1,965 research outputs found
Neutrino treatment in multidimensional astrophysical simulations : a new spectral scheme
No full textNeutrinos play a central role in modern physics and astrophysics. Their extremely weak interaction
rate with baryons and other leptons makes their detection on the Earth difficult and challenging.
At the same time, it implies that the emission and the absorption of neutrinos are the dominant
radiative processes in hot and dense astrophysical environment (such as core-collapse supernovae
and the merger of binary compact objects), where photons are completely trapped and diffuse out on
much longer timescales. The implementation of neutrino-transport schemes in hydrodynamics simulations
is a subtle problem, especially in multi-dimensions, where an accurate solution of the transport
equations can be, computationally speaking, extremely expensive.
In this work, we have developed a new approximate neutrino-radiation treatment, the Advanced Spectral
Leakage (ASL) scheme; after having tested and calibrated it, we have shown a variety of applications,
both in the context of core-collapse supernovae and of binary neutron star mergers.
The ASL scheme was derived from previous grey leakage schemes, and it retains the conceptual and the
computational simplicity that characterize leakage schemes. Different from its predecessors, the new
treatment is spectral (i.e. it retains information on the particle energy), and it includes the modeling
of a neutrino trapped component in optically thick conditions and of neutrino absorption terms in
optically thin conditions. The scheme has been tested against detailed neutrino-transport in the context
of spherically symmetric models of collapsing stellar cores. We have shown that it is able to capture,
with reasonable accuracy, the main expected features during the collapse phase, at core bounce and in the
first hundreds of millisecond after bounce, both for the fundamental hydrodynamics and neutrino quantities.
The optical depth is a central quantity in leakages schemes. We have also developed a new algorithm to
compute the optical depth in multi-dimensional domains, without any symmetry constraint.
We called it Multidimensional Optical Depth Algorithm (MODA).
The major application of the ASL scheme in this work has been the study of the development of a neutrino-driven
wind from the hot and dense disc resulting from the merger of two neutron star. This process has been studied
for the first time in 3D Cartesian simulations, performed with the FISH code. The intense (10^53 erg/s) neutrino
emission coming from the (probably, unstable) hyper massive neutron star and from the disc itself is partially
re-absorbed by low density (< 10^10 g/cm^3), neutron-rich (Y_e < 0.1) matter inside disc. This energy deposition
drives a baryonic wind, mainly perpendicular to the disc plane, on a timescale of ~50 ms. Neutrino-matter
interactions in the wind modify significantly the electron fraction of matter: the resulting distribution shows
a broad range of Y_e, from 0.2 to 0.4, with larger values along the polar directions than along the equatorial one.
At ~100ms after the merger, the amount of ejecta is of the order of 2% the initial mass of the disc, thus it
represents a significant channel for mass ejection from binary neutron star mergers. The broad range in Y_e represents
an interesting signature in the context of r-process nucleosynthesis.
Furthermore, we have shown other applications of the ASL scheme.
First, we have implemented it in the MHD version of the FISH code to study the electron fraction in jets resulting from
magneto-rotationally driven supernovae. In the case of strong and fast jet formation (~30 ms after core bounce),
the electron fraction in the ejecta is low enough (Y_e < 0.3) to produce robust r-process nucleosynthesis.
Second, we have implemented the ASL scheme inside the SPH code SPHYNCS, to perform core-collapse simulations.
The results we have obtained are compatible with what we have obtained with grid codes. This model shows that the
scheme is of easy implementation also in Lagrangian, multidimensional SPH codes.
Finally, we have designed a new prescription to explode artificially spherically symmetric core-collapse models,
using the IDSA scheme for electron neutrinos and the ASL scheme for mu and tau neutrinos. The extra energy deposition
required to trigger the explosion is obtained by the parametrized absorption of heavy flavour neutrinos inside the
gain region. The model has shown promising results and it is well suited to study the explosive nucleosynthesis for
broad stellar progenitor samples, including detailed neutrino treatment
Ask questions, get sales : close the deak and create long-term relationships / Stephan Schiffman.
No full textIncludes index.v, 168 pages ;In Ask Questions, Get Sales, the author and sales guru Stephan Schiffman helps readers boost their careers to the gold-medal level by teaching them how to strengthen their questioning skills during the sales process. The premise is simple yet effective: In order to be successful, salespeople need to change their mindset from "need-orientated" to "do-orientated". The message of the book centers around six core "do" questions: What do you do? How do you do it? When and where do you do it? Why do you do it that way? Who do you do it with? How can we help you do it better? With this indispensable guide in their briefcase, salespeople will have information at the ready to score big sales over the short term and the long term
Unemployment Benefits and Unemployment Rates of Low-Skilled and Elder Workers in West Germany: A Search Equilibrium Approach
No full textApproach Author & abstract Download 16 References 1 Citations Related works & more Corrections Author Listed: Launov, Andrey ([email protected]) (University of Kent) Wolff, Joachim ([email protected]) (Institute for Employment Research (IAB), Nuremberg) Klasen, Stephan ([email protected]) (University of Göttingen) Registered: Stephan Klasen Abstract In this paper we investigate whether the extension of the entitlement to unemployment benefits in the mid 80s can explain the increase in the unemployment rates of unskilled and elder workers in western Germany. To answer this question we estimate a version of the Burdett-Mortensen search equilibrium model and analyze how workers’ search behaviour responded to these reforms. We try both nonparametric and fully-parametric estimation methods and identify the cases in which the nonparametric approach cannot be applied. We find that the entitlement reforms are largely responsible for the increase of unemployment among unskilled workers
Unemployment Benefits and Unemployment Rates of Low-Skilled and Elder Workers in West Germany: A Search Equilibrium Approach
No full textApproach Author & abstract Download 16 References 1 Citations Related works & more Corrections Author Listed: Launov, Andrey ([email protected]) (University of Kent) Wolff, Joachim ([email protected]) (Institute for Employment Research (IAB), Nuremberg) Klasen, Stephan ([email protected]) (University of Göttingen) Registered: Stephan Klasen Abstract In this paper we investigate whether the extension of the entitlement to unemployment benefits in the mid 80s can explain the increase in the unemployment rates of unskilled and elder workers in western Germany. To answer this question we estimate a version of the Burdett-Mortensen search equilibrium model and analyze how workers’ search behaviour responded to these reforms. We try both nonparametric and fully-parametric estimation methods and identify the cases in which the nonparametric approach cannot be applied. We find that the entitlement reforms are largely responsible for the increase of unemployment among unskilled workers
Measuring Vulnerability to Poverty Using Long-Term Panel Data
No full textMeasuring Vulnerability to Poverty Using Long-Term Panel Data Author & abstract Download & other version 16 References 4 Citations Related works & more Corrections Author Listed: Katja Landau (Georg-August-University Göttingen) Stephan Klasen (Georg-August-University Göttingen) Walter Zucchini (Georg-August-University Göttingen) Registered: Stephan Klasen Abstract We investigate the accuracy of ex ante assessments of vulnerability to income poverty using cross-sectional data and panel data. We use long-term panel data from Germany and apply di fferent regression models, based on household covariates and previous-year equivalence income, to classify a household as vulnerable or not. Predictive performance is assessed using the Receiver Operating Characteristics (ROC), which takes account of false positive as well as true positive rates. Estimates based on cross-sectional data are much less accurate than those based on panel data, but for Germany, the accuracy of vulnerability predictions is limited even when panel data are used. In part this low accuracy is due to low poverty incidence and high mobility in and out of poverty
Measuring Vulnerability to Poverty Using Long-Term Panel Data
No full textMeasuring Vulnerability to Poverty Using Long-Term Panel Data Author & abstract Download & other version 16 References 4 Citations Related works & more Corrections Author Listed: Katja Landau (Georg-August-University Göttingen) Stephan Klasen (Georg-August-University Göttingen) Walter Zucchini (Georg-August-University Göttingen) Registered: Stephan Klasen Abstract We investigate the accuracy of ex ante assessments of vulnerability to income poverty using cross-sectional data and panel data. We use long-term panel data from Germany and apply di fferent regression models, based on household covariates and previous-year equivalence income, to classify a household as vulnerable or not. Predictive performance is assessed using the Receiver Operating Characteristics (ROC), which takes account of false positive as well as true positive rates. Estimates based on cross-sectional data are much less accurate than those based on panel data, but for Germany, the accuracy of vulnerability predictions is limited even when panel data are used. In part this low accuracy is due to low poverty incidence and high mobility in and out of poverty
Europium production: neutron star mergers versus core-collapse supernovae
No full textWe have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star-black hole) binaries as well as in Type II supernovae. We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as (i) the time-scale of coalescence, (ii) the Eu yields and (iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from Type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can be summarized as follows: (i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time-scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around 3 × 10-7 M⊙ and the mass range of progenitors of neutron stars is 9-50 M⊙; (ii) both Type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce 2 × 10-7 M⊙ per system and Type II supernovae, with progenitors in the range 20-50 M⊙, produce yields of Eu of the order of 10-8-10-9 M⊙; (iii) either models with only neutron stars producing Eu or mixed ones can reproduce the observed Eu abundance gradient along the Galactic disc
Evaluation of in-store processes related to returnable packaging services offered in grocery stores - the store management perspective
No full textAuthor Stephan LehnerMasterarbeit Universität Linz 202
Evaluation of in-store processes related to returnable packaging services offered in grocery stores - the store management perspective
No full textAuthor Stephan LehnerMasterarbeit Universität Linz 202
Nucleosynthesis in explosive environments : neutron star mergers and core-collapse supernovae
No full textSeveral nucleosynthesis processes are responsible for the production of the chemical elements in the universe. Explosive ejecta in core-collapse supernovae typically produce intermediate-mass elements up to the iron-group nuclei, although the exact compositions depend on the parameters of the supernova, such as the structure of the pre-supernova progenitor, the explosion energy and neutrino luminosities. Understanding the connection between these parameters and the nuclear composition of the ejecta is an ongoing field of research in nuclear astrophysics. Open questions also remain surrounding the late-time ejecta (the neutrino-driven wind), which could host either a weak r-process or the νp-process.
Research on the rapid neutron-capture process requires the knowledge of the properties of exotic nuclei far from stability. Since these nuclei cannot be produced under laboratory conditions, we have to rely on theoretical predictions (e.g., mass models), introducing large uncertainties. In addition, the astrophysical environment of the r-process is still unknown, although recent observational data support mergers of two neutron stars as a promising site. Furthermore, observations of metal-poor stars enriched with r-process material suggest a robust abundance pattern for the strong r-process, which provides a solid benchmark to test our models against.
In this thesis, several aspects of explosive nucleosynthesis are studied. In the first part, the theoretical framework of nucleosynthesis calculations is discussed, with a focus on the r-process and fission reactions. The second part highlights the role of fission on r-process calculations. Finally, we report on nucleosynthesis calculations for core-collapse supernova models, on the one hand in spherical symmetry in order to contrain the so-called PUSH method, as well as in axisymmetric models
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