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 ``$\alpha$-poor'' halo stars, we analyze kinematic and abundance data for 73 intermediate metallicity stars ($-1 >$ [Fe/H] $\geq -2$) 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 ``$\alpha$-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 M⊙M_{\odot} 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 $M_{\rm MS}=13-50$ $M_{\odot}$ and the explosion energy of $E=1-40\times10^{51}$ 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 ($-4< {\rm [Fe/H]}< -3$), while those of very metal-poor (VMP) stars ($-3<{\rm [Fe/H]} < -2$) 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 $M_{\rm MS}$ and $E$. 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] $< -3$, 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 $Y_{\rm e}$ 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