc we also
calculate the minor contributions from long-lived radioactivities (26Al, 44Ti,
60Co, and electron-positron pair annihilation). Although progenitor evolution
for SNIa is not yet fully understood, various arguments suggest delays of order
1-2 Gy between star formation and the production of SNIa's. The effect of this
delay on the CGB is discussed. We emphasize the value of gamma-ray observations
of the CGB in the MeV range as an independent tool for studies of the cosmic
star formation history. If the delay between star formation and SNIa activity
exceeds 1 Gy substantially, and/or the peak of the cosmic star formation rate
occurs at a redshift much larger than unity, the gamma-ray production of SNIa
would be insufficient to explain the observed CGB. Alternatively, the cosmic
star formation rate would have to be higher (by a factor 2-3) than commonly
assumed, which is in accord with several upward revisions reported in the
recent literature.Comment: Minor changes, 26 pages, 9 figures, Accepted by Ap
Anti-microbial Use in Animals: How to Assess the Trade-offs
Antimicrobials are widely used in preventive and curative medicine in animals. Benefits from curative use are clear â it allows sick animals to be healthy with a gain in human welfare. The case for preventive use of antimicrobials is less clear cut with debates on the value of antimicrobials as growth promoters in the intensive livestock industries. The possible benefits from the use of antimicrobials need to be balanced against their cost and the increased risk of emergence of resistance due to their use in animals. The study examines the importance of animals in society and how the role and management of animals is changing including the use of antimicrobials. It proposes an economic framework to assess the trade-offs of anti-microbial use and examines the current level of data collection and analysis of these trade-offs. An exploratory review identifies a number of weaknesses. Rarely are we consistent in the frameworks applied to the economic assessment anti-microbial use in animals, which may well be due to gaps in data or the prejudices of the analysts. There is a need for more careful data collection that would allow information on (i) which species and production systems antimicrobials are used in, (ii) what active substance of antimicrobials and the application method and (iii) what dosage rates. The species need to include companion animals as well as the farmed animals as it is still not known how important direct versus indirect spread of resistance to humans is. In addition, research is needed on pricing antimicrobials used in animals to ensure that prices reflect production and marketing costs, the fixed costs of anti-microbial development and the externalities of resistance emergence. Overall, much work is needed to provide greater guidance to policy, and such work should be informed by rigorous data collection and analysis systems
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