214 research outputs found
The Chemical Evolution of the Solar Neighbourhood: the Effect of Binaries
In this paper we compute the time evolution of the elements (4He, 12C, 14N,
16O, 20Ne, 24Mg, 28Si, 32S, 40Ca and 56Fe) and of the supernova rates in the
solar neighbourhood by means of a galactic chemical evolutionary code that
includes in detail the evolution of both single and binary stars. Special
attention is payed to the formation of black holes.
Our main conclusions: in order to predict the galactic time evolution of the
different types of supernovae, it is essential to compute in detail the
evolution of the binary population, the observed time evolution of carbon is
better reproduced by a galactic model where the effect is included of a
significant fraction of intermediate mass binaries, massive binary mass
exchange provides a possible solution for the production of primary nitrogen
during the very early phases of galactic evolution, chemical evolutionary
models with binaries or without binaries but with a detailed treatment of the
SN Ia progenitors predict very similar age-metallicity relations and very
similar G-dwarf distributions whereas the evolution of the yields as function
of time of the elements 4He, 16O, 20Ne, 24Mg, 28Si, 32S and 40Ca differ by no
more than a factor of two or three, the observed time evolution of oxygen is
best reproduced when most of the oxygen produced during core helium burning in
ALL massive stars serves to enrich the interstellar medium. This can be used as
indirect evidence that (massive) black hole formation in single stars and
binary components is always preceded by a supernova explosion.Comment: 59 page
Delay time distribution of type Ia supernovae: theory vs. observation
Two formation scenarios are investigated for type Ia supernovae in elliptical
galaxies: the single degenerate scenario (a white dwarf reaching the
Chandrasekhar limit through accretion of matter transferred from its companion
star in a binary) and the double degenerate scenario (the inspiraling and
merging of two white dwarfs in a binary as a result of the emission of
gravitational wave radiation). A population number synthesis code is used,
which includes the latest physical results in binary evolution and allows to
differentiate between certain physical scenarios (such as the description of
common envelope evolution) and evolutionary parameters (such as the mass
transfer efficiency during Roche lobe overflow). The thus obtained theoretical
distributions of type Ia supernova delay times are compared to those that are
observed, both in morphological shape and absolute number of events. The
critical influence of certain parameters on these distributions is used to
constrain their values. The single degenerate scenario alone is found to be
unable in reproducing the morphological shape of the observational delay time
distribution, while use of the double degenerate one (or a combination of both)
does result in fair agreement. Most double degenerate type Ia supernovae are
formed through a normal, quasi-conservative Roche lobe overflow followed by a
common envelope phase, not through two successive common envelope phases as is
often assumed. This may cast doubt on the determination of delay times by using
analytical formalisms, as is sometimes done in other studies. The theoretical
absolute number of events in old elliptical galaxies lies a factor of at least
three below the rates that are observed. While this may simply be the result of
observational uncertainties, a better treatment of the effects of rotation on
stellar structure could mitigate the discrepancy.Comment: 5 pages, 4 figures, to appear in proceedings of "Binary Star
Evolution: Mass Loss, Accretion, and Mergers
An allegory of the political influence of the top 1%
We study how rich shareholders can use their economic power to deregulate firms that they own, thus skewing the income distribution towards themselves. Agents differ in productivity and choose how much labor to supply. High productivity agents also own shares in the productive sector and thus earn capital income. All vote over a linear tax rate on (labor and capital) income whose proceeds are redistributed lump sum. Capital owners also lobby in order to ease the price cap imposed on the private firm. We solve analytically for the Kantian equilibrium of this lobbying game together with the majority voting equilibrium over the tax rate, and we perform simulations. We obtain numerically that, as the capital income distribution becomes more concentrated among the top productivity individuals, their increased lobbying effort generates efficiency as well as equity costs, with lower labor supply and lower average utility levels in society
The dynamics of capital accumulation in the US: Simulations after Piketty
We calibrate a sequence of four nested models to study the dynamics of wealth accumulation. Individuals maximize a utility function whose arguments are consumption and investment. They desire to accumulate wealth for its own sake – this is not a life-cycle model. A competitive firm produces a single good from labor and capital; the rate of return to capital and the wage rate are market-clearing. The second model introduces political lobbying by the wealthy, whose purpose is to reduce the tax rate on capital income. The third model introduces differential rates of return to capitals of different sizes. The fourth model introduces inheritance and intergenerational mobility
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