New Stellar Sources for High‐Density, Presolar Graphite Grains

Abstract

We present C, N, O, Si, Al-Mg, K, Ca, and Ti isotopic analyses of seven high-density (ORG1f, $ 2:02 2:04 g cm À3 ) graphite grains from Orgueil with 12 C/ 13 C ratios smaller than 20. The presence of 44 Ti 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 have multiple stellar sources: SNe and born-again AGB stars, in addition to the previously established low-metallicity AGB stars.

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

“…This could also be the case for gasrich presolar graphite grain 15-2, which has the highest inferred 26 Al/ 27 Al ratio (8.17 ± 0.86) × 10 −3 , consistent with what is observed for AB type SiC grains (Zinner et al 2007), and 22 Nerich grains 43-2 and 73-3. A born-again AGB star origin has also been proposed for four high-density graphite grains from the meteorite Orgueil (Jadhav et al 2008). In contrast to these three grains, the low-12 C/ 13 C-grains 10-1, 53-2, 56-1, and 57-3 are 22 22 Ne is below our detection limit), and, using the same speculation, these grains could then originate from J-type carbon stars.…”

Decoding the Message From Meteoritic Stardust Silicon Carbide Grains

“…This result is in qualitative agreement with Spitzer observations of C-rich AGB stars in the Large and Small Magellanic Clouds (Sloan et al 2008), which indicate that the mid-infrared emission from SiC and silicate dust decreases with the metallicity, while the emission from amorphous carbon dust does not. On the basis of this evidence, we tentatively predict that the MDF of stardust silicates (Mostefaoui & Hoppe 2004;Nguyen et al 2010) should be similar to that of SiC grains, while the MDF of stardust graphite grains from C-rich AGB stars (Jadhav et al 2008) should be similar to that of the G-C survey. Nittler (2009) (Nguyen et al 2010).…”

Presolar Stardust in the Solar System: Implications for Nucleosynthesis and Galactic Chemical Evolution

“…As in the case of mixing in low-mass AGB stars (previous section), the solution to this discrepancy between theory and observation may lie in multiple dimensions: models of rotating massive stars appear to produce larger amounts of 15 N during core He-burning than 1-d models predict [41], but clearly this problem requires much more attention. Finally, extremely unusual Ca and Ti isotopic compositions were recently reported in presolar SN graphite grains [42]. It remains to be seen whether these can be adequately explained by nucleosynthesis models or if they will point to new insights about massive star evolution.…”