484 research outputs found
The metal and dust yields of the first massive stars
We quantify the role of Population (Pop) III core-collapse supernovae (SNe)
as the first cosmic dust polluters. Starting from a homogeneous set of stellar
progenitors with masses in the range [13 - 80] Msun, we find that the mass and
composition of newly formed dust depend on the mixing efficiency of the ejecta
and the degree of fallback experienced during the explosion. For standard Pop
III SNe, whose explosions are calibrated to reproduce the average elemental
abundances of Galactic halo stars with [Fe/H] < -2.5, between 0.18 and 3.1 Msun
(0.39 - 1.76 Msun) of dust can form in uniformly mixed (unmixed) ejecta, and
the dominant grain species are silicates. We also investigate dust formation in
the ejecta of faint Pop III SN, where the ejecta experience a strong fallback.
By examining a set of models, tailored to minimize the scatter with the
abundances of carbon-enhanced Galactic halo stars with [Fe/H ] < -4, we find
that amorphous carbon is the only grain species that forms, with masses in the
range 2.7 10^{-3} - 0.27 Msun (7.5 10^{-4} - 0.11 Msun) for uniformly mixed
(unmixed) ejecta models. Finally, for all the models we estimate the amount and
composition of dust that survives the passage of the reverse shock, and find
that, depending on circumstellar medium densities, between 3 and 50% (10 - 80%)
of dust produced by standard (faint) Pop III SNe can contribute to early dust
enrichment.Comment: Accepted by MNRAS, 22 pages, 12 figures, 12 table
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
Discovery Prospects for a Supernova Signature of Biogenic Origin
Approximately 2.8 Myr before the present our planet was subjected to the
debris of a supernova explosion. The terrestrial proxy for this event was the
discovery of live atoms of 60Fe in a deep-sea ferromanganese crust. The
signature for this supernova event should also reside in magnetite Fe3O4
microfossils produced by magnetotactic bacteria extant at the time of the
Earth-supernova interaction, provided the bacteria preferentially uptake iron
from fine-grained iron oxides and ferric hydroxides. Using estimates for the
terrestrial supernova 60Fe flux, combined with our empirically derived
microfossil concentrations in a deep-sea drill core, we deduce a conservative
estimate of the ^{60}{Fe} fraction as 60Fe/Fe ~ 3.6 x 10^{-15}. This value sits
comfortably within the sensitivity limit of present accelerator mass
spectrometry capabilities. The implication is that a biogenic signature of this
cosmic event is detectable in the Earth's fossil record.Comment: As it appears in Icaru
Early metal enrichment in high-redshift quasars
Quasars are powerful systems whose spectrum is rich of metal features that
allow us to investigate the chemical evolution of galaxies at very high
redshift, even close to the reionization epoch. I review the main observational
constraints on the metallicity of quasars host galaxies at high redshift and
discuss the implications and issues for models of galaxy evolution in the early
universe.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe
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
Knowing when to stop: Aberrant precision and evidence accumulation in schizophrenia
Predictive coding and active inference formulations of the dysconnection hypothesis suggest that subjects with schizophrenia (SZ) hold unduly precise prior beliefs to compensate for a failure of sensory attenuation. This implies that SZ subjects should both initiate responses prematurely during evidence-accumulation tasks and fail to inhibit their responses at long stop-signal delays. SZ and healthy control subjects were asked to report the timing of billiards-ball collisions and were occasionally required to withhold their responses. SZ subjects showed larger temporal estimation errors, which were associated with premature responses and decreased response inhibition. To account for these effects, we used hierarchical (Bayesian) drift-diffusion models (HDDM) and model selection procedures to adjudicate among four hypotheses. HDDM revealed that the precision of prior beliefs (i.e., starting point) rather than increased sensory precision (i.e., drift rate) drove premature responses and impaired response inhibition in patients with SZ. From the perspective of active inference, we suggest that premature predictions in SZ are responses that, heuristically, are traded off against accuracy to ensure action execution. On the basis of previous work, we suggest that the right insular cortex might mediate this trade-off
Isochrones and Luminosity Functions for Old White Dwarfs
Using a new grid of models of cooling white dwarfs, we calculate isochrones
and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for
systems containing old white dwarfs. These new models incorporate a non-grey
atmosphere which is necessary to properly describe the effects of molecular
opacity at the cool temperatures of old white dwarfs. The various functions
calculated and extensively tabulated and plotted are meant to be as utilitarian
as possible for observers so all results are listed in quantities that
observers will obtain. The tables and plots developed should eventually prove
critical in interpreting the results of HST's Advanced Camera observations of
the oldest white dwarfs in nearby globular clusters, in understanding the
results of searches for old white dwarfs in the Galactic halo, and in
determining ages for star clusters of all ages using white dwarfs. As a
practical application we demonstrate the use of these results by deriving the
white dwarf cooling age of the old Galactic cluster M67.Comment: 7 pages, 8 tables, accepted for publication in the Astrophysical
Journa
Supernova dust yields: the role of metallicity, rotation, and fallback
Supernovae (SNe) are considered to have a major role in dust enrichment of
high redshift galaxies and, due to the short lifetimes of interstellar grains,
in dust replenishment of local galaxies. Here we explore how SN dust yields
depend on the mass, metallicity, and rotation rate of the progenitor stars, and
on the properties of the explosion. To this aim, assuming uniform mixing inside
the ejecta, we quantify the dust mass produced by a sample of SN models with
progenitor masses , metallicity , rotation rate and ~km/s, that
explode with a fixed energy of ~erg (FE models) or with
explosion properties calibrated to reproduce the - relation
inferred from SN observations (CE models). We find that rotation favours more
efficient dust production, particularly for more massive, low metallicity
stars, but that metallicity and explosion properties have the largest effects
on the dust mass and its composition. In FE models, SNe with are more efficient at forming dust: between 0.1 and 1 is
formed in a single explosion, with a composition dominated by silicates, carbon
and magnetite grains when , and by carbon and magnetite grains
when . In CE models, the ejecta are massive and metal-rich and
dust production is more efficient. The dust mass increases with and it is
dominated by silicates, at all [Fe/H].Comment: MNRAS, in pres
Biohydrogen from microalgae: Production and applications
The need to safeguard our planet by reducing carbon dioxide emissions has led to a significant development of research in the field of alternative energy sources. Hydrogen has proved to be the most promising molecule, as a fuel, due to its low environmental impact. Even if various methods already exist for producing hydrogen, most of them are not sustainable. Thus, research focuses on the biological sector, studying microalgae, and other microorganisms’ ability to produce this precious molecule in a natural way. In this review, we provide a description of the biochemical and molecular processes for the production of biohydrogen and give a general overview of one of the most interesting technologies in which hydrogen finds application for electricity production: fuel cells
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