Neutron capture cross section uncertainties and the weak s-process in massive stars

1
Neutron capture cross section uncertainties and
the weak s-process in massive stars

• Marco Pignatari
• Roberto Gallino
• Carla Baldovin

V Riunione Nazionale di Astrofisica Nucleare,
Teramo 20-22 Aprile 2005
2
The Weak s-process
3
Kippenhans Diagram for a star with M 25 M(sun)
and solar metallicity (Woosley, Heger Weaver
2002)
4
Principal isotopes (pre-supernova)
4He
16O
12C
20Ne
1H
12C
28Si
4He
M25 Msun, ZZsun (Nucl. Data Page, A. Heger)
5
Pre-Supernova and Post-Supernova composition
67Zn
68Zn
70Zn
T1/2(69Zn) 56 m
M25 Msun, ZZsun (Nucl. Data Page, A. Heger)
6
Post-Supernova composition (yields)
Rb
M25 Msun, ZZsun (Nucl. Data Page, A. Heger)
7
Post-Supernova production factors
M25 Msun, ZZsun (Nucl. Data Page, A.
Heger), Rauscher et al. 2002, ApJ
8
Models we use Hydrostatic nucleosynthesis in
massive stars

• FRANEC code (Chieffi Straniero 1989)
• Post-processing models follow
• Convective Core He-burning and
• Convective Shell C-burning
• (Raiteri et al. 1991, 1993, AA)
• Updated network
• Jaeger et al. 2001 22Ne(a,n) 25Mg
• Kubono et al. 2003 13C(a,n) 16O ...
• Bao et al. 2000 for (n,?),
• ß decay rates from various sources,
• (n,p) and (n,a) channels....

9
First results

• The Weak s component is mainly produced by
  convective shell C-burning (M 20Msun).
  Different neutron sources
• High neutron density (1011 1012 n/cm3) with
  respect to the core He-burning (106 n/cm3)
• Pignatari et al. (in preparation)

10
Production factors in the Convective Shell
C-burning region at different metallicity
11
Production factors in the Convective Shell
C-burning region after Shell C-burning and Core
He-burning
12
The weak s-component summary Convective
Convective Core He-burning
Shell C-burning
Low neutron density (106 n/cm3) T3-3.5 108 K
Classical s-process See Lamb et al., Couch et
al., Raiteri et al., Prantzos et al. ......
Peak neutron density (1011- 1012 n/cm3) down to
109 n/cm3 T 1 109 K The convective shell
works over the ashes of the core He-burning
(Raiteri et al. 91)
The final weak s component is an overposition of
two different s(sr) components
13
Propagationeffects of the neutron capture cross
sections uncertainties on the weak s component
14
Cross section uncertainties in the Iron group
isotopes and beyond

• Typical uncertainties 10
• Propagation of the cross section uncertainties on
  the s-nucleosynthesis yields
• For a complete understanding of the weak s
  process, neutron capture cross sections need to
  be remeasured with a much better precision!!!!

15
The case of the 62Ni
Two discrepant estimates of the Maxwellian cross
section at 30 KeV in the literature, based on the
same experiment 35.5 mb Bao et al.
1987 13.5 mb Bao et al. 2000 A new
measurement provides
30.52.8 mb Nassar et al.
2005
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
Neutron poisons of the weak s-processeffect of
cross section uncertainties

• The light isotopes capture the major fraction of
  the available neutrons, behaving as poisons for
  the weak s-process.
• The major poison is 16O
• Other important poisons 25Mg, 12C, 23Na,
  17O(n,a).

20
Standard case/sigma(16O)1.1
sn(16O) µbarn!!!
21
Conclusions

• For M 20Msun the convective shell C-burning is
  the most important nucleosynthesis site (also at
  low metallicity).
• Different neutron sources work during convective
  shell C-burning.
• The weak s component is strongly affected by the
  uncertainties the neutron capture cross sections
  of both seeds and poisons. Need for a
  remeasurement of neutron capture cross section
  for all isotopes involved (new n-TOF program at
  CERN).