[P-PS07] Formation and evolution of planetary materials in the Solar System

[PPS07-19] High-density presolar graphite grains with low ¹²C/¹³C ratios

*Sachiko Amari¹, Manavi Jadhav^1,2 (1.Washington University, 2.International Health Research Institute)

Keywords:presolar graphite, isotopic anomalies

Presolar grains have a range of densities (1.6 - 2.2 g/cm³). Although core-collapse supernovae and low-metallicity AGB (Asymptotic Giant Branch) stars are the main stellar sources for low-density grains and high-density grains, respectively, there are other stellar sources of graphite grains (Jadhav et al., 2013a; Amari et al., 2014).
Some of the Orgueil high-density grains with ¹²C/¹³C ratios < 20 and no signatures of supernovae show pronounced Ca and Ti excesses (Jadhav et al., 2008; 2013b) (Fig. 1). Jadhav et al. (2013b) proposed that those grains formed in born-again AGB stars. Born-again AGB stars undergo a very late thermal pulse (VLTP) after leaving the AGB track (Schönberner, 1979; Iben, 1982; Iben and MacDonald, 1995; Herwig et al., 1999; 2001). During this VLTP phase of such post-AGB stars, the convection zone (rich in freshly produced ¹²C) extends into the residual H-rich envelope and causes convective H burning via the CN cycle. This converts ¹²C to ¹³C and reduces the ¹²C/¹³C ratio. Astronomical observations of Sakurai’s object, such a born-again AGB star, shows that it is C-rich and has a ¹²C/¹³C ratio of 1.5 - 5 (e.g., Duerbeck and Benetti, 1996).
In order to explain those large Ca and Ti isotopic patterns that are too high to be accounted for by the envelope composition of AGB stars [predicted by the F.R.U.I.T.Y. database (http://fruity.oa-abruzzo.inaf.it/modelli.pl)], Jadhav et al. (2013b) invoked the three-dimensional models by Herwig et al. (2011), where significantly higher neutron densities (~ 10¹⁵ cm^-3) can be achieved, typical of the i-process (intermediate neutron capture process) (Cowan and Rose, 1977), than those (< few 10¹¹ cm^-3) in one-dimensional stellar evolution models. Jadhav et al. (2013b) concluded that the high neutron densities could account for the extreme Ca and Ti anomalies in some ¹³C-enriched graphite grains and that they originated from post-AGB stars.
Amari and Jadhav (2025) used a different dataset for AGB nucleosynthesis and found that Ca and Ti isotopes of the He-shell reaches comparable excesses after the last thermal pulse. Born-again AGB stars are needed to explain grains with low ¹²C/¹³C ratios. However, if most of the envelope is lost at the end of the thermal pulse, we may not need to invoke the process where high neutron densities need to be achieved. The grain data may be able to constrain parameters of the various stellar models.

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