Abundances of Actinides and Short-lived Nonactinides in the Interstellar Medium: Diverse Supernova Sources for the r-Processes (original) (raw)
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Earth and Planetary Science Letters, 2006
The short-lived nuclide 247 Cm is produced by r-process nucleosynthesis. When the presolar nebula formed, 247 Cm became isolated from r-process production and its abundance diminished as a result of radioactive decay. Given its short half-life of only ∼ 16 million years, 247 Cm is presently extinct, but its former presence should be detectable as small variations in 235 U/ 238 U in primitive meteoritic material, provided Cm was chemically fractionated from U at the time these solid objects formed. The magnitude of U isotopic anomalies in meteorites can thus be used to elucidate the timing and character of the last r-process nucleosynthetic event for input into models describing the formation and evolution of the early solar system. Here we report coupled U isotopic determinations and Nd/U proxy measurements for Cm/U in a series of acid-etched leachates and mineral assemblages extracted from meteorites containing primitive phases expected to show strong Cm-U fractionations. Using multiplecollector ICPMS, we are able to determine 235 U/ 238 U with 2σ analytical uncertainties of ± 1 epsilon (1 epsilon = 1 part in 10,000) on sample sizes consisting of b 3 ng of 238 U and b 20 pg of 235 U. A double-spiking procedure using a mixed 236 U-233 U spike was employed to allow instrumental mass fractionation to be reliably corrected internally and at high precision. Uranium isotopic results for almost 40 different phases show no resolvable deviations in 235 U/ 238 U from the chondritic value, at the ∼ 1-2 epsilon level. These data supplement our previous observations for a suite of bulk meteorite samples [C.H. Stirling, A.N. Halliday, D. Porcelli, In search of live 247 Cm in the early solar system, Geochim. Cosmochim. Acta 69 (2005) 1059-1071] and provide evidence for a solar system initial 247 Cm/ 235 U of b 8 × 10 − 5 . Such a low value is difficult to explain without a long time-scale of ∼ 2.3 × 10 8 years between the last actinide producing r-process event and the formation of the solar system. As such it is difficult to reconcile with a model of actinide production in the same r-process forming event as 182 Hf with a half-life of 8.9 My [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the rprocesses, Astrophys. J. 466 (1996) L109-L113]. The alternative models of 182 Hf production via a neutron-rich fast s-process, occurring, for example, in the helium burning shell in a 25 solar mass star during explosive nucleosynthesis [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109-L113], or via a distinct r-process event that is separate from actinide production [G.J. Wasserburg, M. Busso, R. Gallino, K.M. Nollet, Short-lived nuclei in the early solar system: Possible AGB sources, Nucl.
Production of solar abundances for nuclei beyond Sr: The s- and r-process perspectives
Frontiers in Astronomy and Space Sciences
We present the status of nucleosynthesis beyond Sr, using up-to-date nuclear inputs for both the slow (s-process) and rapid (r-process) scenarios of neutron captures. It is now widely accepted that at least a crucial part of the r-process distribution is linked to neutron star merger (NSM) events. However, so far, we have found only a single direct observation of such a link, the kilonova GW170817. Its fast evolution could not provide strict constraints on the nucleosynthesis details, and in any case, there remain uncertainties in the local r-process abundance patterns, which are independent of the specific astrophysical site, being rooted in nuclear physics. We, therefore, estimate the contributions from the r-process to solar system (S.S.) abundances by adopting the largely site-independent waiting-point concept through a superposition of neutron density components normalized to the r-abundance peaks. Nuclear physics inputs for such calculations are understood only for the trans-F...
Nucleosynthesis in the early Galaxy
2008
Recent observations of r-process-enriched metal-poor star abundances reveal a non-uniform abundance pattern for elements Z ≤ 47. Based on non-correlation trends between elemental abundances as a function of Eu-richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent non-uniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron capture process within a site-independent approach. It is suggested that scenarios with neutron densities n n ≤ 10 13 cm −3 or in the range n n ≥ 10 24 cm −3 best explain the observations.
Short-Lived Nuclei in the Early Solar System: A Low Mass Stellar Source
Publications of The Astronomical Society of Australia, 2003
We discuss possible stellar origins of short-lived radioactive nuclei with meanlifeτ ≤ 100 Myr, which were shown to be alive in the Early Solar System (ESS). We first review current ideas on the production of nuclides having 10 ≤τ ≤ 100 Myr, which presumably derive from the continuous interplay of galactic astration, nucleosynthesis from massive supernovae and free decay in the interstellar medium. The abundance of the shorter lived 53 Mn might be explained by this same scenario. Then we consider the nuclei 107 Pd, 26 Al, 41 Ca and 60 Fe, whose early solar system abundances are too high to have originated in this way. Present evidence favours a stellar origin, particularly for 107 Pd, 26 Al and 60 Fe, rather than an in situ production by energetic solar particles. The idea of an encounter (rather close in time and space) between the forming Sun and a dying star is therefore discussed: this star may or may not have also triggered the solar formation. Recent nucleosynthesis calculations for the yields of the relevant short-lived isotopes and of their stable reference nuclei are discussed. Massive stars evolving to type II supernovae (either leaving a neutron star or a black hole as a remnant) seem incapable of explaining the four most critical ESS radioactivities in their observed abundance ratios. An asymptotic giant branch (AGB) star seems to be a viable source, especially if of relatively low initial mass (M ≤ 3 M ) and with low neutron exposure: this model can provide a solution for 26 Al, 41 Ca and 107 Pd, with important contributions to 60 Fe, which are inside the present uncertainty range of the 60 Fe early solar system abundance. Such a model requires that 26 Al is produced substantially on the AGB by cool bottom processing. The remaining inventory of short-lived species in the solar nebula would then be attributed to the continuous galactic processing, with the exception of 10 Be, which must reflect production by later proton bombardment at a low level during early solar history.
Nucleosynthesis: Stellar and Solar Abundances and Atomic Data
2006
Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., sand r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy-the progenitors of the halo stars-responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis.
Short-lived nuclei in the early Solar System: Possible AGB sources
Nuclear Physics A, 2006
The abundances of short-lived radionuclides in the early Solar System (ESS) are reviewed, as well as the methodology used in determining them. These results are compared with the inventory estimated for a uniform galactic production model. It is shown that, to within a factor of two, the observed abundances of 238 U, 235 U, 232 Th, 244 Pu, 182 Hf, 146 Sm, Mn are roughly compatible with long-term galactic nucleosynthesis. 129 I is an exception, with an ESS inventory much lower than expected from uniform production. The isotopes 107 Pd, 60 Fe, 41 Ca, 36 Cl, 26 Al, and 10 Be require late addition to the protosolar nebula. 10 Be is the product of energetic particle irradiation of the Solar System as most probably is 36 Cl. Both of these nuclei appear to be present when 26 Al is absent. A late injection by a supernova (SN) cannot be responsible for most of the short-lived nuclei without excessively producing 53 Mn; it can however be the source of 53 Mn itself and possibly of 60 Fe. If a late SN injection is responsible for these two nuclei, then there remains the problem of the origin of 107 Pd and several other isotopes. Emphasis is given to an AGB star as a source of many of the nuclei, including 60 Fe; this possibility is explored with a new generation of stellar models. It is shown that if the dilution factor (i.e. the ratio of the contaminating mass to the solar parental cloud mass) is f 0 ∼ 4 × 10 −3 , a reasonable representation for many nuclei is obtained; this requires that ( 60 Fe/ 56 Fe) ESS ∼ 10 −7 to 2 × 10 −6 . The nuclei produced by an AGB source do not include 53 Mn, 10 Be or 36 Cl if it is very abundant. The role of irradiation is discussed with regard to 26 Al, 36 Cl and 41 Ca, and the estimates of bulk solar abundances of these isotopes are commented on. The conflict between various scenarios is emphasized as well as the current absence of an astrophysically plausible global interpretation for all the existing data. Examination of abundances for the actinides indicates that a quiescent interval of ∼ 10 8 yr is required for actinide group production. This is needed in order to explain the data on 244 Pu and the new bounds on 247 Cm. Because this quiescent interval is not compatible with the 182 Hf data, a separate type of r-process event is needed for at least the actinides, distinct from the two types that have previously been identified. The apparent coincidence of the 129 I and trans-actinide time scales suggests that the last heavy r contribution was from an r-process that produced very heavy nuclei but without fission recycling so that the yields at Ba and below (including I) were governed by fission.
R-Process Nucleosynthesis in Supernovae
A lmost all of the hydrogen and helium in the cosmos, along with some of the lithium, was created in the first three minutes after the Big Bang. Two more light elements, beryllium and boron, are synthesized in interstellar space by collisions between cosmic rays and gas nuclei. All of the other elements in nature are formed by nuclear reactions inside stars.
Galactic Chemical Evolution of Radioactive Isotopes with an s-process Contribution
2021
Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives 0.1− 100 Myr existed in the early Solar System (ESS). We investigate the ESS origin of Pd, Cs, and Hf, which are produced by slow neutron captures (the s-process) in asymptotic giant branch (AGB) stars. We modelled the galactic abundances of these SLRs using the OMEGA+ galactic chemical evolution (GCE) code and two sets of massand metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean life τ of the SLR to the average length of time between the formation of AGB progenitor γ, we calculate timescales relevant for the birth of the Sun. If τ/γ & 2, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted Pd/Pd, Cs/Cs, and Hf/Hf ratios to their respective ESS ratios. The predicted Pd/Hf ratio indicates that our GCE models are missing 9− 73% of Pd and Pd in the ESS. This missing component ...