137 research outputs found
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Prospects for obtaining an r-process from Gamma Ray Burst Disk Winds
We discuss the possibility that r-process nucleosynthesis may occur in the
winds from gamma ray burst accretion disks. This can happen if the temperature
of the disk is sufficiently high that electron antineutrinos are trapped as
well as neutrinos. This implies accretion disks with greater than a solar mass
per second accretion rate, although lower accretion rates with higher black
hole spin parameters may provide viable environments as well. Additionally, the
outflow from the disk must either have relatively low entropy, e.g. around s =
10, or the initial acceleration of the wind must be slow enough that it is
neutrino and antineutrino capture as opposed to electron and positron capture
that sets the electron fraction.Comment: 8 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8
proceeding
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
Supernova Nucleosynthesis in Population III 13 -- 50 Stars and Abundance Patterns of Extremely Metal-Poor Stars
We perform hydrodynamical and nucleosynthesis calculations of core-collapse
supernovae (SNe) and hypernovae (HNe) of Population (Pop) III stars. We provide
new yields for the main-sequence mass of and the
explosion energy of ergs to apply for chemical evolution
studies. Our HN yields based on the mixing-fallback model of explosions
reproduce the observed abundance patterns of extremely metal-poor (EMP) stars
(), while those of very metal-poor (VMP) stars () are reproduced by the normal SN yields integrated over the
Salpeter initial mass function. Moreover, the observed trends of abundance
ratios [X/Fe] against [Fe/H] with small dispersions for the EMP stars can be
reproduced as a sequence resulting from the various combination of
and . This is because we adopt the empirical relation that a larger amount
of Fe is ejected by more massive HNe. Our results imply that the observed
trends with small dispersions do not necessarily mean the rapid homogeneous
mixing in the early galactic halo at [Fe/H] , but can be reproduced by
the ``inhomogeneous'' chemical evolution model. In addition, we examine how the
modifications of the distributions of the electron mole fraction
and the density in the presupernova models improve the agreement with
observations. In this connection, we discuss possible contributions of
nucleosynthesis in the neutrino-driven wind and the accretion disk.Comment: 45 pages, 14 figures. Accepted for publication in the Astrophysical
Journal (10 May 2007, v660n2 issue). Moved from Supplements to Part
Explosive Nucleosynthesis in Hypernovae
We examine the characteristics of nucleosynthesis in 'hypernovae', i.e.,
supernovae with very large explosion energies ( \gsim 10^{52} ergs). We
carry out detailed nucleosynthesis calculations for these energetic explosions
and compare the yields with those of ordinary core-collapse supernovae. We find
that both complete and incomplete Si-burning takes place over more extended,
lower density regions, so that the alpha-rich freezeout is enhanced and
produces more Ti in comparison with ordinary supernova nucleosynthesis. In
addition, oxygen and carbon burning takes place in more extended, lower density
regions than in ordinary supernovae. Therefore, the fuel elements O, C, Al are
less abundant while a larger amount of Si, S, Ar, and Ca ("Si") are synthesized
by oxygen burning; this leads to larger ratios of "Si"/O in the ejecta.
Enhancement of the mass ratio between complete and incomplete Si-burning
regions in the ejecta may explain the abundance ratios among iron-peak elements
in metal-poor stars. Also the enhanced "Si"/O ratio may explain the abundance
ratios observed in star burst galaxies. We also discuss other implications of
enhanced [Ti/Fe] and [Fe/O] for Galactic chemical evolution and the abundances
of low mass black hole binaries.Comment: Accepted for publication in the Astrophysical Journal (13 March 2001)
Tables 6 - 9 are available at
http://www.astron.s.u-tokyo.ac.jp/~nakamura/research/papers/nakamuratab.ps.g
NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Aims: Aluminium is a key element to constrain the models of the chemical
enrichment and the yields of the first supernovae. But obtaining precise Al
abundances in extremely metal-poor (EMP) stars requires that the non-LTE
effects be carefully taken into account.
Methods: The NLTE profiles of the blue resonance aluminium lines have been
computed in a sample of 53 extremely metal-poor stars with a modified version
of the program MULTI applied to an atomic model of the Al atom with 78 levels
of Al I and 13 levels of Al II, and compared to the observations.
Results: With these new determinations, all the stars of the sample show a
ratio Al/Fe close to the solar value: [Al/Fe] =-0.06 +- 0.10 with a very small
scatter. These results are compared to the models of the chemical evolution of
the halo using different models of SN II and are compatible with recent
computations. The sodium-rich giants are not found to be also aluminium-rich
and thus, as expected, the convection in these giants only brings to the
surface the products of the Ne-Na cycle.Comment: To be published on A&
Interstellar Turbulence II: Implications and Effects
Interstellar turbulence has implications for the dispersal and mixing of the
elements, cloud chemistry, cosmic ray scattering, and radio wave propagation
through the ionized medium. This review discusses the observations and theory
of these effects. Metallicity fluctuations are summarized, and the theory of
turbulent transport of passive tracers is reviewed. Modeling methods, turbulent
concentration of dust grains, and the turbulent washout of radial abundance
gradients are discussed. Interstellar chemistry is affected by turbulent
transport of various species between environments with different physical
properties and by turbulent heating in shocks, vortical dissipation regions,
and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered
and accelerated in turbulent magnetic waves and shocks, and they generate
turbulence on the scale of their gyroradii. Radio wave scintillation is an
important diagnostic for small scale turbulence in the ionized medium, giving
information about the power spectrum and amplitude of fluctuations. The theory
of diffraction and refraction is reviewed, as are the main observations and
scintillation regions.Comment: 46 pages, 2 figures, submitted to Annual Reviews of Astronomy and
Astrophysic
Linking the Metallicity Distribution of Galactic Halo Stars to the Enrichment History of the Universe
We compare the metallicity distribution of Galactic Halo stars with 3D
realizations of hierarchical galaxy formation. Outflows from dwarf galaxies
enrich the intergalactic medium inhomogeneously, at a rate depending on the
local galaxy density. Consequently, the first stars created in small
early-forming galaxies are less metal-rich that the first stars formed in more
massive galaxies which typically form later. As most halo stars are likely to
originate in accreted dwarfs, while disk stars formed out of outflow-enriched
gas, this scenario naturally generates a ``metallicity floor'' for old disk
stars, which we find to be roughly coincident with the higher end of our
predicted metallicity distribution of halo stars, in agreement with
observations. The broad and centrally peaked distribution of halo star
metallicities is well reproduced in our models, with a natural dispersion
depending on the exact accretion history. Our modeling includes the important
``baryonic stripping'' effect of early outflows, which brush away the tenuously
held gas in neighboring pre-virialized density perturbations. This stripping
process does not significantly modify the predicted shape of the halo star
metal distribution but inhibits star-formation and hence the number of accreted
stars, helping to reproduce the observed total Galactic halo luminosity and
also the lack of low-luminosity local dwarf galaxies relative to N-body
predictions.Comment: 5 pages, 1 figure, ApJ Letters, accepte
Lead: Asymptotic Giant Branch production and Galactic Chemical Evolution
The enrichment of Pb in the Galaxy is followed in the framework of a detailed
model of Galactic chemical evolution that already proved adequate to reproduce
the chemical enrichment of O and of the elements from Ba to Eu. The stellar
yields are computed through nucleosynthesis calculations in the Asymptotic
Giant Branch (AGB) phase of low- and intermediate-mass stars, covering a wide
range of metallicities. The physical parameters of the stellar structure were
derived from full stellar evolutionary models previously computed. We show that
low-mass AGB stars are the main producers of Pb in the Galaxy, with a complex
dependence on metallicity and a maximum efficiency at [Fe/H] ~ -1. Our
calculations succeed in reproducing the abundances of Pb isotopes in the solar
system: the role attributed by the classical analysis of the s-process to the
'strong component', in order to explain more than 50% of solar 208Pb, is
actually played by the high production of Pb in low-mass and low-metallicity
AGB stars. We then follow the Galactic chemical evolution of Pb isotopes and
give our expectations on the s-process contribution to each of them at the
epoch of the solar system formation. Finally, we present new spectroscopic
estimates of Pb abundance on a sample of field stars and compare them, together
with a few other determinations available, with the predicted trend of [Pb/Fe]
in the Galaxy.Comment: Accepted for ApJ, 19 pages, 4 figure
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