Shinya Wanajo

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.

NUCLEOSYNTHESIS IN ELECTRON CAPTURE SUPERNOVAE OF ASYMPTOTIC GIANT BRANCH STARS

W eexam inenucleosynthesisin theelectron capturesupernovaeofprogenitorAGB starswith an O- Ne-M g core(with theinitialstellarm assof8:8M � ).Therm odynam ictrajectoriesfortherst810m s aftercore bounce are taken from a recentstate-of-the-arthydrodynam ic sim ulation. The presented nucleosynthesisresults are characterized by a num ber ofdistinct features that are not shared with thoseofothersupernovaefrom thecollapseofstarswith iron core(with initialstellarm assesofm ore than 10M � ).Firstisthe sm allam ountof 56 Ni(= 0:002 0:004M � )in theejecta,which can bean explanation forobserved propertiesoffaintsupernovaesuch asSNe2008S and 1997D.In addition,the largeNi/Feratioisin reasonableagreem entwith thespectroscopicresultoftheCrab nebula(therelic ofSN 1054).Second isthelargeproduction of 64 Zn, 70 Ge,light p-nuclei( 74 Se, 78 Kr, 84 Sr,and 92 M o), and in particular, 90 Zr,which originatesfrom the low Ye (= 0:46 0:49,thenum berofelectronsper nucleon) ejecta. W end,however,that only a 1 2% increase ofthe m inim um Ye m oderates the overproduction of 90 Zr.In contrast,the production of 64 Zn isfairly robustagainsta sm allvariation of Ye. Thisprovidesthe upperlim itofthe occurrence ofthistype ofeventsto be about30% ofall core-collapsesupernovae. Subjectheadings: nuclearreactions,nucleosynthesis,abundances| stars:abundances| supernovae: general| supernovae:individual(SN 1054,SN 1997D,SN 2008S)| nebulae:Crab Nebula

The rp-Process in Neutrino-driven Winds

Recent hydrodynamic simulations of core-collapse supernovae with accurate neutrino transport suggest that the bulk of the early neutrino-heated ejecta is proton rich, in which the production of some interesting proton-rich nuclei is expected. As suggested in recent nucleosynthesis studies, the rapid proton-capture (rp) process takes place in such proton-rich environments by bypassing the waiting point nuclei with β+-lives of a few minutes via the faster capture of neutrons continuously supplied from the neutrino absorption by protons. In this study, the nucleosynthesis calculations are performed with a wide range of neutrino luminosities and electron fractions (Ye), using semianalytic models of proto-neutron-star winds. The masses of proto-neutron stars are taken to be 1.4 and 2.0 M☉, where the latter is regarded as the test for somewhat high-entropy winds (about a factor of 2). For Ye > 0.52, the neutrino-induced rp-process takes place in many wind trajectories, and p-nuclei up to A ~ 130 are synthesized in interesting amounts. However, 92Mo is somewhat underproduced compared to other p-nuclei with similar mass numbers. For 0.46 < Ye < 0.49, on the other hand, 92Mo is significantly enhanced by the nuclear flows in the vicinity of the abundant 90Zr that originates from the α-process at higher temperature. The nucleosynthetic yields are averaged over the ejected masses of winds, and further, over the Ye distribution predicted by a recent hydrodynamic simulation of a core-collapse supernova. Comparison of the Ye- and mass-averaged yields to the solar compositions implies that the neutrino-driven winds can potentially be the origin of light p-nuclei up to A ~ 110, including 92,94Mo and 96,98Ru, that cannot be explained by other astrophysical sites.

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.

Cold r-Process in Neutrino-Driven Winds

The r-process in a low-temperature environment is explored, in which the neutron emission by photodisintegration does not play a role (cold r-process). A semianalytic neutrino-driven wind model is utilized for this purpose. The temperature in a supersonically expanding outflow can quickly drop to a few 108 K, where the (n,γ)-(γ,n) equilibrium is never achieved during the heavy r-nuclei synthesis. In addition, the neutron capture competes with the β-decay owing to the low matter density. Despite such nonstandard physical conditions for the cold r-process, a solar-like r-process abundance curve can be reproduced. The cold r-process predicts, however, low lead production compared to that expected in the traditional r-process conditions, which can be a possible explanation for the low lead abundances found in a couple of r-process-rich Galactic halo stars.

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

Recent spectroscopic studies have revealed the presence of numerous carbon-enhanced, metal-poor stars with [Fe/H] < -2.0 that exhibit strong enhancements of s-process elements. These stars are believed to be the result of a binary mass transfer episode from a former asymptotic giant branch (AGB) companion that underwent s-process nucleosynthesis. However, several such stars exhibit significantly lower Ba/Eu ratios than solar s-process values. This might be explained if there were an additional contribution from the r-process, thereby diluting the Ba/Eu ratio by extra production of Eu. We propose a model in which the double enhancements of r-process and s-process elements originate from a former 8-10 M☉ companion in a wide binary system, which may undergo s-processing during an AGB phase, followed by r-processing during its subsequent supernova explosion. The mass of Eu (as representative of r-process elements) captured by the secondary through the wind from the supernova is estimated and is assumed to be proportional to the geometric fraction of the secondary (low-mass, main-sequence) star with respect to the primary (exploding) star. We find that the estimated mass is in good agreement with a constraint on the Eu yield per supernova event obtained from a Galactic chemical evolution study, when the initial orbital separation is taken to be ~1 yr. If one assumes an orbital period on the order of 5 yr, the efficiency of wind pollution from the supernova must be enhanced by a factor of ~10. This may, in fact, be realized if the expansion velocity of the supernovas innermost ejecta, in which the r-process has taken place, is significantly slow, resulting in an enhancement of accretion efficiency by gravitational focusing.

The r-Process in Supersonic Neutrino-driven Winds: The Role of the Wind Termination Shock

Recent hydrodynamic studies of core-collapse supernovae imply that the neutrino-heated ejecta from a nascent neutron star develop to supersonic outflows. These supersonic winds are influenced by the reverse shock from the preceding supernova ejecta, forming the wind termination shock. We investigate the effects of the termination shock in neutrino-driven winds and its role in the r-process. Supersonic outflows are calculated with a semianalytic neutrino-driven wind model. Subsequent termination-shocked, subsonic outflows are obtained by applying the Rankine-Hugoniot relations. We find a couple of effects that can be relevant for the r-process. First is the sudden slowdown of the temperature decrease by the wind termination. Second is the entropy jump by termination-shock heating, up to several hundred NAk. Calculations of nucleosynthesis in the obtained winds are performed to examine these effects on the r-process. We find that the slowdown of the temperature decrease plays a decisive role in determining the r-process abundance curves. This is due to the strong dependences of the nucleosynthetic path on the temperature during the r-process freezeout phase. Our results suggest that only the termination-shocked winds with relatively small shock radii (~500 km) are relevant for the bulk of the solar r-process abundances (A ≈ 100-180). The heaviest part of the solar r-process curve (A ≈ 180-200), however, can be reproduced in both shocked and unshocked winds. These results may help to constrain the mass range of supernova progenitors relevant for the r-process. We find, on the other hand, a negligible role of the entropy jump in the r-process. This is because the sizable entropy increase takes place only at a large shock radius (10,000 km), where the r-process has already ceased.

The R-process in proto-neutron star winds with anisotropic neutrino emission

The astrophysical origin of the r-process nuclei is still unknown. Even the most promising scenario, the neutrino-driven winds from a nascent neutron star, encounters severe difficulties in obtaining the requisite entropy and short dynamic timescale for the r-process. In this study, the effect of anisotropy in neutrino emission from a proto-neutron star surface is examined with semianalytic neutrino-driven wind models. The increase of neutrino number density in the wind owing to the anisotropy is modeled schematically by enhancing the effective neutrino luminosity. It is shown that the neutrino heating rate from neutrino-antineutrino pair annihilation into electron-positron pairs can significantly increase owing to the anisotropy and can play a dominant role in the heating of wind material. A factor of 5 increase in the effective neutrino luminosity results in a 50% higher entropy and a factor of 10 shorter dynamic timescale owing to this enhanced neutrino heating. The nucleosynthesis calculations show that this change is enough for the robust r-process, producing the third abundance peak (A = 195) and beyond. Future multidimensional studies with accurate neutrino transport will be needed if such anisotropy relevant to the current scenario (more than a factor of a few) is realized during the wind phase (~1-10 s).

Light-Element Production in the Circumstellar Matter of Energetic Type Ic Supernovae

We investigate energetic Type Ic supernovae as production sites for 6Li and Be in the early stages of the Milky Way. Recent observations have revealed that some very metal-poor stars with [Fe/H] < -2.5 possess unexpectedly high abundances of 6Li. Some also exhibit enhanced abundances of Be as well as N. From a theoretical point of view, recent studies of the evolution of metal-poor massive stars show that rotation-induced mixing can enrich the outer H and He layers with C, N, and O (CNO) elements, particularly N, and at the same time cause the intense mass loss of these layers. Here we consider energetic supernova explosions occurring after the progenitor star has lost all but a small fraction of the He layer. The fastest portion of the supernova ejecta and the circumstellar matter (CSM), both of which are composed of He and CNO, can interact directly and induce light-element production through spallation and He-He fusion reactions. The CSM should be sufficiently thick to energetic particles so that the interactions terminate within its innermost regions. We calculate the resulting 6Li/O and 9Be/O ratios in the ejecta + CSM material out of which the very metal-poor stars may form. We find that they are consistent with the observed values if the mass of the He layer remaining on the preexplosion core is ~0.01-0.1 M☉ and if the mass fraction of N mixed in the He layer is ~0.01. Further observations of 6Li, Be, and N at low metallicity should provide us with critical tests of this production scenario.