Primary production of light elements in very metalpoor AGB stars

1
Primary production of light elements in very
metal-poor AGB stars

• Laura Husti
• University of Torino
• (Collaborators Roberto Gallino, Sara Bisterzo,
  Sergio Cristallo, Oscar Straniero)
• GIANTS 27-28 November 2006
  Perugia

2
Very low metallicity AGB stars

• 12C and s-elements enhanced envelope
• H and He alternatively burning shells
• Neutron sources
• 13C(a,n)16O 8 keV radiative
  conditions,
• interpulse
  phase
• 22Ne(a,n)25Mg 23 keV convective conditions,
  
• during the
  thermal pulse

3
Third Dredge Up event (TDU)
Busso, Gallino Wasserburg A.R. AA 1999
4
M1.5 M_sun Fe/H-2.6
5
Primary Light Isotopes

• A large abundance of primary 12C is produced by
  partial He burning during each thermal pulse (3-a
  reaction)
• 12C is mixed with the envelope by third dredge up
  episodes
• H-shell burning converts CNO into 14N, including
  primary 12C
• During the next thermal pulse 14N(a,g)18F(bn)18O
  (a,g)22Ne(a,n)25Mg
• A large fraction of 22Ne survives

6
22Ne and primary Light Isotopes

• Large abundance of primary 22Ne at very low
  metallicities
• 22Ne(n,g)23Ne CROSS SECTION VERY LOW!
• s(30 KeV) 0.059 0.0057 mbarn (Beer et al.
  1991)
• TEST
• 22Ne(n,g)23Ne cross section put to zero gt the
  abundances of 23Na and its progenies decrease
  drastically

7
M1.5 M_sun Fe/H-2.6 ST case
Ne
Mg
Na
8
M1.5 M_sun Fe/H-2.6 No 13C pocket case
Ne
Mg
Na
9
Primary Light Isotopes
10
Light elements and the Huge first TDU
F
Ne
N
Na
C
Mg
Al
O
Straniero et al., Mem. S.A.It. (2004)
11
Conclusions

• 22Ne is a primary neutron source and a strong
  neutron poison for the s-process
• At very low metallicities, light isotopes are
  produced by neutron captures starting from 22Ne
• The effect of 22Ne decreases with increasing the
  metallicity
• Primary 19F and 14N derive from the Huge first TDU

12
related references

• Chieffi, A. et al., 2001, ApJ 554, 1159
• Gallino,R. et al., 2006, PoS(Nic-XI)100
• Herwig, F. et al., 2004, ApJS 155, 651
• Siess, L. et al., 2002, ApJ 570, 329
• Straniero, O. et al., 2004, Mem. S.A.It. 75, 665
• Suda, T. et al., OMEG05, Tokyo
• Suda, T. et al., 2005 NIC-VIII, Nucl. Phys. A
• Suda, T. et al., 2004, ApJ 611, 476