Statistics – Computation
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.235h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 235
Statistics
Computation
Scientific paper
Recent determinations of the isotopic composition of a series of heavy elements in small-sized (micron) SiC grains have provided nuclear astrophysics with a wealth of compositional clues as to the astrophysical origin of these grains. In particular, the isotopic compositions of Si, Ca, Ti, Sr, Kr, Xe, Ba, Nd, and Sm (1,2,3,4) have been measured and exhibit significant deviations from their solar system counterparts. Although various astrophysical sources of these anomalous compositions have been proposed, e.g., AGB or Wolf-Rayet stars, no consistent picture has emerged. We t parametrized model in order to investigate in as a broad way as possible the neutron exposures or ranges of such exposures that could replicate the series of measured isotopic compositions without relying on any specific (and highly uncertain) astrophysical scenario. This study is in the same spirit in which one has studied the s-process as well as the r- and p- processes with parametrized models in order to understand the bulk solar system composition. We find that we can reproduce the general trends of the isotopic data for the elements from krypton through samarium with a rather simple neutron exposure history. We also show how one can relax the classical assumption of a two- component mixing curve. We interpret the linear correlations in the three-isotope plots as a mixture of components with slightly different neutron exposures. The neutron captures are calculated by a method described earlier (5,6), where we use the latest compilation of neutron capture rates and of the temperature and density dependent beta-decay rates. For the initial composition we take a solar-like composition (7). We define a standard model by tau = 0.006 mb-1, a neutron exposure timescale of 5x10**6 s and a temperature kT = 30 keV, this implying a neutron density of 5x10**9 cm-3. This model defines a standard set of abundances. A series of computations has been performed with different tau values and initial compositions in order to analyze the sensitivity of the derived abundances to changes in these quantities. We find that such a small exposure replicates many of the features of the abundance pattern for the elements Kr, Sr, Xe, Ba, Nd, and Sm found in the KJ series of SiC grains in Murchison. Table 1 gives a comparison between solar-system values (7) the SiC values (4) and our standard model results for Ba. Furthermore, we reproduce many of the linear trends found in the three-isotope plots with a variation of neutron exposure about our standard model conditions. For an astrophysical site, we suggest a star that has undergone a last pulse during its post planetary nebula phase. It is believed that such a star has been observed (8) and is enriched in s-process elements. That star has presumably suffered significant mass loss prior to the s- processing, so that dilution effects are minimized. REFERENCES (1) Lewis, R. S., Amari, S., and Anders, E. (1990) Nature, 348, 293-298. (2) Amari, S., Lewis, R. S., and Anders, E. (1992) Interstellar Grains in Meteorites. II. SiC and its noble gases, preprint. (3) Richter, S., Ott, U., and Begemann, F. (1992) LPSC XXIII, to be published. (4) Prombo, C. A., Podosek, F. A., Amari, S., and Lewis, R. S. (1992) LPSC XXIII, to be published. (5) Howard, W.M., Meyer, B. S., and Clayton, D. D. (1992) Meteoritics, in press. (6) Howard, W. M., Mathews, G. J., Takahashi, K., and Ward, R. A. (1986) Astrophys. J., 309, 633- 652. (7) Anders, E. and Grevesse, N. (1989) Geochim. Acta 53, 197-214. (8) Langer, G. E., Kraft, R. P., and Anderson, K. S. (1974) Astrophys. J., 189, 509-521. TABLE 1 Barium Ratio Solar SiC Value Standard Model 130Ba/136Ba 0.013 0.003 0.00076 132Ba/136Ba 0.013 0.003 0.0044 134Ba/136Ba 0.31 0.34 0.37 135Ba/136Ba 0.84 0.31 0.24 137Ba/136Ba 1.4 0.87 0.85 138Ba/136Ba 9.1 6.9 5.3
Arnould Marcel
Howard Michael W.
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