THE Cs-Ba CHRONOMETER AND THE ORIGIN OF EXTINCT NUCLIDES IN THE SOLAR

Introduction: A number of now extinct shortlived nuclides, with half-lives ranging from ~10 (Ca) to 10 years (Sm), were present in the early Solar System and provide constraints on chronometry, nucleosynthetic sources of early Solar System materials and provide a tool for identifying the environment where the Solar System formed (such as isolated star formation versus formation in a cluster of stars). The average ratio of such a short-lived radioactive (R) isotope to a stable (S) reference isotope (NR/NS) in the interstellar medium (ISM) can be estimated from well established principles [1]. Also, various stellar sources have been suggested to explain the observed ratios in the early Solar System [cf. 2]. Normalizing such ratios (NR/NS) to the nucleosynthetic production ratio (PR/PS), defines the parameter αR/S = (NR/NS)/(PR/PS) which is useful in plotting such results versus the mean-life (τ) of the short-lived nuclides under consideration. Jacobsen [3] showed that the data for the early Solar System yield three groups of short-lived nuclides in such a plot of logα vs. logτ as shown in Figure 1. The three groups are: (i) Mn, Hf, Pu and Sm lie on a trend with a slope somewhat less than 2. These nuclides are likely produced by type II supernova (SN) sources injected into the interstellar medium over the history of our galaxy, (ii) I and Pd are substantially below this trend and may have been produced by SN sources which appear to have been more common earlier in the history of our galaxy and (iii) Ca, Al and Fe are too high to be of average galactic production; these must be a contamination from young stellar sources (dissimilar to typical SNII sources) that possibly formed within the proto-Solar molecular cloud (MC). Thus, some nuclides (such as Al, Ca, Fe) may be from contamination of the pre-solar molecular cloud by a single stellar source [supernova (SN), TPAGB star or W-R star] close to the time of Solar System origin. Other nuclides (such as Sm, Pu, Hf and I) may primarily be related to the average composition of the ISM at the origin of the Solar System. Here we add new constraints using the Cs-Ba chronometer based on new high precision Ba isotopic measurements. Mean life (Ma) 0.1 1 10 10