141 research outputs found
The Explosive Yields Produced by the First Generation of Core Collapse Supernovae and the Chemical Composition of Extremely Metal Poor Stars
We present a detailed comparison between an extended set of elemental
abundances observed in some of the most metal poor stars presently known and
the ejecta produced by a generation of primordial core collapse supernovae. We
used five stars which form our initial database and define a "template" ultra
metal poor star which is then compared to the theoretical predictions. Our main
findings are as follows: a) the fit to [Si/Mg] and [Ca/Mg] of these very metal
poor stars seems to favor the presence of a rather large C abundance at the end
of the central He burning; in a classical scenario in which the border of the
convective core is strictly determined by the Schwarzschild criterion, such a
large C abundance would imply a rather low C12(alpha,gamma)O16 reaction rate;
b) a low C abundance left by the central He burning would imply a low [Al/Mg]
(<-1.2 dex) independently on the initial mass of the exploding star while a
rather large C abundance would produce such a low [Al/Mg] only for the most
massive stellar model; c) at variance with current beliefs that it is difficult
to interpret the observed overabundance of [Co/Fe], we find that a mildly large
C abundance in the He exhausted core (well within the present range of
uncertainty) easily and naturally allows a very good fit to [Co/Fe]; d) our
yields allow a reasonable fit to 8 out of the 11 available elemental
abundances; e) within the present grid of models it is not possible to find a
good match of the remaining three elements, Ti, Cr and Ni (even for an
arbitrary choice of the mass cut); f) the adoption of other yields available in
the literature does not improve the fit; g) since no mass in our grid provides
a satisfactory fit to these three elements, even an arbitrary choice of the
initial mass function would not improve their fit.Comment: 30 pages, 8 figures, 8 tables. Accepted for publication on Ap
Isolated Star Formation: A Compact HII Region in the Virgo Cluster
We report on the discovery of an isolated, compact HII region in the Virgo
cluster. The object is located in the diffuse outer halo of NGC 4388, or could
possibly be in intracluster space. Star formation can thus take place far
outside the main star forming regions of galaxies. This object is powered by a
small starburst with an estimated mass of \sim 400\msun and age of \sim
3\myr. From a total sample of 17 HII region candidates, the present rate of
isolated star formation estimated in our Virgo field is small, \sim 10^{-6}
Msun arcmin}^{-2} yr^{-1}. However, this mode of star formation might have
been more important at higher redshifts and be responsible for a fraction of
the observed intracluster stars and total cluster metal production. This object
is relevant also for distance determinations with the planetary nebula
luminosity function from emission line surveys, for high-velocity clouds and
the in situ origin of B stars in the Galactic halo, and for local enrichment of
the intracluster gas by Type II supernovae.Comment: 5 pages, LaTeX, 1 figure. ApJ Letters, in press (scheduled Dec 1,
2002
nu-Process Nucleosynthesis in Population III Core-Collapse Supernovae
We investigate the effects of neutrino-nucleus interactions (the nu-process)
on the production of iron-peak elements in Population III core-collapse
supernovae. The nu-process and the following proton and neutron capture
reactions produce odd-Z iron-peak elements in complete and incomplete Si
burning region. This reaction sequence enhances the abundances of Sc, Mn, and
Co in the supernova ejecta. The supernova explosion models of 15 M_sol and 25
M_sol stars with the nu-process well reproduce the averaged Mn/Fe ratio
observed in extremely metal-poor halo stars. In order to reproduce the observed
Mn/Fe ratio, the total neutrino energy in the supernovae should be 3 - 9 x
10^{53} ergs. Stronger neutrino irradiation and other production sites are
necessary to reproduce the observed Sc/Fe and Co/Fe ratios, although these
ratios increase by the nu-process.Comment: 10 pages, 12 figures, Accepted for publication in The Astrophysical
Journa
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
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
Abundances and Kinematics of Field Stars II: Kinematics and Abundance Relationships
As an investigation of the origin of ``-poor'' halo stars, we analyze
kinematic and abundance data for 73 intermediate metallicity stars (
[Fe/H] ) selected from Paper I of this series. We find evidence for a
connection between the kinematics and the enhancement of certain
element-to-iron ([X/Fe]) ratios in these stars. Statistically significant
correlations were found between [X/Fe] and galactic rest-frame velocities
(\vrf{}) for Na, Mg, Al, Si, Ca and Ni, with marginally significant
correlations existing for Ti and Y as well. We also find that the [X/Fe] ratios
for these elements all correlate with a similar level of significance with
[Na/Fe]. Finally, we compare the abundances of these halo stars against those
of stars in nearby dSph galaxies. We find significant differences between the
abundance ratios in the dSph stars and halo stars of similar metallicity. From
this result, it is unlikely that the halo stars in the solar neighborhood,
including even the ``-poor'' stars, were once members of disrupted dSph
galaxies similar to those studied to date.Comment: Accepted for publication in January 2002 AJ. 29 pages, 12 figures, 2
table
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
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
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
- …