Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.315a&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 315
Astronomy and Astrophysics
Astrophysics
Carbide, Cm Chondrites, Interstellar Dust, Isotopic Anomalies
Scientific paper
The ^21Ne cosmic ray exposure ages of 8 presolar SiC size fractions from Murchison increase with size from 13 Ma up to 133 Ma [1]. The ^22Ne and ^4He concentrations in these fractions also increase with size [1], although the origin of these gases (from the He-shell of AGB stars [1]) is independent of the cosmogenic ^21Ne. Here a model is presented that demonstrates that the apparently correlated behavior of ^22Ne, ^4He and cosmogenic ^21Ne is understandable, despite their disparate origins, if the exposure ages reflect the typical lifetime of grains of different size in the ISM. It is assumed that numerous stars contributed SiC [2,3] to some presolar reservoir at a more or less constant rate. While SiC grains reside in the reservoir, they are bombarded by cosmic rays and their ^21Ne contents rise accordingly. At the same time, grains are being continuously destroyed by shocks etc. [4] and are replaced by SiC that has zero age. Thus, the concentration and average exposure age of the SiC in the reservoir will evolve, but given enough time both will reach a steady state. The ^4He and ^22Ne, at least in the coarser fractions, are carried by only 5% of the grains [5], perhaps because the stars that form gas-rich grains are relatively rare. The majority of the grains are assumed to contain only cosmogenic ^21Ne [5]. The concentration of grains injected into the reservoir by a particular star will decay with time at rates that depend on size. As a result when the solar system forms, the grains from the particular star will be depleted in the finer fractions compared to the coarser ones, the extent of the depletion depending on the age of the grains. Here, then, is a means of varying the ^4He and ^22Ne with size and in the same direction as the exposure ages. Changes in the ^4He/^22Ne ratio with size require that a minimum of two gas-rich populations be present [1] and that they have different ages. For simplicity, it is also assumed that the gas concentration in the gas-rich grains is size independent and that when formed all SiC has the same initial size distribution. A mathematical description of this model has been derived and, despite the simplifications, has been successfully fitted to the ^4He and ^22Ne data from the various size fractions (see Fig. for Ne). For the steady state case the exposure age of the size fractions is their typical lifetimes in the ISM. The ^4He and ^22Ne data require that the two gas-rich populations have presolar exposure ages of 38 +- 7 Ma and 198 +- 28 Ma and that the older grains have higher gas concentrations and/or initial abundances. Although not included here, the Kr isotopes may also be explained by these two populations. If this interpretation is correct, destruction rates for refractory grains in the ISM are size dependent and for submicron grains are up to an order of magnitude faster than theoretical estimates [3]. References: [1] Lewis R. S. et al. (1993) GCA, submitted. [2] Alexander C. M. O'D. (1993) GCA, in press. [3] Hoppe P. et al. (1993) Astrophys. J., submitted. [4] McKee C. F. (1989) In Interstellar Dust, pp. 431-443. [5] Nichols R. H. Jr. et al. (1993) Meteoritics, submitted.
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