Studying the galactic chemical evolution of isotopes by analyzing presolar grains

• Nanodiamonds: ~10³ - 10⁴ atoms
• Silicon carbide: The hardy ones
• Graphites: Large but fragile
• Silicates & Oxides: Small and fragile
• Larry's talk Friday!

Silicon Carbide (SiC) are the best studied phase due to their size and hardiness

• δ-units: Deviation from solar in ‰
• Presolar grains have extreme isotope compositions
• Classified by analyzing Si, C, & N isotopes
• Fingerprint of the parent star's composition
• Hands-on astrophysical samples
  • Galactic chemical evolution
  • Stellar nucleosynthesis

• δ-units: Deviation from solar in ‰
• Presolar grains have extreme isotope compositions
• Classified by analyzing Si, C, & N isotopes
• Fingerprint of the parent star's composition
• Hands-on astrophysical samples
  • Galactic chemical evolution
  • Stellar nucleosynthesis

• Cosmic-rays induce production of nuclides
• Ideal proxy for exposure age:
  ²¹Ne concentration
  • Does not condense into grain
  • Produced from Si
• Most grains formed <1 Ga prior to solar system
• Some grains are several billion years old!

• Presolar grains are older than the solar system
• However, many of them are enriched in ²⁹Si and ³⁰Si compared to the sun
• Heterogeneous GCE
• Models predict a slope ~1 line for correlation
• Actual measurements show slope 1.34 (Stephan et al., 2024)

• Start with defined initial composition
• Add randomly a few supernovae to create a new composition
• Record the Si isotopic composition and repeat
• Supernovae produce too little ²⁹Si^*
• Overeall, this model can explain the Si isotopic composition, but...

• Analyzed SiC X grains from supernovae
• Does not trace GCE, but supernova nucleosynthesis
• Too little ²⁹Si when all other isotope ratios matched
• Proposed to enhance ²⁶Mg(α,n)²⁹Si

Clearly, nuclear reaction rates and their uncertainties play a key role!

Fok et al. (2024)

• New study by Stephan et al. (2025)
• Measured 55 presolar SiC grains for Mo, Ru, and Ba isotopes
• All grains from AGB stars allow deciphering the s-process composition
• r- and p-composition can be determined as the s component

• p-nuclei destroyed in s-process
• Example Molybdenum
  • ⁹⁶Mo: Pure s-process nuclei
  • δ⁹²Mo₉₆ in s-process: -1000‰
• High precision presolar grain data:

The r- and p-processes contributed at fixed rates to the various parent star's initial composition as well as to the solar composition

Stephan et al. (2025)

• Presolar grains from AGB stars: valuable proxies for GCE
• Isotopes are a very fine probe!
• The Si isotope chonundrum
  • ²⁹Si underproduced in models
  • Grain formation bias
  • Nuclear reaction rate uncertainties play a major role
• Mo, Ru, Ba: Tracing the r/p-ratio through (recent) GCE

SiC grain imaged in the secondary electron microscope

• Presolar grains from AGB stars: valuable proxies for GCE
• Isotopes are a very fine probe!
• The Si isotope chonundrum
  • ²⁹Si underproduced in models
  • Grain formation bias
  • Nuclear reaction rate uncertainties play a major role
• Mo, Ru, Ba: Tracing the r/p-ratio through (recent) GCE

NASA, ESA, NRAO/AUI/NSF and G. Dubner (Univ. of Buenos Aires)