Oxygen and Aluminum-Magnesium Isotopic Systematics of Presolar
  Nanospinel Grains from CI Chondrite Orgueil
  Presolar oxide grains have been previously divided into several groups (Group
1 to 4) based on their isotopic compositions, which can be tied to several
stellar sources. Much of available data was acquired on large grains, which may
not be fully representative of the presolar grain population present in
meteorites. We present here new O isotopic data for 74 small presolar oxide
grains (~200 nm in diameter on average) from Orgueil and Al-Mg isotopic
systematics for 25 of the grains. Based on data-model comparisons, we show that
(i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent
(red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red
giant stars and (ii) Group 1 grains with (26Al/27Al)0 >= 5x10^-3 and Group 2
grains with (26Al/27Al)0 <= 1x10^-2 all likely experienced extra circulation
processes in their parent low-mass stars but under different conditions,
resulting in proton-capture reactions occurring at enhanced temperatures. We do
not find any large 25Mg excess in Group 1 oxide grains with large 17O
enrichments, which provides evidence that 25Mg is not abundantly produced in
low-mass stars. We also find that our samples contain a larger proportion of
Group 4 grains than so far suggested in the literature for larger presolar
oxide grains (~400 nm). We also discuss our observations in the light of
stellar dust production mechanisms.
