402 research outputs found
The Explosive Yields Produced by the First Generation of Core Collapse Supernovae and the Chemical Composition of Extremely Metal Poor Stars
We present a detailed comparison between an extended set of elemental
abundances observed in some of the most metal poor stars presently known and
the ejecta produced by a generation of primordial core collapse supernovae. We
used five stars which form our initial database and define a "template" ultra
metal poor star which is then compared to the theoretical predictions. Our main
findings are as follows: a) the fit to [Si/Mg] and [Ca/Mg] of these very metal
poor stars seems to favor the presence of a rather large C abundance at the end
of the central He burning; in a classical scenario in which the border of the
convective core is strictly determined by the Schwarzschild criterion, such a
large C abundance would imply a rather low C12(alpha,gamma)O16 reaction rate;
b) a low C abundance left by the central He burning would imply a low [Al/Mg]
(<-1.2 dex) independently on the initial mass of the exploding star while a
rather large C abundance would produce such a low [Al/Mg] only for the most
massive stellar model; c) at variance with current beliefs that it is difficult
to interpret the observed overabundance of [Co/Fe], we find that a mildly large
C abundance in the He exhausted core (well within the present range of
uncertainty) easily and naturally allows a very good fit to [Co/Fe]; d) our
yields allow a reasonable fit to 8 out of the 11 available elemental
abundances; e) within the present grid of models it is not possible to find a
good match of the remaining three elements, Ti, Cr and Ni (even for an
arbitrary choice of the mass cut); f) the adoption of other yields available in
the literature does not improve the fit; g) since no mass in our grid provides
a satisfactory fit to these three elements, even an arbitrary choice of the
initial mass function would not improve their fit.Comment: 30 pages, 8 figures, 8 tables. Accepted for publication on Ap
Pre-suprenova evolution of rotating massive stars
The Geneva evolutionary code has been modified to study the advanced stages
(Ne, O, Si burnings) of rotating massive stars. Here we present the results of
four 20 solar mass stars at solar metallicity with initial rotational
velocities of 0, 100, 200 and 300 km/s in order to show the crucial role of
rotation in stellar evolution. As already known, rotation increases mass loss
and core masses (Meynet and Maeder 2000). A fast rotating 20 solar mass star
has the same central evolution as a non-rotating 26 solar mass star. Rotation
also increases strongly net total metal yields. Furthermore, rotation changes
the SN type so that more SNIb are predicted (see Meynet and Maeder 2003 and N.
Prantzos and S. Boissier 2003). Finally, SN1987A-like supernovae progenitor
colour can be explained in a single rotating star scenario.Comment: To appear in proceedings of IAU Colloquium 192, "Supernovae (10 years
of 1993J)", Valencia, Spain 22-26 April 2003, eds. J.M. Marcaide, K.W.
Weiler, 5 pages, 8 figure
Role of glutathionylation in infection and inflammation
Glutathionylation, that is, the formation of mixed disulfides between protein cysteines and
glutathione (GSH) cysteines, is a reversible post-translational modification catalyzed by dierent
cellular oxidoreductases, by which the redox state of the cell modulates protein function. So far, most
studies on the identification of glutathionylated proteins have focused on cellular proteins, including
proteins involved in host response to infection, but there is a growing number of reports showing
that microbial proteins also undergo glutathionylation, with modification of their characteristics and
functions. In the present review, we highlight the signaling role of GSH through glutathionylation,
particularly focusing on microbial (viral and bacterial) glutathionylated proteins (GSSPs) and host
GSSPs involved in the immune/inflammatory response to infection; moreover, we discuss the
biological role of the process in microbial infections and related host responses
Evolution and nucleosynthesis of primordial low mass stars
We discuss in detail the evolutionary properties of low mass stars M< 1
M(Solar) having metallicity lower than Z=10^(-6) from the pre- main sequence up
to (almost) the end of the Asymptotic Giant Branch phase. We also discuss the
possibility that the large [C,N/Fe] observed on the surface of the most Iron
poor star presently known, HE0107-5240, may be attributed to the autopollution
induced by the penetration of the He convective shell into the H rich mantle
during the He core flash of a low mass, very low metallicity star. On the basis
of a quite detailed analysis, we conclude that the autopollution scenario
cannot be responsible for the observed chemical composition of HE0107-5240
New Stationary Frame Control Scheme for Three Phase PWM Rectifiers Under Unbalanced Voltage Dips Conditions
A new stationary frame control scheme for three-phase pulsewidth-modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper. The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage ripple. This paper's novelty is the development of a new current-reference generator implemented directly in stationary reference frame. This allows using proportional sinusoidal signal integrator (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components. No phase-locked loop (PLL) strategies and coordinate transformations are needed for the proposed current-reference generator. Experimental results are presented for a 20-kV A alternative current (ac)/direct current (dc) converter prototype to demonstrate the effectiveness of the proposed control scheme. A comparison with two other existing control techniques is also performed. Fast dynamic performance with small dc-link voltage ripple and input sinusoidal currents are obtained with this control scheme, even under severe voltage dips operating conditions
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
Recent improvements in stellar models for intermediate-mass and massive stars
are recalled, together with their expectations for the synthesis of radioactive
nuclei of lifetime Myr, in order to re-examine the origins
of now extinct radioactivities, which were alive in the solar nebula. The
Galactic inheritance broadly explains most of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly Fe require
nucleosynthesis events close to the solar formation. We outline the persisting
difficulties to account for these nuclei by Intermediate Mass Stars (2
M/M). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in Pd. This is shown for two recent
approaches to Deep Mixing. Even a late contamination by a Massive Star meets
problems. In fact, inhomogeneous addition of Supernova debris predicts
non-measured excesses on stable isotopes. Revisions invoking specific low-mass
supernovae and/or the sequential contamination of the pre-solar molecular cloud
might be affected by similar problems, although our conclusions here are
weakened by our schematic approach to the addition of SN ejecta. The limited
parameter space remaining to be explored for solving this puzzle is discussed.Comment: Accepted for publication on Ap
The Chemical Evolution of Magnesium Isotopic Abundances in the Solar Neighbourhood
The abundance of the neutron-rich magnesium isotopes observed in metal-poor
stars is explained quantitatively with a chemical evolution model of the local
Galaxy that considers - for the first time - the metallicity-dependent
contribution from intermediate mass stars. Previous models that simulate the
variation of Mg isotopic ratios with metallicity in the solar neighbourhood
have attributed the production of Mg25 and Mg26 exclusively to hydrostatic
burning in massive stars.
These models match the data well for [Fe/H]>-1.0 but severely underestimate
Mg25/Mg24 and Mg26/Mg24 at lower metallicities. Earlier studies have noted that
this discrepancy may indicate a significant role played by intermediate-mass
stars. Only recently have detailed calculations of intermediate-mass stellar
yields of Mg25 and Mg26 become available with which to test this hypothesis. In
an extension of previous work, we present a model that successfully matches the
Mg isotopic abundances in nearby Galactic disk stars through the incorporation
of nucleosynthesis predictions of Mg isotopic production in asymptotic giant
branch stars.Comment: 9 pages, 6 figures, to appear in Publications of the Astronomical
Society of Australia (PASA) in 2003, vol. 20, No.
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
Progressive changes in descriptive discourse in First Episode Schizophrenia: a longitudinal computational semantics study
Computational semantics, a branch of computational linguistics, involves automated meaning analysis that relies on how words occur together in natural language. This offers a promising tool to study schizophrenia. At present, we do not know if these word-level choices in speech are sensitive to the illness stage (i.e., acute untreated vs. stable established state), track cognitive deficits in major domains (e.g., cognitive control, processing speed) or relate to established dimensions of formal thought disorder. In this study, we collected samples of descriptive discourse in patients experiencing an untreated first episode of schizophrenia and healthy control subjects (246 samples of 1-minute speech; n = 82, FES = 46, HC = 36) and used a co-occurrence based vector embedding of words to quantify semantic similarity in speech. We obtained six-month follow-up data in a subsample (99 speech samples, n = 33, FES = 20, HC = 13). At baseline, semantic similarity was evidently higher in patients compared to healthy individuals, especially when social functioning was impaired; but this was not related to the severity of clinically ascertained thought disorder in patients. Across the study sample, higher semantic similarity at baseline was related to poorer Stroop performance and processing speed. Over time, while semantic similarity was stable in healthy subjects, it increased in patients, especially when they had an increasing burden of negative symptoms. Disruptions in word-level choices made by patients with schizophrenia during short 1-min descriptions are sensitive to interindividual differences in cognitive and social functioning at first presentation and persist over the early course of the illness
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