414 research outputs found
s/alpha/Fe Abundance Ratios in Halo Field Stars: Is there a Globular Cluster Connection?
We try to understand the s- and r-process elements vs Ti/Fe plots derived by
Jehin et al. (1999) for mildly metal-poor stars within the framework of the
analytical semi-empirical models for these elements by Pagel & Tautvaisiene
(1995, 1997). Jehin et al. distinguished two Pop II subgroups: IIa with
alpha/Fe and s-elements/Fe increasing together, which they attribute to pure
SNII activity, and IIb with constant alpha/Fe and a range in s/Fe which they
attribute to a prolonged accretion phase in parent globular clusters. However,
their sample consists mainly of thick-disk stars with only 4 clear halo
members, of which two are `anomalous' in the sense defined by Nissen & Schuster
(1997). Only the remaining two halo stars (and one in Nissen & Schuster's
sample) depart significantly from Y/Ti (or s/alpha) ratios predicted by our
model.Comment: 6 pages, 5 figures To appear in: Roma-Trieste Workshop 1999: `The
Chemical Evolution of the Milky Way: Stars vs Clusters', Vulcano Sept. 1999.
F. Giovanelli & F. Matteucci (eds), Kluwer, Dordrech
Observational Constraints to the Evolution of Massive Stars
We consider some aspects of the evolution of massive stars which can only be
elucidated by means of "indirect" observations, i.e. measurements of the
effects of massive stars on their environments. We discuss in detail the early
evolution of massive stars formed in high metallicity regions as inferred from
studies of HII regions in external galaxies.Comment: 6 pages, 1 figure; Invited Paper presented at the Roma-Trieste
Workshop 1999 "The Chemical Evolution of the Milky Way: Stars versus
Clusters", Vulcano Island (ME, Italy), 20-24 September, 1999, eds. F.
Giovannelli & F. Matteucci, Kluwer-Holland (in press
Controlling passively-quenched single photon detectors by bright light
Single photon detectors based on passively-quenched avalanche photodiodes can
be temporarily blinded by relatively bright light, of intensity less than a
nanowatt. I describe a bright-light regime suitable for attacking a quantum key
distribution system containing such detectors. In this regime, all single
photon detectors in the receiver Bob are uniformly blinded by continuous
illumination coming from the eavesdropper Eve. When Eve needs a certain
detector in Bob to produce a click, she modifies polarization (or other
parameter used to encode quantum states) of the light she sends to Bob such
that the target detector stops receiving light while the other detector(s)
continue to be illuminated. The target detector regains single photon
sensitivity and, when Eve modifies the polarization again, produces a single
click. Thus, Eve has full control of Bob and can do a successful
intercept-resend attack. To check the feasibility of the attack, 3 different
models of passively-quenched detectors have been tested. In the experiment, I
have simulated the intensity diagrams the detectors would receive in a real
quantum key distribution system under attack. Control parameters and side
effects are considered. It appears that the attack could be practically
possible.Comment: Experimental results from a third detector model added. Minor
corrections and edits made. 11 pages, 10 figure
CRISPR-Cas system:A new paradigm for bacterial stress response through genome rearrangement
Bacteria can receive genetic material from other bacteria or invading bacteriophages primarily through horizontal gene transfer. These genetic exchanges can result in genome rearrangement and the acquisition of novel traits that assist cells with stresses and adverse environmental conditions. Bacteria have a relatively small genome with >90% of sequences consisting of protein coding genes, stable RNA biomolecules, and gene regulatory sequences. The remaining genome fraction is primarily large repeat elements, such as retrotransposons, interspersed repeat elements, insertion sequences, and the more recently discovered clustered regularly interspaced short palindromic repeats (CRISPRs), with CRISPR-associated gene sequences (cas) that code for various Cas proteins. The CRISPR genetic locus is a series of direct repeats that are interspersed by unique spacer sequences. These unique spacer sequences represent signatures of bacteriophage genomes as the "working memory" for a bacterium to identify and destroy an invading phage genome that has previously infected the host. The protective function of the CRISPR-Cas systems are found in âŒ40% of sequenced bacterial genomes, and it is often defined as bacterial acquired immunity. This chapter will elaborate the origin, structure, and function of CRISPR-Cas genetic systems acquired by bacteria, and their role in adaptive fitness while being subjected to environmental stress conditions
Atherogenic Lipid Stress Induces Platelet Hyperactivity Through CD36-Mediated Hyposensitivity To Prostacyclin-; The Role Of Phosphodiesterase 3A
Prostacyclin (PGI2) controls platelet activation and thrombosis through a cyclic adenosine monophosphate (cAMP) signalling cascade. However, in patients with cardiovascular diseases this protective mechanism fails for reasons that are unclear. Using both pharmacological and genetic approaches we describe a mechanism by which oxidised low density lipoproteins (oxLDL) associated with dyslipidaemia promote platelet activation through impaired PGI2 sensitivity and diminished cAMP signalling. In functional assays using human platelets, oxLDL modulated the inhibitory effects of PGI2, but not a PDE-insensitive cAMP analogue, on platelet aggregation, granule secretion and in vitro thrombosis. Examination of the mechanism revealed that oxLDL promoted the hydrolysis of cAMP through the phosphorylation and activation of phosphodiesterase 3A (PDE3A), leading to diminished cAMP signalling. PDE3A activation by oxLDL required Src family kinases, Syk and protein kinase C. The effects of oxLDL on platelet function and cAMP signalling were blocked by pharmacological inhibition of CD36, mimicked by CD36-specific oxidised phospholipids and ablated in CD36-/- murine platelets. The injection of oxLDL into wild type mice strongly promoted FeCl3 induced carotid thrombosis in vivo, which was prevented by pharmacological inhibition of PDE3A. Furthermore, blood from dyslipidaemic mice was associated with increased oxidative lipid stress, reduced platelet sensitivity to PGI2 ex vivo and diminished PKA signalling. In contrast, platelet sensitivity to a PDE-resistant cAMP analogue remained normal. Genetic deletion of CD36, protected dyslipidaemic animals from PGI2 hyposensitivity and restored PKA signalling. These data suggest that CD36 can translate atherogenic lipid stress into platelet hyperactivity through modulation of inhibitory cAMP signalling. â
The effects of a Variable IMF on the Chemical Evolution of the Galaxy
In this work we explore the effects of adopting an initial mass function
(IMF) variable in time on the chemical evolution of the Galaxy. In order to do
that we adopt a chemical evolution model which assumes two main infall episodes
for the formation of the Galaxy. We study the effects on such a model of
different IMFs. First, we use a theoretical one based on the statistical
description of the density field arising from random motions in the gas. This
IMF is a function of time as it depends on physical conditions of the site of
star formation. We also investigate the behaviour of the model predictions
using other variable IMFs, parameterized as a function of metallicity. Our
results show that the theoretical IMF when applied to our model depends on time
but such time variation is important only in the early phases of the Galactic
evolution, when the IMF is biased towards massive stars. We also show that the
use of an IMF which is a stronger function of time does not lead to a good
agreement with the observational constraints suggesting that if the IMF varied
this variation should have been small. Our main conclusion is that the G-dwarf
metallicity distribution is best explained by infall with a large timescale and
a constant IMF, since it is possible to find variable IMFs of the kind studied
here, reproducing the G-dwarf metallicity but this worsens the agreement with
other observational constraints.Comment: 7 pages, to appear in "The Chemical Evolution of the Milky Way: Stars
vs Clusters", Vulcano, September 1999, F. Giovannelli and F. Matteucci eds.
(Kluwer, Dordrecht) in pres
The mixed problem for the Laplacian in Lipschitz domains
We consider the mixed boundary value problem or Zaremba's problem for the
Laplacian in a bounded Lipschitz domain in R^n. We specify Dirichlet data on
part of the boundary and Neumann data on the remainder of the boundary. We
assume that the boundary between the sets where we specify Dirichlet and
Neumann data is a Lipschitz surface. We require that the Neumann data is in L^p
and the Dirichlet data is in the Sobolev space of functions having one
derivative in L^p for some p near 1. Under these conditions, there is a unique
solution to the mixed problem with the non-tangential maximal function of the
gradient of the solution in L^p of the boundary. We also obtain results with
data from Hardy spaces when p=1.Comment: Version 5 includes a correction to one step of the main proof. Since
the paper appeared long ago, this submission includes the complete paper,
followed by a short section that gives the correction to one step in the
proo
A High Deuterium Abundance at z=0.7
Of the light elements, the primordial abundance of deuterium, (D/H)_p,
provides the most sensitive diagnostic for the cosmological mass density
parameter Omega_B. Recent high redshift (D/H) measurements are highly
discrepant, although this may reflect observational uncertainties. The larger
(D/H) values, which imply a low Omega_B and require the Universe to be
dominated by non-baryonic matter (dynamical studies indicate a higher total
density parameter), cause problems for galactic chemical evolution models since
they have difficulty in reproducing the large decline down to the lower
present-day (D/H). Conversely, low (D/H) values imply an Omega_B greater than
derived from ^7Li and ^4He abundance measurements, and may require a deuterium
abundance evolution that is too low to easily explain. Here we report the first
measurement at intermediate redshift, where the observational difficulties are
smaller, of a gas cloud with ideal characteristics for this experiment. Our
analysis of the z = 0.7010 absorber toward 1718+4807 indicates (D/H) = 2.0 +/-
0.5 x 10^{-4} which is in the high range. This and other independent
observations suggests there may be a cosmological inhomogeneity in (D/H)_p of
at least a factor of ten.Comment: 6 pages, 1 figur
The Cosmological Baryon Density from the Deuterium Abundance at a redshift z = 3.57
We present a measurement of the deuterium to hydrogen ratio in a quasar
absorption system at redshift z = 3.57 towards QSO 1937-1009. We use a two
component fit, with redshifts determined from unsaturated metal lines, to fit
the hydrogen and deuterium features simultaneously. We find a low value of D/H
= 2.3 \pm 0.6 \times 10^{-5}, which does not agree with other measurements of
high D/H (Songaila et al. 1994, Carswell et al. 1994). The absorption system is
very metal poor, with metallicities less than 1/100 solar. Standard models of
chemical evolution show the astration of deuterium is limited to a few percent
from primordial for systems this metal-poor, so we believe our value represents
the primordial one. Using predictions of standard big-bang nucleosynthesis and
measurements of the cosmic microwave background, our measurement gives the
density of baryons in units of the critical density, , where H_0 = 100 h km s^{-1] Mpc^{-1}.Comment: 10 pages, 2 Figures, also available at http://nately.ucsd.edu/ ;
submitted to Natur
Very Cold Gas and Dark Matter
We have recently proposed a new candidate for baryonic dark matter: very cold
molecular gas, in near-isothermal equilibrium with the cosmic background
radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed
in a fractal structure, resembling the hierarchical structure of the detected
interstellar medium.
We present some perspectives of detecting this very cold gas, either directly
or indirectly. The H molecule has an "ultrafine" structure, due to the
interaction between the rotation-induced magnetic moment and the nuclear spins.
But the lines fall in the km domain, and are very weak. The best opportunity
might be the UV absorption of H in front of quasars. The unexpected cold
dust component, revealed by the COBE/FIRAS submillimetric results, could also
be due to this very cold H gas, through collision-induced radiation, or
solid H grains or snowflakes. The -ray distribution, much more
radially extended than the supernovae at the origin of cosmic rays
acceleration, also points towards and extended gas distribution.Comment: 16 pages, Latex pages, crckapb macro, 3 postscript figures, uuencoded
compressed tar file. To be published in the proceeedings of the
"Dust-Morphology" conference, Johannesburg, 22-26 January, 1996, D. Block
(ed.), (Kluwer Dordrecht
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