Studying the galactic chemical evolution of isotopes by analyzing presolar grains

Reto Trappitsch
April 11, 2025

The Big Bang made mainly hydrogen and helium

All the "metals" formed subsequently in stars

Credit: Frank Timmes

Observations allow us to decipher the details

Stellar messengers in our solar system

A zoo of presolar grains

Silicon Carbide grains: Are they presolar?

Silicon Carbide grains: Are they presolar?

Cosmic-ray induced spallation in the interstellar medium

Production rates can be modeled with cosmic-ray spectrum in interstellar medium

Trappitsch and Leya (2016)

How old are presolar grains? At least 4.5 billion years!

Sample preparation

Chemical separation

• Dissolve meteorite with acids
• Heavy liquid separation

Sample imaging

• Mapping sample mount with secondary electron microscope
• Identify presolar grains by energy dispersive X-Ray spectroscopy

Sample classification

• Analyze C, N, and Si isotopes
• Nanoscale secondary ion imaging
• Ideal instrument to measure major element isotopes

Trace element isotopes

• Need for a high-sensitivity technique
• Resonance Ionization Mass Spectrometry (RIMS)
  • Useful yield up to 40%
  • No isobaric interferences
• Currently only two instruments world-wide
• RIMS allows measuring most of the periodic table

Asymptotic giant branch (AGB) stars

• Expand rapidly and cool
• Cycles between H and He burning
• Copious dust producers
• Host of the s-process

V838 Monocerotis (Credit: ESA/Hubble)

The galactic chemical evolution (GCE) puzzle

Some early ideas to solve this puzzle

• GCE models do not reproduce the solar silicon abundances. Recommendation: Renormalize! (Timmes and Clayton, 1996)

• Combination of AGB stars and supernovae yields (Timmes and Clayton, 1996)

• Nuclear uncertainties might play a crucial role (Timmes and Clayton, 1996)

• Different migration histories for stars vs. dust (Clayton, 1997)

• Presolar galactic merger induced starburst (Clayton, 2003)

Heterogeneous GCE of Si - Lugaro et al. (1999)

Heterogeneous GCE of Si and Ti - Nittler (2005)

More problems with ²⁹Si - Hoppe et al. (2009)

Supersolar Si isotopes: A selection effect?

Figure from Lewis et al., 2013, but see also Lugaro et al. (2020)

Re-visiting nuclear reaction rate uncertainties and their influence on the slope of the mainstream line

Fok et al. (2024)

Stellar nucleosynthesis effects

C-O shell mergers complicate the picture further

• Ritter et al. (2017) proposed shell mergers solve abundance of odd-Z elements
• Isotopes do not agree and are a much finer probe!

Fok et al. (2024)

Nuclear reaction rate uncertainties are important!

Fok et al. (2024)

GCE of rare isotopes - r/p-ratio for Mo and Ru

r and p-nuclei in the solar neighborhood were well mixed

Mo, Ru, Ba correlations

What metallicities are we actually sampling?

• Compare δ²⁹Si₂₈ with GCE
  (Kobayashi et al. (2011)
• Determine [Fe/H] of a grain's parent star
• Details in Lewis et al. (2013)
• Distribution as expected slightly supersolar

Compare with observations

• [Eu/Fe]: -process tracer
• Figure after Molero et al. (2023)
• Metallicity spread from Stephan et al. (2025): orange bars
• Observations show spread, but what about /-ratio?

Presolar grain analysis: Hands-on astrophysics...

Presolar grain analysis: Hands-on astrophysics...

... or astronomy with a microscope

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