Nobuyuki Iwamoto

Neutron Capture Elements in s -Process-rich, Very Metal-poor Stars

We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly aUected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the —nal abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z 38) to Pb (Z 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the eUects of hyper—ne splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any speci—c stellar model. The neutron exposure q is estimated to be about 0.7 mbarn~1, signi—cantly larger than that which best —ts solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also —nd that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r D 0.1).

Flash-Driven Convective Mixing in Low-Mass, Metal-deficient Asymptotic Giant Branch Stars: A New Paradigm for Lithium Enrichment and a Possible s-Process

We have calculated models for low-mass, metal-deficient ([Fe/H] = -2.7) stars from the zero-age main sequence through the thermally pulsing asymptotic giant branch (TP-AGB) phase. We confirm that the entropy barrier between the H-rich envelope and the He intershell can be surmounted by the energy released by thermal pulses during the early phase of the TP-AGB. For models in the mass range of 1 ≤ M/M☉ < 3, this energy release causes the top of the flash-driven convective shell to reach into the bottom of the overlying H-rich envelope. Protons are then carried downward into the hotter He- and 12C-rich layer, while He intershell material is mixed upward. This phenomenon causes the surface chemical composition to change dramatically. In particular, surface abundances are enriched in CNO elements by as much as 1 to 3 orders of magnitude. Lithium is also enhanced by this event in the 1, 1.5, and 2 M☉ models. We have also studied the formation and reactions of 13C as protons are mixed into the He intershell. We find that this mixed material experiences the s-process through the α-capture reaction on newly synthesized 13C under convective conditions during the thermal pulse. This results in neutron-capture nucleosynthesis under relatively high neutron density environments. In lower mass models, the s-abundance distributions would be characterized by the small number of neutron irradiations through the standard s-process, which occurs under radiative conditions in a 13C pocket as a result of the immediate termination of the third dredge-up. Accordingly, in extremely metal-poor stars, we may observe the s-element distributions mainly created by the s-processing relevant to the proton-mixing event. Furthermore, we discuss possible observational signatures of the mixing of protons into He-burning regions.

Europium Isotope Ratios in s-Process Element-enhanced Metal-poor Stars: A New Probe of the 151Sm Branching*

We report on the first measurement of the Eu isotope fractions (151Eu and 153Eu) in s-process element-enhanced metal-poor stars. We use these ratios to investigate the 151Sm branching of s-process nucleosynthesis. The measurement was made by detailed study of Eu II lines that are significantly affected by hyperfine splitting and isotope shifts in spectra of the carbon-rich very metal poor stars LP 625-44 and CS 31062-050, observed with the Subaru Telescope High Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu + 153Eu)] derived for LP 625-44 and CS 31062-050 are 0.60 and 0.55, respectively, with uncertainties of about ±0.05. These values are higher than found in solar system material but agree well with the predictions of recent s-process models. We derive new constraints on the temperature and neutron density during the s-process based on calculations of pulsed s-process models for the 151Eu fraction.

Nucleosynthesis in baryon-rich outflows associated with gamma-ray bursts

Robust generation of gamma-ray bursts (GRBs) implies the formation of outflows with very low baryon loads and highly relativistic velocities, but more baryon-rich, slower outflows are also likely to occur in most GRB central engine scenarios, either as circumjet winds or as failed GRBs. Here we study the possibility of nucleosynthesis within such baryon-rich outflows by conducting detailed reaction network calculations in the framework of the basic fireball model. It is shown that high baryon load fireballs attaining mildly relativistic velocities can synthesize appreciable quantities of heavy neutron capture elements with masses up to the platinum peak and beyond. Small but interesting amounts of light elements such as deuterium and boron can also be produced. Depending on the neutron excess and baryon load, the combination of high entropy, rapid initial expansion, and gradual expansion at later times can cause the reaction flow to reach the fission regime, and its path can be intermediate between those of the r- and s-processes (n-process). The nucleosynthetic signature of these outflows may be observable in the companion stars of black hole binary systems and in the most metal-poor stars, potentially offering an important probe of the inner conditions of the GRB source. Contribution to the solar abundances for some heavy elements may also be possible. The prospects for further developments in various directions are discussed.

A new model for s-process nucleosynthesis in low-mass, low-metallicity AGB stars

Abstract We have investigated evolution of low-mass, low-metallicity AGB stars. We evolved models with masses ranging from 1 to 3 M ⊙ and metallicity of [Fe/H] = −2.7. We found for M ≲ 2.5 M ⊙ models that in the early phase of the AGB evolution convective shell triggered by thermal runaway of He shell burning extends upwards and reaches into the bottom of the overlying H-rich envelope. Protons are mixed into hotter He intershell. Proton capture reactions occur under He burning conditions and lead to release of huge amounts of energy. This phenomenon causes the surface chemical composition to change dramatically. We present surface abundance changes of CNO elements and lithium in this event.

Nucleosynthesis in low-mass, low-metallicity AGB stars

Abstract The evolution of stars with masses ranging between 1 and 3M⊙, metallicity [ Fe H ] = − 2.7 and He mass fraction Y = 0.24 is followed from the zero-age main sequence through core He burning up to the thermally pulsing asymptotic giant branch phase. We find that the second thermal pulse leads to the injection of protons into a flash-driven convective shell, the outer part of which reaches the lower part of the hydrogen tail. The energy produced by proton capture reactions then induces various changes in the stellar interior. The main results of this event are that the surface abundances of CNO and 7Li become highly enhanced relative to solar abundance. We describe a new production mechanism for 7Li by this phenomenon.