New Stellar sources for high-density, presolar graphite grains

We present C, N, O, Si, Al-Mg, K, Ca, and Ti isotopic analyses of seven high-density (ORG1f, ρ ∼ 2.02-2.04 g cm^-3) graphite grains from Orgueil with ¹²C/¹³C ratios smaller than 20. The presence of ⁴⁴Ti in three of these grains indicates an origin in Type II supemovae (SNe). The ¹³C 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 ¹²C/¹³C 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 ¹²C/ ¹³C 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.