Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0333a&link_type=abstract
American Astronomical Society, AAS Meeting #212, #3.33; Bulletin of the American Astronomical Society, Vol. 40, p.190
Astronomy and Astrophysics
Astronomy
Scientific paper
There are several isotopically distinct noble gas components in meteorites. Of them, Ne-E(L), heavily enriched in 22Ne, is carried by graphite with a range of density (1.6 - 2.2 g/cm3). Bulk (=aggregates) noble gas analysis of graphite separates from the Murchison meteorite indicate that a dominant source of 22Ne is 22Na (T1/2 = 2.6 a) with varying proportions of 22Ne via 14N(α,γ)18F(e+ν)18O(α,γ)22Ne with density. Low-density graphite grains, from their isotopic signatures, are believed to have formed in supernovae. Examinations of both bulk and single-grain analyses of low-density graphite grains (Amari et al., 1995; Nichols et al., 1994) indicate that all 22Ne in low-density graphite grains is from the decay of 22Na that was produced in the O/Ne zone in supernovae.
One may argue why implanted 20,22Ne was not observed in the grains, considering the fact that the mass fraction of 20Ne is 5 orders of magnitude larger than that of 22Na. Croat et al. (2003) observed TiC subgrains inside low-density graphite grains have amorphous rims with the thickness of 3 to 15 nm, indicating atom bombardment from the surrounding gas. Assuming the gas is He, they estimated the velocity is 50 km/s or less. If the relative velocities between the Ne and the graphite grains are in that range, the penetration depth into the graphite grains is 2nm. Such shallow surface layers would be sputtered once the grains hit the reverse shock and keep traveling into the hot H-rich region (Nozawa et al, 2007).
It remains to be seen whether or not 22Na in higher-density graphite is from supernovae or novae, or both.
Amari, S. et al. 1995, Geochim. Cosmochim. Acta, 59, 1411
Croat, T.K. et al. 2003, Geochim. Cosmochim. Acta, 67, 4705
Nichols R.H. et al. 1994, Meteoritics, 29, 510
Nozawa, T. et al. 2007 ApJ, 666, 955
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