Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.261m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 261
Astronomy and Astrophysics
Astrophysics
Scientific paper
A basaltic achondrite, Chervony Kut (CK), shows a distinct ^60Ni excess that is probably due to decay of live ^60Fe in the early solar system (Shukolyukov and Lugmair, 1992). The inferred ^60Fe/^56Fe abundance ratio at the time of solidification of the Allende meteorite is 1.6 x 10^-6 (Birck and Lugmair, 1988). Since the mass of the interstellar medium (ISM) is roughly 10^10 solar masses and the mass fraction of ^56Fe is about 1.3 x 10^-3, a uniform, steady-state distribution of ^60Fe (at the ratio ^60Fe/^56Fe = 1.6 x 10^-6) would imply that there are some 17 solar masses of ^60Fe in the ISM today. If this mass were concentrated near the galactic center (10 kiloparsecs), the gamma-ray flux from decay of ^60Fe would be about 10^-3 cm^-2 s^- 1. The flux from a uniform distribution of the mass is similar (Clayton, 1982). Such a large flux is not seen by the SMM Gamma- Ray Observatory (Leising, 1992), which indicates that either ^60Fe/^56Fe has changed by more than a factor of ten in the ISM over the last 5 Gyr or that the solar system formed in a site in the galaxy over-abundant in ^60Fe. The former scenario is unlikely since the ^56Fe mass fraction has only grown by a factor of roughly 1.5 in the last 5 Gyr (Clayton, 1988), and the latter agrees with conclusions from ^26Al (e.g. Clayton, 1985). ^60Fe is produced in the freezeout from neutron-rich nuclear statistical equilibrium (NSE) (Hartmann, Woosley, and El Eid, 1985). It may also be produced during carbon burning by a small burst of neutrons that capture on ^56Fe (e.g. Lee et al., 1979). As a preliminary part of a larger, ongoing project (Hartmann et al., 1991), we have computed the production of ^48Ca, ^50Ti, ^54Cr, and ^60Fe produced during the freezeout from NSE for an initial temperature of 10^10 K, various initial densities and neutron excesses, and an expansion timescale given by the hydrodynamical timescale. In fitting solar ratios of the first three isotopes, we find at most in the steady-state ISM 1-2 solar masses of ^60Fe. This is far less than the 17 solar masses needed to explain the high ^60Fe/^56Fe ratio in CK and lends support to the conclusion that the sun formed in a region of enhanced ^60Fe. We are continuing with a larger parameter survey of ^60Fe production in freezeout from neutron-rich NSE to confirm these conclusions. We are also exploring the production of ^60Fe during carbon burning, although we do not expect it to make a significantly greater contribution to the galactic abundance than neutron-rich NSE (e.g. Lee, 1988). References Birck J. L. and Lugmair G. W. (1988) Earth Planet. Sci. Lett. 90, 131-143. Clayton D. D. (1982) In Essays in Nuclear Astrophysics (eds. C. A. Barnes, D. D. Clayton, and D. N. Schramm), pp. 401- 426. Cambridge Univ. Press, Cambridge. Clayton D. D. (1985) Nature 315, 633-634. Clayton D. D. (1988) In Meteoritics and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews), pp. 1021-1062. Univ. Arizona Press, Tucson, Arizona. Hartmann D. H., Hoffman R. D., Howard W. M., and Meyer B. S. (1991) Meteoritics (abstract) 26, 342. Hartmann, D. H., Woosley S. E., and El Eid M. (1985) Astrophys. J. 297, 837-845. Lee T. (1988) In Meteorites and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews), pp. 1063-1089. Univ. Arizona Press, Tucson, Arizona. Lee, T., Schramm D. N., and Wefel, J. P. (1979) Astrophys. J. 232, 854-862. Leising M. D. (1992) private communication. Shukolyukov A. and Lugmair G. W. (1992) Lunar Planet. Sci. (abstract) 23, 1295.
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