2,666 research outputs found
GRB progenitors at low metallicities
We calculated pre-supernova evolution models of single rotating massive
stars. These models reproduce observations during the early stages of the
evolution very well, in particular Wolf--Rayet (WR) populations and ratio
between type II and type Ib,c supernovae at different metallicities (Z). Using
these models we found the following results concerning long and soft gamma--ray
burst (GRB) progenitors:
- GRBs coming from WO--type (SNIc) WR stars are only produced at low Z (LMC
or lower).
- The upper metallicity limit for GRBs is reduced to Z ~ 0.004 (SMC) when the
effects of magnetic fields are included.
- GRBs are predicted from the second (and probably the first) stellar
generation onwards.Comment: 5 pages, 1 figure, to appear in the proceedings of "Swift and GRBs:
Unveiling the Relativistic Universe", San Servolo, Venice, 5-9 June 200
Looking for imprints of the first stellar generations in metal-poor bulge field stars
Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] ≈−1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations.
Aims. The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ~ 45 000) and high-signal-to-noise (S/N> 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu.
Methods. High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe I and Fe II. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code.
Results. We confirm that the analysed stars are moderately metal-poor (−1.04 ≤ [Fe/H] ≤−0.43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤ + 0.2, and α-enhanced. We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na − O, Al − O, Al − Mg anti-correlations) were detected in our sample. The heavy elements Y, Zr, Ba, La, and Eu also exhibit oversolar abundances. Three out of the five stars analysed here show slightly enhanced [Y/Ba] ratios similar to those found in other metal-poor bulge globular clusters (NGC 6522 and M 62).
Conclusions. This sample shows enhancement in the first-to-second peak abundance ratios of heavy elements, as well as dominantly s-process element excesses. This can be explained by different nucleosynthesis scenarios: (a) the main r-process plus extra mechanisms, such as the weak r-process; (b) mass transfer from asymptotic giant branch stars in binary systems; (c) an early generation of fast-rotating massive stars. Larger samples of moderately metal-poor bulge stars, with detailed chemical abundances, are needed to better constrain the source of dominantly s-process elements in the early Universe
Very Massive Stars and the Eddington Limit
We use contemporary evolutionary models for Very Massive Stars (VMS) to
assess whether the Eddington limit constrains the upper stellar mass limit. We
also consider the interplay between mass and age for the wind properties and
spectral morphology of VMS, with reference to the recently modified
classification scheme for O2-3.5If*/WN stars. Finally, the death of VMS in the
local universe is considered in the context of pair instability supernovae.Comment: 6 pages, 4 figures, from "Four Decades of Massive Star Research"
(Quebec, Jul 2011), ASP Conf Ser, in press (L. Drissen, C. Robert, N.
St-Louis, A.F.J. Moffat, eds.
Production of 92Nb, 92Mo, and 146Sm in the gamma-process in SNIa
The knowledge of the production of extinct radioactivities like 92Nb and
146Sm by photodisintegration processes in ccSN and SNIa models is essential for
interpreting abundances in meteoritic material and for Galactic Chemical
Evolution (GCE). The 92Mo/92Nb and 146Sm/144Sm ratios provide constraints for
GCE and production sites. We present results for SNIa with emphasis on nuclear
uncertainties.Comment: 6 pages, 4 figures, Proceedings of the 13th Symposium on Nuclei in
the Cosmos (NIC XIII), July 2014, Debrecen, Hungar
Stellar evolution of massive stars at very low metallicities
Recently, measurements of abundances in extremely metal poor (EMP) stars have
brought new constraints on stellar evolution models. In an attempt to explain
the origin of the abundances observed, we computed pre--supernova evolution
models, explosion models and the related nucleosynthesis. In this paper, we
start by presenting the pre-SN models of rotating single stars with
metallicities ranging from solar metallicity down to almost metal free. We then
review key processes in core-collapse and bounce, before we integrate them in a
simplistic parameterization for 3D MHD models, which are well underway and
allow one to follow the evolution of the magnetic fields during collapse and
bounce. Finally, we present explosive nucleosynthesis results including
neutrino interactions with matter, which are calculated using the outputs of
the explosion models.
The main results of the pre-SN models are the following. First, primary
nitrogen is produced in large amount in models with an initial metallicity
. Second, at the same metallicity of and for models with
an initial mass larger than about 60 Mo, rotating models may experience heavy
mass loss (up to more than half of the initial mass of the star). The chemical
composition of these winds can qualitatively reproduce the abundance patterns
observed at the surface of carbon-rich EMP stars. Explosive nucleosynthesis
including neutrino-matter interactions produce improved abundances for iron
group elements, in particular for scandium and zinc. It also opens the way to a
new neutrino and proton rich process (p-process) able to contribute to the
nucleosynthesis of elements with A > 64. (Abridged)Comment: 29 pages, 10 figures, Reviews of Modern Astronomy 19, proceedings for
79th Annual Scientific Meeting of the Deutsche Astronomische Gesellschaft
200
The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars
The goal of this paper is to analyze the impact of a primary neutron source
on the s-process nucleosynthesis in massive stars at halo metallicity. Recent
stellar models including rotation at very low metallicity predict a strong
production of primary N14. Part of the nitrogen produced in the H-burning shell
diffuses by rotational mixing into the He core where it is converted to Ne22
providing additional neutrons for the s process. We present nucleosynthesis
calculations for a 25 Msun star at [Fe/H] = -3, -4, where in the convective
core He-burning about 0.8 % in mass is made of primary Ne22. The usual weak
s-process shape is changed by the additional neutron source with a peak between
Sr and Ba, where the s-process yields increase by orders of magnitude with
respect to the yields obtained without rotation. Iron seeds are fully consumed
and the maximum production of Sr, Y and Zr is reached. On the other hand, the
s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the
amount of Ne22 and by nuclear uncertainties, first of all by the
Ne22(alpha,n)Mg25 reaction. Finally, assuming that Ne22 is primary in the
considered metallicity range, the s-process efficiency decreases with
metallicity due to the effect of the major neutron poisons Mg25 and Ne22. This
work represents a first step towards the study of primary neutron source effect
in fast rotating massive stars, and its implications are discussed in the light
of spectroscopic observations of heavy elements at halo metallicity.Comment: Accepted for publication in ApJ Letters, 11 pages, 2 figures, 1 tabl
Certain General Anesthetics Used in Pediatrics Hinder Neurological Development in Infants
General anesthetics act by either blocking N-methyl-d-aspartate (NMDA) receptors or over stimulating γ-aminobutyric acid (GABA) receptors.1–3 The actions of these receptors are responsible for the anesthetized state and are also crucial in the neurological development of infants.2,4–8 Animal studies, although limited, provide vital information about general anesthesia’s neurotoxicity its hindrance of neurological development. Exposure to general anesthesia can severely hinder proper neuronal migration, synaptogenesis, and can drastically increase neuronal apoptosis in infant animals.9–16 General anesthetics are more neurotoxic to infant animals in combination compared to individually.10,16,17 Additionally, multiple exposures to general anesthesia tend to have compounding deleterious effects on neurological development in infant animals.12 It is likely that repeated exposure and combinational exposure to general anesthetics are the most detrimental to neurological development. Many retrospective studies on human infants show a correlation between exposure to general anesthesia and an increased risk of neurodevelopment disorders.18–25 However, these studies are statistically limited due to confounding factors. These confounding factors are the reason direct evidence of the neurotoxic effects of general anesthetics has been so elusive. In vitro human stem cell models provide an ethical alternative to clinical studies. However, clinical trials are necessary and are the most promising methods for obtaining direct evidence of the deleterious effects of general anesthesia on the developing human brain. Different methods used in clinical trials on infants help to minimalize ethical dilemmas, increase recruitment rates, and maximize safety and expediency. Specifically, this paper will evaluate the efficacy and safety of different methods used in clinical trials and will propose how clinical trials can be designed for future studies
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