Multi-element isotopic analyses of presolar graphite grains from the Orgueil meteorite

This dissertation presents the results and implications of the isotopic analyses of presolar graphite grains from the primitive carbonaceous chondrite, Orgueil. Graphite grains from low- and high-density fractions were analyzed for C, N, O, Si, Al-Mg, K, Ca, and Ti isotopes. These analyses indicate that isotopic properties are density dependent. Most low-density grains come from supernovae as indicated by large 18O, 15N, 28Si excesses, high inferred 26Al/27Al and 41Ca/40Ca ratios, and the initial presence of the short-lived radionuclide 44Ti in some grains. Some high-density grains also show supernovae signatures, but a majority seems to originate from low-metallicity asymptotic giant branch stars. Evidence for this comes from correlated 12C and 30Si enrichments. In low-metallicity asymptotic giant branch stars, 12C and 29,30Si that are produced in the He shell are mixed into the envelope by the third dredge-up during the thermally pulsing phase. This scenario also increases the C/O ratio of the envelope and favours the condensation of graphite grains over SiC grains. A minor fraction of high-density graphite grains exhibits very low 12C/13C ratios and extremely large 42,43Ca and 46,47,49,50Ti excesses. These excesses are much larger than those expected in the envelopes of asymptotic giant branch stars and are as large as those predicted for pure He-shell material in those stars and the interior, O-rich zones of type II supernovae. However, these zones have almost pure 12C, making the low 12C/13C ratios enigmatic. We propose that born-again, asymptotic giant branch stars that have undergone a very late thermal pulse, such as Sakurai\u27s object, might be the stellar source for these grains. In such stars, limited mixing of the He-burning intershell with the thin, residual hydrogen envelope leads to the production of 13C and enables material with s-process enrichments and low 12C/13C ratios to occur on the surface simultaneously. This study concludes that low-density graphite grains originate from supernovae while high-density graphite grains have multiple stellar sources: low-metallicity and born-again asymptotic giant branch stars, as well as supernovae

New stellar sources for high-density, presolar graphite grains

WepresentC,N,O,Si,Al-Mg,K,Ca,andTiisotopicanalyses ofsevenhigh-density(ORG1f, � � 2:02 2:04 gcm � 3 ) graphite grains from Orgueil with 12 C/ 13 C ratios smaller than 20. The presence of 44Ti in three of these grains indicates an origin in Type II supernovae (SNe). The 13 C excesses in these SNe grains, however, remain enigmatic. The remaining grains have extremely large Ca and Ti isotopic anomalies, some of which are much larger than those predicted for envelopes of asymptotic giant branch (AGB) stars. These anomalies in conjunction with low 12 C/ 13 C ratios can only be explained by pure nucleosynthetic He-shell components of AGB stars. Born-again AGB stars that experience a late He flash are able to explain the low 12 C/ 13 C ratios of some of the grains along with the presence of extreme enrichments in the Ca and Ti isotopes. This study indicates that high-density graphite grains havemultiple stellar sources: SNe and born-again AGB stars, in addition to the previously established low-metallicity AGB stars. Subject headingg dust, extinction — meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — supernovae: genera