Na and Mg sprocess in low metallicity AGB stars

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Na and Mg s-process in low metallicity AGB stars

• Sara Bisterzo
• Roberto Gallino

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AGB s-process nucleosynthesis

• M 1.3, 1.5, 2, 3, 5 Msun
• Different 13C pocket efficiencies
• At various Fe/H

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ENVELOPE COMPOSITION AT THE END OF THE AGB
5 TPs with TDU
Fe/H - 2.60
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ENVELOPE COMPOSITION AT THE END OF THE AGB
20 TPs with TDU
Fe/H - 2.60
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ENVELOPE COMPOSITION AT THE END OF THE AGB
26 TPs with TDU
Fe/H - 2.60
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ENVELOPE COMPOSITION AT THE END OF THE AGB
35 TPs with TDU
Fe/H - 2.60
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ENVELOPE COMPOSITION AT THE END OF THE AGB
48 TPs with TDU
Fe/H - 2.60
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Range of Masses M 1.3 5 Msun For the same
13C pocket (ST case)
Fe/H - 2.60
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Primary 22Ne in low metallicity stars

• The third dredge-up episode mixes with the
  envelope primary 12C produced by partial He
  burning in the thermal pulse
• The growing H burning shell converts CNO nuclei
  into 14N, part of which derives from primary 12C
• This 14N is further converted to 22Ne at the
  beginning of the next thermal pulse
• In the thermal pulse 22Ne is marginally affected
  by a captures

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• ?A strong primary production of 22Ne results,
  which increases with pulse number

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Primary Na

• During the thermal pulse, 23Na is produced by
  22Ne(n,g)23Na
• Slight depletion of 23Na derives from its low n
  capture cross section (2mb at 30 keV)
• ? The strong primary production of 22Ne implies a
  strong primary production of Na

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Primary Mg

• Solar Mg is made of about 80 24Mg, 10 25Mg, 10
  26Mg.
• In the TP, the 22Ne(a,g)26Mg and 22Ne(a,n)25Mg
  reactions feed a strong primary production of
  25Mg and 26Mg
• Also some primary production of 24Mg derives from
  23Na(n,g) 24Na(b-) 24Mg

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Na and Mg at different metallicities
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CONCLUSIONS

• AGB models of low metallicities predict an
  important production of primary Na and Mg which
  may constraint the initial mass of spectroscopic
  s-enhanced stars