8) Carbon Burning & Late Stellar Evolution - ISNAP

Carbon Burning and Late Stellar Evolution
H-shell burning
Carbon-core &
shell burning
• Dredge-up to the surface
• mass loss by solar wind
nucleosynthesis contribution?

Nuclear burning & stellar evolution
10
O-ignition
Ne-ignition
Si-ignition
9
C-ignition
log (T c
)
He-ignition
m/M
8
7
H-ignition
0 2 4 6 8 10
log (ρ c
)
6 4 2 0 -2 -4 -6 -8
log (time until core collapse) [y]
Each burning phase is characterized in terms of nuclear particle configurations
hydrogen burning ⇔ CNO proton capture reactions
helium burning ⇔ alpha capture reactions
carbon burning ⇔ C,O fusion reactions and capture reactio
Mass
cut

Subsequent burning sequences
Takes place in environment of increasing density
Carbon burning:
Neon burning:
heavy ion burning 12 C+ 12 C ➱ 24 Mg
➱ 20 Ne+α and 23 Na+p
photodissociation of 20 Ne to 16 O and 4 He
because of low α binding energy of 20 Ne
Oxygen burning: heavy ion burning 16 O+ 16 O➱ 28 Si
➱ 24 Mg+α and 27 Al+p
Silicon burning: photodissociation of weakly bound 28 Si
with subsequent p-, α-capture to Fe

Carbon Burning
Conversion of 4 He into 12 C and 16 O
depending on the 12 C(α,γ) 16 O reaction
12
12
12
16
16
16
C(
C(
C(
O(
O(
O(
12
12
12
16
16
16
C,
C,
α)
C,
n)
O,
p)
p)
O,
α)
O,
n)
23
20
23
31
28
31
Na
Ne
Mg
P
Si
S
Q
Q
= 2.240 MeV
= 4.617 MeV
Q = −2.598MeV
Q = 7.628 MeV
Q = 9.594 MeV
Q = 1.499 MeV
16
16
16
O(
O(
O(
12
12
12
C,
p)
C,
α)
C,
n)
27
24
27
Al
Mg
Si
Q
Q
= 7.170 MeV
= 6.771MeV
Q = −0.424
MeV
Wide range of possible
heavy ion
reactions at low energies

The Gamow window for
heavy ion reactions
E
G
ΔE
= 0.122⋅
G
= 0.236⋅
(
2 2 2
Z Z T )
1/3
⋅ ⋅ μ ⋅ [ MeV ]
1
(
2 2 5
Z Z T )
1/ 6
⋅ ⋅ μ ⋅ [ MeV ]
1
1
1
9
9
E G
[MeV]
12
10
8
6
4
2
16O+16O
12C+16O
12C+12C
0
12
C+
12
C
:
E
G
=
2.42⋅T
2/3
9
±
0.75⋅T
5/ 6
9
0 1 2 3
temperature [GK]
16
O+
12
C
:
E
G
=
3.06⋅T
2/3
9
±
0.86⋅T
5/ 6
9
16
O+
16
O
:
E
G
=
3.90⋅T
2/3
9
± 1.34⋅T
5/ 6
9

12
C+ 12 C
different reaction
channels open at
higher temperature
dY
12
dt
C
= −Y
12
C
⋅Y
12
C
⎛
⎜
N
A
συ 12
⋅ ρ ⋅
⎜
⎜
+ N
A
συ
⎜
+ N
⎝
A
συ
C(
12
12
12
C,
α )
C (
C(
12
12
20
C,
p)
C,
n)
Ne
23
23
Na
Mg
⎞
⎟
⎟
⎟
⎟
⎠

Experimental status and goals
unknown S-factor towards lower energies!
new generation of initiatives has started!

S-factor extrapolations for different
nuclear models
The main uncertainty
components are: overall
S-factor extrapolation,
low energy resonances,
exit channel branching!
Presently assumed is
an overall uncertainty
of a factor 2-5 in the
total reaction rate!

Problems of heavy ion burning
20
Ne+α
12
C+ 12 C ⇒ 23 Na+p
23
Mg+n
How to extrapolate, what are branchings
28
Si+α
16
O+ 16 O ⇒ 31 P+p
31
S+n
Times scale for stellar C, O burning
Neutron production in C-burning
& weak s-process nucleosynthesis
White dwarf abundance distribution
& nucleosynthesis in Ne-novae
Ignition conditions for type I SN
Ignition condition for Superbursts

What potential model is appropriate?
Different potential models leads
to different extrapolation of low
energy cross section (S-factor).
Extreme case,
standard potential model
hindrance potential model
Caughlan & Fowler 1988
Gasques et al. 2005
Yakovlev et al. 2006
Jiang et al. 2007

Density & Screening Dependence of Fusion
Towards higher densities there is an increase in the rate due to
electron screening effects. The free electrons reduce the deflective
Coulomb potential between the two fusioning ions.

Consequences for late stellar burning
Simulations performed for 20 and 60 M ☼
stars, no major differences for
nucleosynthesis except a significant increase in abundance for the longlived
radioactive isotopes 26 Al and 60 Fe and some s-process nuclei. This
suggests impact on n production. That needs further investigation.

Production of Galactic Radioactivity?
Standard
Reduced rates
Chieffi, Limongi 2006
Gasques et al. 2007
Gasques et al. PRC 2007 in print

Resonance Structures in 12 C+ 12 C
Recent data suggest strong but
narrow resonance structures in the
12
C+ 12 C reaction system, which may
enhance the reaction rate
substantially.
Spillane et al. PRL 2007

Reaction rate formalism
N
A
συ
=
N
A
⋅
f
screen
⋅
2
⋅
μ
ΔE
( kT )
G
3/ 2
⋅
S
ij
⋅
e
⎛ 3E
⎜ −
⎝ kT
G
⎞
⎟
⎠
new resonance
nonres
total
N
A
συ = N
A
⋅ f
screen
⋅
2
⋅
μ
ΔE
( kT )
G
3/ 2
⋅ S
ij
⋅e
⎛ 3E
⎜ − G
⎝ kT
⎞
⎟
⎠
Additional resonance causes significant enhancement of low
temperature rate, more resonances could contribute more strongly.