Neutron Star Mergers as the Origin of r-Process Elements in the Galactic Halo Based on the Sub-halo Clustering Scenario

Binary mergers (NSMs) of double neutron star (and black hole-neutron star) systems are suggested to be major sites of r-process elements in the Galaxy by recent hydrodynamical and nucleosynthesis studies. It has been pointed out, however, that the estimated long lifetimes of neutron star binaries are in conflict with the presence of r-process-enhanced halo stars at metallicities as low as [Fe/H] ~ -3. To resolve this problem, we examine the role of NSMs in the early Galactic chemical evolution on the assumption that the Galactic halo was formed from merging sub-halos. We present simple models for the chemical evolution of sub-halos with total final stellar masses between 10^4 M_solar and 2 x 10^8 M_solar. Typical lifetimes of compact binaries are assumed to be 100 Myr (for 95% of their population) and 1 Myr (for 5%), according to recent binary population synthesis studies. The resulting metallcities of sub-halos and their ensemble are consistent with the observed mass-metallicity relation of dwarf galaxies in the Local Group, and the metallicity distribution of the Galactic halo, respectively. We find that the r-process abundance ratios [r/Fe] start increasing at [Fe/H] <= -3 if the star formation efficiencies are smaller for less massive sub-halos. In addition, the sub-solar [r/Fe] values (observed as [Ba/Fe] ~ -1.5 for [Fe/H] < -3) are explained by the contribution from the short-lived (~1 Myr) binaries. Our results indicate that NSMs may have a substantial contribution to the r-process element abundances throughout the Galactic history.Comment: 5 pages, 2 figures, accepted for publication in ApJ

Roles of SNIa and SNII in ICM Enrichment

Based on ASCA observations Mushotzky et al. (1996, ApJ 466, 686) have recently derived the relative-abundance ratios of $\alpha$-elements to iron, [\alpha/Fe] \simeq 0.2-0.3$, for four rich clusters, and have suggested that the origin of metals in an intra-cluster medium (ICM) is not a type-Ia supernovae (SNIa), but a type-II supernovae (SNII). However, these authors used the solar photospheric iron abundance for ASCA data reduction, while the meteoritic iron abundance is usually adopted in chemical-evolution studies. It is true that although the photospheric and meteoritic solar abundances are consistent for most of the elements, a serious discrepancy is known to exist for iron; indeed, the photospheric abundance of iron is$N_{Fe}/N_H = 4.68 10^{-5}$by number, while the meteoritic value is$3.24 10^{-5}$. The argument concerning the relative roles of SNIa and SNII in ICM enrichment is quite sensitive to the precise values of [\alpha/Fe], and one should use an identical solar iron abundance in data reduction as well as in theoretical arguments. We therefore adopt the meteoritic iron abundance, which is consistent with chemical-evolution studies, and shift Mushotzky et al.'s ASCA data by$\Delta[\alpha/Fe] \simeq -0.16$ dex. By comparing the corrected [\alpha/Fe] values with theoretical nucleosynthesis prescriptions of SNIa and SNII, we reach a conclusion that an SNIa iron contribution of 50% or higher in the ICM enrichment could not be ruled out, and might indeed be favoured based on the ASCA spectra.Comment: 8 pages, 2 figures, requires PASJ LaTeX macros. To appear in PAS

Neutron-capture elements in the very metal-poor star HD88609: another st ar with excesses of light neutron-capture elements

We obtained a high resolution, high signal-to-noise UV-blue spectrum of the extremely metal-poor red giant HD88609 to determine the abundances of heavy elements. Nineteen neutron-capture elements are detected in the spectrum. Our analysis revealed that this object has large excesses of light neutron-capture elements while heavy neutron-capture elements are deficient. The abundance pattern shows a continuously decreasing trend, as a function of atomic number, from Sr to Yb, which is quite different from those in stars with excesses of r-process elements. Such an abundance pattern is very similar to that of HD122563 that was studied by our previous work. The results indicate that the abundance pattern found in the two stars could represent the pattern produced by the nucleosynthesis process that provided light neutron-capture elements in the very early Galaxy.Comment: 18 pages, 6 figures, accepted for publication in Ap

Enrichment of r-process elements in dwarf spheroidal galaxies in chemo-dynamical evolution model

The rapid neutron-capture process (r-process) is a major process to synthesize elements heavier than iron, but the astrophysical site(s) of r-process is not identified yet. Neutron star mergers (NSMs) are suggested to be a major r-process site from nucleosynthesis studies. Previous chemical evolution studies however require unlikely short merger time of NSMs to reproduce the observed large star-to-star scatters in the abundance ratios of r-process elements relative to iron, [Eu/Fe], of extremely metal-poor stars in the Milky Way (MW) halo. This problem can be solved by considering chemical evolution in dwarf spheroidal galaxies (dSphs) which would be building blocks of the MW and have lower star formation efficiencies than the MW halo. We demonstrate that enrichment of r-process elements in dSphs by NSMs using an N-body/smoothed particle hydrodynamics code. Our high-resolution model reproduces the observed [Eu/Fe] by NSMs with a merger time of 100 Myr when the effect of metal mixing is taken into account. This is because metallicity is not correlated with time up to ~ 300 Myr from the start of the simulation due to low star formation efficiency in dSphs. We also confirm that this model is consistent with observed properties of dSphs such as radial profiles and metallicity distribution. The merger time and the Galactic rate of NSMs are suggested to be <~ 300 Myr and ~ $10^{-4}$ yr$^{-1}$, which are consistent with the values suggested by population synthesis and nucleosynthesis studies. This study supports that NSMs are the major astrophysical site of r-process.Comment: 16 pages, 16 figures, accepted for publication in Ap

Enrichment of the r-process Element Europium in the Galactic Halo

We investigate the enrichment of europium, as a representative of r-process elements, in the Galactic halo. In present chemical evolution models, stars are assumed to be formed through shock processes by supernovae (SNe). The enrichment of the interstellar medium is calculated by a one-zone approach. The observed large dispersions in [Eu/Fe] for halo stars, converging with increasing metallicity, can be explained with our models. In addition, the mass range of SNe for the {\it r}-process site is constrained to be either stars of $8-10 M_\odot$ or $\gtrsim 30 M_\odot$.Comment: 5 pages (including 4 figures), LaTeX, uses aas2pp4.sty, accepted to ApJ

The r-process in the neutrino winds of core-collapse supernovae and U-Th cosmochronology

The discovery of the second highly $r$-process-enhanced, extremely metal-poor star, CS 31082-001 ([Fe/H] $= -2.9$) has provided a powerful new tool for age determination, by virtue of the detection and measurement of the radioactive species uranium and thorium. One of the serious limitations of this approach, however, is that predictions of the production ratio of U and Th have not been made in the context of a realistic astrophysical model of the $r$-process. We have endeavored to produce such a model, based on the ``neutrino winds'' that are expected to arise from the nascent neutron star of a core-collapse supernova. The mass-integrated $r$-process yields, obtained by assuming a simple time evolution of the neutrino luminosity, are compared to the available spectroscopic elemental abundance data of CS 31082-001. As a result, the ``age'' of this star is determined to be $14.1 \pm 2.5$ Gyr, in excellent agreement with lower limits on the age of the universe estimated by other dating techniques, as well as with other stellar radioactive age estimates. Future measurements of Pt and Pb in this star, as well as expansion of searches for additional $r$-process-enhanced, metal-poor stars (especially those in which both U and Th are measurable), are of special importance to constrain the current astrophysical models for the $r$-process.Comment: 23 pages, 7 figures, accepted for publication in Ap

Detection of low Eu abundances in extremely metal-poor stars and the origin of r-process elements

We report abundance analyses of three extremely metal-poor stars with [Fe/H] $\lesssim -3$, using the Subaru High Dispersion Spectrograph (HDS). All are found to have sub-solar values of [Eu/Fe]. Comparison with our chemical evolution model of the Galactic halo implies the dominant source of Eu to be the low-mass end of the supernova mass range. Future studies of stars with low Eu abundances will be important to determine the r-process site.Comment: 7 pages, 2 figures, accepted for publication in the Astrophysical Journal Letter

The r-process in supernova explosions from the collapse of O-Ne-Mg cores

We examine r-process nucleosynthesis in a "prompt supernova explosion" from an 8-10 Msun progenitor star, as an alternative scenario to the "neutrino wind" mechanism. In the present model, the progenitor star has formed an oxygen-neon-magnesium core at its center. The core-collapse simulations are performed with a one-dimension, Newtonian hydrodynamic code. We obtain a very weak prompt explosion, in which no r-processing occurs. We further simulate energetic prompt explosions by enhancement of the shock-heating energy, in order to investigate conditions necessary for the production of r-process nuclei in such events. The highly neutronized ejecta (Ye = 0.14-0.20) leads to robust production of r-process nuclei; their relative abundances are in excellent agreement with the solar r-process pattern. Our results suggest that prompt explosions of 8-10 Msun stars with oxygen-neon-magnesium cores can be a promising site of r-process nuclei.Comment: 32 pages, 9 figures, accepted for publication in Ap

r-Process Calculations and Galactic Chemical Evolution

While the origin of r-process nuclei remains a long-standing mystery, recent spectroscopic studies of extremely metal-poor stars in the Galactic halo strongly suggest that it is associated with core-collapse supernovae. In this article, an overview of the recent theoretical studies of the r-process is presented with a special emphasis on the astrophysical scenarios related to core-collapse supernovae. We also review a recent progress of the Galactic chemical evolution studies as well as of the spectroscopic studies of extremely metal-poor halo stars, which provide us important clues to better understanding of the astrophysical r-process site.Comment: 31 pages, 17figures, Nuclear Physics A (Special Issue on Nuclear Astrophysics / eds. K. Langanke, F.-K. Thielemann, & M. Wiescher), in pres