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 <super>18</super>O, <super>15</super>N, <super>28</super>Si excesses, high inferred <super>26</super>Al/<super>27</super>Al and <super>41</super>Ca/<super>40</super>Ca ratios, and the initial presence of the short-lived radionuclide <super>44</super>Ti 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 <super>12</super>C and <super>30</super>Si enrichments. In low-metallicity asymptotic giant branch stars, <super>12</super>C and <super>29,30</super>Si 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 <super>12</super>C/<super>13</super>C ratios and extremely large <super>42,43</super>Ca and <super>46,47,49,50</super>Ti 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 <super>12</super>C, making the low <super>12</super>C/<super>13</super>C ratios enigmatic. We propose that born-again, asymptotic giant branch stars that have undergone a very late thermal pulse, such as Sakurai's 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 <super>13</super>C and enables material with s-process enrichments and low <super>12</super>C/<super>13</super>C 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.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Earth and Planetary Sciences

Year dc:date.available

2009

Author dc:creator

• Jadhav, Manavi

Contributors dc:contributor

• Ernst Zinner

• Physics
• Astronomy and Astrophysics
• Geochemistry
• asymptotic giant branch stars
• born-again AGB stars
• NanoSIMS
• presolar graphite grains
• short-lived radionuclides
• supernovae

Language dc:language

English (en)

DOI

https://doi.org/10.7936/K78P5XK9

OAI identifier oai:identifier

oai:openscholarship.wustl.edu:etd-1169