Folie 1

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(He and) Ne ages of circumstellar silicon carbide
grains
Philipp Heck, Frank Gyngard, Ulrich Ott, Matthias
Meier, Janaina Avila, Sachiko Amari, Ernst
Zinner, Roy Lewis, Heinrich Baur, Rainer
Wieler Uli Ott Torino, September
27,2008
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• STRUCTURE
• motivation ?
• historical notes
• the recoil problem
• new results for Jumbo grains
• conclusions /outlook

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Historical notes, the pioneers Tang and Anders
(1988) Lewis, Amari and Anders (1994)
Ne analysis of Murchison KJ grain size fractions
rightward shift relative to G-solar mixing
line evidence for spallation 21Ne produced on Si
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inferred presolar ageslt 10 up to 100 Ma-
scaling with Ne-G content (?!?)
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• Historical problems / mistaken assumptions
  and mistakes
  
• problem production rate of cosmogenic Ne
  in ISM (cross sections, cosmic ray flux
  and spectrum) ? may never go fully away
• assumption a constant, fix (21Ne/22Ne)G
  ratio ? a task for Roberto and colleagues
• mistake recoil loss correction assuming
  recoil range of cosmogenic 21Ne 0.1 µm,
  where correct value on the order of 2-3 µm

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Tang and Anders (and subsequently Lewis et al.)
severely underestimated recoil losses several
things went wronga) application of incorrect
approach of Ray and Völkb) wrong scaling to
range of 100 keV ions (Ar like RV instead of Ne)
corrected shown experiment range 2.5 µm
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The Greiner experiment

• fragmentation of C, O nuclei to produce He, Li,
• momentum distribution Gaussian isotropic in
  system where C,O nuclei at rest
• like in grains? good data base for He, Li recoil
• mistake of Ray and Völkaveraging over momentum
  including direction
• however two nuclei going into opposite direction
  with large momentum will be lost, even if their
  average momentum is close to zero!!!

best approach fold momentum /energy
distribution with energy-range relationship ?
recoil losses
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• mistake of Ray and Völkaveraging over momentum
  including direction
• however two nuclei going into opposite direction
  with large momentum (energy) will be lost- even
  if their average momentum is close to zero!!!-
  and their average position will be within the
  grain!

best approach do not use an average momentum
/energy, but fold momentum /energy distribution
with energy-range relationship ? recoil losses
as function of grain size
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also Greiner data (crosses at lower left) plot
below empirical line that describes well
situation for fragmentation with larger
target-product mass difference (survey by
Morissey, 1989) ? good for He, Li on C, O, but
not for Ne on Si
slope 150 (MeV/c) / u1/2
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New results Jumbo grains

large grains 5-35 µm (except for one small
2 µm grain not considered here) ? recoil
correction unproblematic
in this size range perfect agreement of recoil
losses for fixed range of 2.5 µm (experiment,
Morissey relation similar) and from theoretical
predictions of recoil energy distribution
(Wrobel)
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moreover fraction of 21Ne that is cosmogenic
is much larger than for the Lewis et al. (94)
grain size separates

• always more than 30, mostly gt50 , up to 97 of
  21Ne cosmogenic
• choice of (21Ne/22Ne)-G, whether 0.00059 (Lewis
  et al., 1994) or 0.0033 (for maximum 18O(a,n)21Ne
  rate Karakas et al, 2008) uncritical

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• analytical problems related to the shape of the
  grains
• spherical assumed for recoil loss calculations
• even more serious determination of volume /
  mass? concentrations

two examples
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The results

• 21Ne ages mostly lt 200 Ma, distinctly shorter
  than expected interstellar grain lifetimes
  of 500 Ma
• no obvious trend with grain size

production rates from Reedy (1989), x1.33 for
a-induced reactions

• maybe one really old grain at 1070?460 Ma
• two expected ages
• many young ages

• AB grains older?

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remarkable reasonable agreement (within factor
2) between Ne and He ages, where both could be
determined
relative retention over range of interest
0.3-0.4
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comparison - Li ages generally older
also included upper limits from grains with no
detected 21Ne
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• SUMMARY / OUTLOOK
• Ne (and He) ages generally much shorter than
  expected 500 Ma lifetime of IS grains
• Ne (and He) ages shorter (?) than Li ages
  needs to be understood / verified
• young ages would be consistent with fresh
  appearance of non-destructively isolated grains
• as well as inferred Xe ages from KJ grain size
  separates and scenario suggested there (Ott et
  al., 05)

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• a scenario for young grains
• starburst 2 Ga before SS formation (as
  suggested by Clayton in connection with Si
  isotopes)
• AGB parent stars of SiC most likely mass
  1.5-2 M? ? at 500 Ma before SS formation not
  yet in AGB phase
• first grains from these sources arrived only
  shortly before SS formed

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• would be nice to have ages for more typical
  smaller grains, not just the exotic Jumbos
• note Jumbos are poor in Ne-G-- KJA (0.4 µm)
  to KJG (3.0 µm) 2000 40,000 (increasing with
  size) -- while Jumbos only between 0 and 1,000
  (in units of 10-8 cc/g)

• a correlation in the Lewis et al. (1994) data
• Roberto et al. can we possibly infer the
  correct 21Ne/22Ne of the G component in a given
  grain / sample using its (simultaneously
  measured) Kr composition as a guide?