Yuhri Ishimaru

Neutron-capture elements in the very metal poor star HD 122563

We obtained high-resolution, high signal-to-noise ratio (S/N) spectroscopy for the very metal poor star HD 122563 with the Subaru Telescope High Dispersion Spectrograph. Previous studies have shown that this object has excesses of light neutron-capture elements, while its abundances of heavy ones are very low. In our spectrum, which covers 3070-4780 A of this object, 19 neutron-capture elements have been detected, including seven for the first time in this star (Nb, Mo, Ru, Pd, Ag, Pr, and Sm). Upper limits are given for five other elements including Th. The abundance pattern shows a gradually 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. This abundance pattern of neutron-capture elements provides new strong constraints on the models of nucleosynthesis responsible for the very metal poor stars with excesses of light neutron-capture elements but without enhancement of heavy ones.

The Iron Discrepancy in Elliptical Galaxies after ASCA

We present estimates for the iron content of the stellar and diffused components of elliptical galaxies, as derived respectively from integrated optical spectra and from ASCA X-ray observations. A macroscopic discrepancy emerges between the expected iron abundances in the hot interstellar medium (ISM) and what is indicated by the X-ray observations, especially when allowance is made for the current iron enrichment by Type Ia supernovae. This strong discrepancy, that in some extreme instances may be as large as a factor of ~20, calls into question our current understanding of supernova enrichment and chemical evolution of galaxies. We discuss several astrophysical implications of the inferred low iron abundances in the ISM, including the chemical evolution of galaxies and clusters of galaxies, the evolution of gas flows in elliptical galaxies, and the heating of the intracluster medium. Some of the consequences appear hard to accept, and in the attempt to avoid some of the difficulties we explore ways of hiding or diluting iron in the ISM of ellipticals. None of these possibilities appears astrophysically plausible, and we alternatively raise the question of the reliability of iron L line diagnostic tools that are currently used to infer abundances from X-ray spectra. Various thin-plasma emission models are shown to give iron abundances that may differ significantly, especially at low temperatures (kT 1 keV), when the iron L complex is dominated by iron ions with still many bound electrons. From a collection of ASCA and other X-ray observatory data, it is shown that current thin-plasma codes tend to give very low iron abundances when the temperature of the objects is below ~1 keV. Such objects include various types of binary stars, supernova remnants, starburst galaxies, and AGNs, with the case of galaxy groups being especially well documented. We conclude that, besides rethinking the chemical evolution of galaxies, one should also consider the possibility that existing thin-plasma models may incorporate inaccurate atomic physics for the ions responsible for the iron L complex.

The r-Process in Supernova Explosions from the Collapse of O-Ne-Mg Cores

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 study we examine r-process nucleosynthesis in a prompt supernova explosion from an 8-10 M☉ progenitor star as an alternative scenario to the neutrino wind mechanism, which has also been considered a promising site of the r-process. In the present model, the progenitor star has formed an oxygen-neon-magnesium (O-Ne-Mg) core (of mass 1.38 M☉) at its center. Its smaller gravitational potential, as well as the smaller core that is in nuclear statistical equilibrium at the time of core bounce, as compared with the iron cores in more massive stars, may allow the star to explode hydrodynamically rather than by delayed neutrino heating. The core-collapse simulations are performed with a one-dimensional, 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 r-process nucleosynthesis is calculated using a nuclear reaction network code including relevant neutron-rich isotopes with reactions among them. The highly neutronized ejecta (Ye ≈ 0.14-0.20) lead 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 M☉ stars with O-Ne-Mg cores can be a promising site of r-process nuclei. The mass of the r-process material per event is about 2 orders of magnitude larger than that expected from Galactic chemical evolution studies. We propose, therefore, that only a small fraction of r-process material is ejected via mixing-fallback mechanism of the core matter, wherein most of the r-process material falls back onto the proto-neutron star. A lower limit on the age of the universe is derived by application of the uranium-thorium (U-Th) chronometer pair by comparison with the observed ratio of these species in the highly r-process-enhanced, extremely metal poor star CS 31082-001. The inferred age is 14.1 ± 2.4 Gyr—the same as that obtained previously based on the neutrino wind scenario with the same nuclear mass formula. This suggests that chronometric estimates obtained using the U-Th pair are independent of the astrophysical conditions considered.

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 r-process site is constrained to be either stars of 8-10 or 30 M☉.

Neutron-Capture Elements in the Very Metal-poor Star HD 88609: Another Star 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 HD 88609 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 HD 122563, which was studied in 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.

r-process calculations and Galactic chemical evolution

Abstract 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.

Roles of SN Ia and SN II in ICM Enrichment

Based on ASCA observations Mushotzky et al. (1996, ApJ 466, 686) have recently derived the relative-abundance ratios of

The Detection of Low Eu Abundances in Extremely Metal-poor Stars and the Origin of r-Process Elements

alpha

Enrichment of Very Metal Poor Stars with Both r-Process and s-Process Elements from 8-10 M☉ Stars

-elements to iron, [alpha/Fe] simeq 0.2-0.3

Deuterium at high redshift: Primordial or evolved?

, 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