15:30 〜 15:45
[PPS07-19] High-density presolar graphite grains with low 12C/13C ratios
キーワード：プレソーラーグラファイト、同位体比異常
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.
References
Amari S., Zinner E. and Gallino R. (2014) Geochem. Cosmochim. Acta 133, 479-522.
Amari S. and Jadhav M. (2025) in: Amari, S. (Ed.), Presolar grains in extra-terrestrial materials: Probing stars with stardust. Elsevier, Amsterdam, p. in press.
Cowan J.J. and Rose W.K. (1977) Astrophys. J. 212, 149-158.
Duerbeck H.W. and Benetti S. (1996) Astrophys. J. 468, L111-L114.
Herwig F. et al. (1999) Astron. Astrophys. 349, L5-L8.
Herwig F. (2001) Astrophys. J. 554, L71-L74.
Herwig F. et al. (2011) Astrophys. J. 727, 89(15pp).
Iben I., Jr. (1982) Astrophys. J. 260, 821-837.
Iben I., Jr. and MacDonald J. (1995) in: Koester, D., Werner, K. (Eds.), White Dwarfs, Lecture Notes in Physics ed. Springer, Berlin Heidelberg, pp. 48-57.
Jadhav M. et al. (2008) Astrophys. J. 682, 1479-1485.
Jadhav M. et al. (2013a) Geochem. Cosmochim. Acta 113, 193-224.
Jadhav M. et al. (2013b) Astrophys. J. Lett. 777, L27 (27pp).
Schönberner D. (1979) Astron. Astrophys. 79, 108-114.
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.
References
Amari S., Zinner E. and Gallino R. (2014) Geochem. Cosmochim. Acta 133, 479-522.
Amari S. and Jadhav M. (2025) in: Amari, S. (Ed.), Presolar grains in extra-terrestrial materials: Probing stars with stardust. Elsevier, Amsterdam, p. in press.
Cowan J.J. and Rose W.K. (1977) Astrophys. J. 212, 149-158.
Duerbeck H.W. and Benetti S. (1996) Astrophys. J. 468, L111-L114.
Herwig F. et al. (1999) Astron. Astrophys. 349, L5-L8.
Herwig F. (2001) Astrophys. J. 554, L71-L74.
Herwig F. et al. (2011) Astrophys. J. 727, 89(15pp).
Iben I., Jr. (1982) Astrophys. J. 260, 821-837.
Iben I., Jr. and MacDonald J. (1995) in: Koester, D., Werner, K. (Eds.), White Dwarfs, Lecture Notes in Physics ed. Springer, Berlin Heidelberg, pp. 48-57.
Jadhav M. et al. (2008) Astrophys. J. 682, 1479-1485.
Jadhav M. et al. (2013a) Geochem. Cosmochim. Acta 113, 193-224.
Jadhav M. et al. (2013b) Astrophys. J. Lett. 777, L27 (27pp).
Schönberner D. (1979) Astron. Astrophys. 79, 108-114.