4,407 research outputs found
Milky Way's Thick and Thin disk: Is there distinct thick disk?
This article is based on our discussion session on Milky Way models at the
592 WE-Heraeus Seminar, Reconstructing the Milky Way's History: Spectroscopic
Surveys, Asteroseismology and Chemodynamical models. The discussion focused on
the following question: "Are there distinct thick and thin disks?". The answer
to this question depends on the definition one adopts for thin and thick disks.
The participants of this discussion converged to the idea that there are at
least two different types of disks in the Milky Way. However, there are still
important open questions on how to best define these two types of disks
(chemically, kinematically, geometrically or by age?). The question of what is
the origin of the distinct disks remains open. The future Galactic surveys
which are highlighted in this conference should help us answering these
questions. The almost one-hour debate involving researchers in the field
representing different modelling approaches (Galactic models such as TRILEGAL,
Besancon and Galaxia, chemical evolution models, extended distribution
functions method, chemodynamics in the cosmological context, and
self-consistent cosmological simulations) illustrated how important is to have
all these parallel approaches. All approaches have their advantages and
shortcomings (also discussed), and different approaches are useful to address
specific points that might help us answering the more general question above.Comment: 7 pages, no figure. To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalban, and M. Steffe
Pregabalin: a range of misuse-related unanswered questions
© 2019 The Authors. CNS Neuroscience & Therapeutics Published by John Wiley & Sons Ltd.Peer reviewedFinal Published versio
Explaining the Ba, Y, Sr, and Eu abundance scatter in metal-poor halo stars: constraints to the r-process
Context. Thanks to the heroic observational campaigns carried out in recent
years we now have large samples of metal-poor stars for which measurements of
detailed abundances exist. [...] These data hold important clues on the nature
of the contribution of the first stellar generations to the enrichment of our
Galaxy. Aims. We aim to explain the scatter in Sr, Ba, Y, and Eu abundance
ratio diagrams unveiled by the metal-poor halo stars. Methods. We computed
inhomogeneous chemical evolution models for the Galactic halo assuming
different scenarios for the r-process site: the electron-capture supernovae
(EC) and the magnetorotationally driven (MRD) supernovae scenario. We also
considered models with and without the contribution of fast-rotating massive
stars (spinstars) to an early enrichment by the s-process. A detailed
comparison with the now large sample of stars with measured abundances of Sr,
Ba, Y, Eu, and Fe is provided (both in terms of scatter plots and number
distributions for several abundance ratios). Results. The scatter observed in
these abundance ratios of the very metal-poor stars (with [Fe/H] < -2.5) can be
explained by combining the s-process production in spinstars, and the r-process
contribution coming from massive stars. For the r-process we have developed
models for both the EC and the MRD scenario that match the observations.
Conclusions. With the present observational and theoretical constraints we
cannot distinguish between the EC and the MRD scenario in the Galactic halo.
Independently of the r-process scenarios adopted, the production of elements by
an s-process in spinstars is needed to reproduce the spread in abundances of
the light neutron capture elements (Sr and Y) over heavy neutron capture
elements (Ba and Eu). We provide a way to test our suggestions by means of the
distribution of the Ba isotopic ratios in a [Ba/Fe] or [Sr/Ba] vs. [Fe/H]
diagram.Comment: 14 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The effects of Population III stars and variable IMF on the chemical evolution of the Galaxy
We studied the effects of a hypothetical initial stellar generation (PopIII)
of only massive and very massive stars (VMS) on the chemical evolution of the
Galaxy. We adopted the two-infall chemical evolution model of Chiappini et al.
and tested several sets of yields for primordial VMS (Pair-Creation SNe), which
produce different amounts of heavy elements than lower mass stars. We focused
on the evolution of alpha-elements, C, N, Fe. The effects of PopIII stars on
the Galactic evolution of these elements is negligible if a few generations of
such stars occurred, whereas they produce different results from the standard
models if they formed for a longer period. Also the effects of a more strongly
variable IMF were discussed, making use of suggestions appeared in the
literature to explain the lack of metal-poor stars in the Galactic halo with
respect to model predictions. The predicted variations in abundances, SN rates,
G-dwarf [Fe/H] distribution are here more dramatic and in contrast with
observations; we concluded that a constant or slightly varying IMF is the best
solution. Our main conclusion is that if VMS existed they must have formed only
for a very short period of time (until the halo gas reached the threshold
metallicity for the formation of very massive objects); in this case, their
effects on the evolution of the studied elements was negligible also in the
earliest phases. We thus cannot prove or disprove the existence of such stars
on the basis of the available data. Due to their large metal production and
short lives, primordial VMS should have enriched the halo gas beyond the
metallicity of the most metal poor stars known in a few Myrs. This constrains
the number of Pair-Creation SNe: we find that a number of 2-20 of such SNe
occurred in our Galaxy depending on the stellar yields.Comment: 30 pages, 10 figures, accepted for publication in New Astronom
FDI and trade: A Granger causality analysis in a heterogeneous panel
This paper will investigate the Granger causality between outward Foreign Direct Investment (FDI) and the exports of goods and services in 11 European countries from 1996 to 2008. Using a new method to evaluate causality in a heterogeneous panel, we find that the causal relationship from FDI to exports is homogeneous among the panel. However, we find strong evidence of a heterogeneity of the causal relationship from exports to FDI in our sample.Foreign direct investment, exports, Granger causality, heterogeneous panel
Stellar Evolution in the Early Universe
Massive stars played a key role in the early evolution of the Universe. They
formed with the first halos and started the re-ionisation. It is therefore very
important to understand their evolution. In this paper, we describe the strong
impact of rotation induced mixing and mass loss at very low . The strong
mixing leads to a significant production of primary nitrogen 14, carbon 13 and
neon 22. Mass loss during the red supergiant stage allows the production of
Wolf-Rayet stars, type Ib,c supernovae and possibly gamma-ray bursts (GRBs)
down to almost Z=0 for stars more massive than 60 solar masses. Galactic
chemical evolution models calculated with models of rotating stars better
reproduce the early evolution of N/O, C/O and C12/C13. We calculated the weak
s-process production induced by the primary neon 22 and obtain overproduction
factors (relative to the initial composition, Z=1.e-6) between 100-1000 in the
mass range 60-90.Comment: 8 pages, 4 figures, proceedings of IAU Symposium 255,
"Low-Metallicity Star Formation: From the First stars to Dwarf Galaxies",
L.K. Hunt, S. Madden & R. Schneider, ed
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