Paper presented by Amanda Karakas & Maria Lugaro at the IAU Symposium 283, Planetary Nebulae: an Eye to the Future, 25-29 July 2011, Tenerife, Spain.

1. Heavy elements in planetary
nebulae: A theorist's gold mine
Amanda Karakas1 & Maria Lugaro2
1) Research School of Astronomy & Astrophysics
Mount Stromlo Observatory, Australia
2) Centre for Stellar and Planetary Astrophysics, Monash
University, Australia

2. Introduction
• The gas in planetary nebulae preserve the surface composition of
the AGB star from the last ~few thermal pulses
• PN abundances can be used to help constrain mixing and
nucleosynthesis in AGB stars
• Recent observations have revealed enrichments of heavy elements
that can be produced by the slow neutron capture process (the s-
process, e.g., Ge, Br, Se, Kr, Xe, Ba, Pb)
• Pequignot & Baluteau (1994); Dinerstein et al. (2001a,b); Sharpee et
al. (2007); Sterling & Dinerstein (2008); Otsuka et al. (2010)
• Heavy element production is a signature of AGB nucleosynthesis
that can be used to study the physics of evolved stars

3. AGB stars and the s-process
The s process is responsible for
the production of about half the
abundances of elements heavier
than iron in the Galaxy
From low-mass stars (~1-3Msun)
s-process peaks
During the s process:
Time scale (n,g) << τβ
Questions:
1. s-process in massive AGB stars?
2. Formation of 13C pockets in low-
mass AGB stars

4. Where in AGB stars?
4He, 12C, s-process elements: Ba, Pb,...
Interpulse phase (t ~ 103-5 years)

5. At the
Where in AGB stars? stellar
surface:
4He, 12C, s-process elements: Ba, Pb,... C>O, s-
process
enhance
ments
Interpulse phase (t ~ 103-5 years)

6. At the
Where in AGB stars? stellar
surface:
4He, 12C, s-process elements: Ba, Pb,... C>O, s-
process
enhance
ments
At the stellar surface:
HBB nucleosynthesis including
14N, 23Na, 26Al, 27Al…
Interpulse phase (t ~ 103-5 years)

7. Questions
• How do nucleosynthesis models compare to the
observations of heavy elements in PNe?
• Take the composition after the final computed thermal
pulse, assume it doesn’t change from there
• Can we constrain the neutron sources operating in AGB
stars of different mass?
• Likewise, can we constrain the progenitor masses using
neutron-capture element abundances?
• Limitations: Few observations for comparison

8. Observations
• From Sterling & Dinerstein (2008)
• Large sample of Se and Kr
abundances from PNe spectra
• Some nebulae have large
overabundances of Se and Kr, with
[Kr/Ar,O] ~ 1.8!
• Type I have lower s-process
enrichments, on average, than their
non-Type I counterparts
• Along with high He/H and N/O
ratios
• More massive progenitors?
• Type I may also be produced by
binary interactions (e.g., Soker
1997)
From Nick Sterling

9. Observations
• Otsuka et al. (2010)
performed a detailed
chemical abundance analysis
BoBn 1
of the metal-poor PN BoBn 1
[Xe or Ba/Ar]
• BoBn 1 is the most F-rich
among F-detected PNe
• Is highly enriched in s-
process elements
• Likely explained by a binary
star model where the
progenitor AGB star had a [C/Ar]
mass ~1.5Msun
From Otsuka et al. (2010)

10. Observations
• Otsuka et al. (2010)
performed a detailed
chemical abundance analysis
BoBn 1
of the metal-poor PN BoBn 1
[Xe or Ba/Ar]
• BoBn 1 is the most F-rich
among F-detected PNe
• Is highly enriched in s-
process elements
• Likely explained by a binary
star model where the
progenitor AGB star had a [C/Ar]
mass ~1.5Msun
From Otsuka et al. (2010)

11. The neutron sources
Low mass AGBs Intermediate mass AGBs
Lower temperature ~4 Msun Higher temperature
In between pulses During thermal pulses
proton
diffusion
13C(α,n)16O
22Ne(α,n)25Mg
Interpulse phase (t ~ 105 years)

12. The neutron sources
Low mass AGBs Intermediate mass AGBs
Lower temperature ~4 Msun Higher temperature
In between pulses During thermal pulses
proton
diffusion
13C(α,n)16O
22Ne(α,n)25Mg
Interpulse phase (t ~ 105 years)

13. s-process yields: the effect of mass
• Little or no s-process production in the 1.25 or 6Msun model; the 1.8
and 3Msun produce copious Sr, Ba and some Pb
• Yields for Z = 0.01 will be published in Karakas, et al. (2011, ApJ, in
preparation) for M = 1.25, 1.8, 3, and 6Msun
1.25Msun, [Fe/H] = -0.14
1.8Msun, [Fe/H] = -0.14
2 3Msun, [Fe/H] = -0.14
6Msun, [Fe/H] = -0.14
1.5
[X/O]
1
0.5
0
Sr = 38 Ba = 56 Pb = 82
-0.5
30 40 50 60 70 80
Atomic Number

14. s-process yields: The effect of metallicity
Decrease in metallicity results in more s-process elements at the 2nd
peak (Ba, La), then at the 3rd (Pb)
3.5
2.5Msun, [Fe/H] = -1.4
3 2.5Msun, [Fe/H] = 0
2.5Msun, [Fe/H] = -2.3
2.5
2
[X/Fe]
1.5
1
0.5
0
-0.5
Sr = 38 Ba = 56 Pb = 82
30 40 50 60 70 80
Atomic Number
This is well known, e.g., Busso et al. (2001)

15. Comparison to Type I PNe
• Type I PNe have [Se,Kr/Ar] enrichments that are typically ≤
0.3 dex
Results:
1. 4-6Msun models of ~Zsolar
are a reasonable match to the
observational data from
Sterling & Dinerstein (2008)
2. Does the spread in Se
reflects the evolution of this
element in the Galaxy?
Karakas et al. (2009, ApJ)

16. Low-mass AGB models
• The whole sample have [Se,Kr/O] enrichments that are
typically 0.2 - 1 dex, but up to 1.8 dex in the case of Kr
Results:
1. The new models can explain
most of the observed spread
2. Except the negative values
New Z =0.01 3. New Z = 0.01 can produce
models [Se/O] ~ 1 and [Kr/O] ~ 1.4
4. Within errors of the most Se
and Kr-enriched objects?
Karakas & Lugaro (2010, PASA) &
Karakas et al. (2011, in prep)

17. The s-process at low metallicity
• The s-process from a low-Z intermediate-mass star is essentially an s-
process with a small neutron flux but a high neutron density (~1013 n/cm3);
produces Rb and less Sr, Ba, Pb
• Yields for Z = 0.0001 ([Fe/H] ~ -2.3) will be published in Lugaro, Karakas,
et al. (2011, ApJ, in preparation) for M = 0.9 to 6Msun
3.5
2Msun, [Fe/H] = -2.3
3 6Msun, [Fe/H] = -2.3
2.5
2
[X/Fe]
1.5
1
0.5
0
Sr = 38 Ba = 56 Pb = 82
-0.5
30 40 50 60 70 80
Atomic Number

18. Low metallicity PN
• There are a few PN found in low-metallicity environments
(e.g., K548 in M15 and BoBn 1 in the Halo)
2 The model:
1.5Msun, [Fe/H] = -2.3
1.5 1. Z = 0.0001 or [Fe/H] = -2.3
1 2. Alpha-enhanced + r-process
0.5 enriched initially
3. Heavy element and fluorine
[X/O]
0
-0.5 Kr Ba abundance best fit by a
~1.5Msun, Z = 10-4 model
-1 Shaded region shows
approximate range of BoBn 1 4. Present day PN evolved from
-1.5
data. Depends on [O/H] a star that accreted material
-2
30 40 50 60 70 80 from a previous AGB star
Atomic Number
Karakas & Lugaro (2010, PASA) and Lugaro et al. (2011, ApJ, in prep)

19. Low metallicity PN
At very low metallicity ([Fe/H] ~ -2.3 or log(O/H) + 12 ~ 6.5), the
progenitor AGB star can produce significant amounts of oxygen
10 From a 2Msun model:
C Ne 2Msun [Fe/H] = -2.3
9
O 1. Final log e(O) ~ 8, from 6.5
8 Na 2. Would have the O of a
7 F
log10 (X/H) + 12
more metal-rich object with
6 Si S halo kinematics (as
5 Ar suggested by Brent M.)
4
Mg 3. Very low O abundance
3
(e.g., Stasińska et al.
2
2010) would imply low
1 Even at 0.9Msun, log e(O) ~ 7.5 mass and/or no TDU 
0
6 8 10 12 14 16 18 20 short AGB phase due to
Atomic Number binarity
Karakas (2010, MNRAS) and Lugaro et al. (2011, in prep)

20. Summary
• Neutron capture elements in planetary nebulae provide
a complimentary data set to abundances from AGB stars
• It has the potential to constrain uncertain mixing and
nucleosynthesis during the AGB phase
• As well as to set limits on the masses of the progenitor
AGB stars
• New models of full s-process element production from
AGB models covering a large range of mass and
metallicity
• Need more observations for comparison!
• Dredge-up of O important at low metallicities – Use Ar
instead!