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

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

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)

AGB stars and the s-process

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

4He, 12C, s-process elements: Ba, Pb,...

Where in AGB stars?

Interpulse phase (t ~ 103-5 years)

4He, 12C, s-process elements: Ba, Pb,...

At the stellar

surface: C>O, s-process enhance

ments

Where in AGB stars?

Interpulse phase (t ~ 103-5 years)

4He, 12C, s-process elements: Ba, Pb,...

At the stellar

surface: C>O, s-process enhance

ments

Where in AGB stars?

Interpulse phase (t ~ 103-5 years)

At the stellar surface: HBB nucleosynthesis including

14N, 23Na, 26Al, 27Al…

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

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

Observations

•  Otsuka et al. (2010) performed a detailed chemical abundance analysis of the metal-poor PN BoBn 1

•  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 mass ~1.5Msun

From Otsuka et al. (2010)

BoBn 1

[C/Ar]

[Xe

or B

a/A

r]

Observations

•  Otsuka et al. (2010) performed a detailed chemical abundance analysis of the metal-poor PN BoBn 1

•  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 mass ~1.5Msun

From Otsuka et al. (2010)

BoBn 1

[C/Ar]

[Xe

or B

a/A

r]

proton diffusion

13C(α,n)16O

22Ne(α,n)25Mg

Low mass AGBs Intermediate mass AGBs Lower temperature ~4 Msun Higher temperature In between pulses During thermal pulses

The neutron sources

Interpulse phase (t ~ 105 years)

proton diffusion

13C(α,n)16O

22Ne(α,n)25Mg

Low mass AGBs Intermediate mass AGBs Lower temperature ~4 Msun Higher temperature In between pulses During thermal pulses

The neutron sources

Interpulse phase (t ~ 105 years)

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

Sr = 38 Ba = 56 Pb = 82 -0.5

0

0.5

1

1.5

2

30 40 50 60 70 80

[X/O

]

Atomic Number

1.25Msun, [Fe/H] = -0.141.8Msun, [Fe/H] = -0.14

3Msun, [Fe/H] = -0.146Msun, [Fe/H] = -0.14

s-process yields: The effect of metallicity

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

30 40 50 60 70 80

[X/F

e]

Atomic Number

2.5Msun, [Fe/H] = -1.42.5Msun, [Fe/H] = 0

2.5Msun, [Fe/H] = -2.3

Decrease in metallicity results in more s-process elements at the 2nd peak (Ba, La), then at the 3rd (Pb)

This is well known, e.g., Busso et al. (2001)

Ba = 56 Pb = 82 Sr = 38

Comparison to Type I PNe

•  Type I PNe have [Se,Kr/Ar] enrichments that are typically ≤ 0.3 dex

Karakas et al. (2009, ApJ)

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?

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

Karakas & Lugaro (2010, PASA) &

Karakas et al. (2011, in prep)

Results: 1.  The new models can explain

most of the observed spread 2.  Except the negative values 3.  New Z = 0.01 can produce

[Se/O] ~ 1 and [Kr/O] ~ 1.4 4.  Within errors of the most Se

and Kr-enriched objects?

New Z =0.01 models

The s-process at low metallicity

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

30 40 50 60 70 80

[X/F

e]

Atomic Number

2Msun, [Fe/H] = -2.36Msun, [Fe/H] = -2.3

•  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

Sr = 38 Ba = 56 Pb = 82

Low metallicity PN

•  There are a few PN found in low-metallicity environments (e.g., K548 in M15 and BoBn 1 in the Halo)

Karakas & Lugaro (2010, PASA) and Lugaro et al. (2011, ApJ, in prep)

The model: 1.  Z = 0.0001 or [Fe/H] = -2.3 2.  Alpha-enhanced + r-process

enriched initially 3.  Heavy element and fluorine

abundance best fit by a ~1.5Msun, Z = 10-4 model

4.  Present day PN evolved from a star that accreted material from a previous AGB star -2

-1.5

-1

-0.5

0

0.5

1

1.5

2

30 40 50 60 70 80

[X/O

]

Atomic Number

1.5Msun, [Fe/H] = -2.3

Ba Kr

Shaded region shows approximate range of BoBn 1

data. Depends on [O/H]

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

Karakas (2010, MNRAS) and Lugaro et al. (2011, in prep)

From a 2Msun model: 1.  Final log e(O) ~ 8, from 6.5 2.  Would have the O of a

more metal-rich object with halo kinematics (as suggested by Brent M.)

3.  Very low O abundance (e.g., Stasińska et al. 2010) would imply low mass and/or no TDU short AGB phase due to binarity

0 1 2 3 4 5 6 7 8 9

10

6 8 10 12 14 16 18 20

log 1

0 (X

/H) +

12

Atomic Number

2Msun [Fe/H] = -2.3C O

Ne

F Na

Mg

Even at 0.9Msun, log e(O) ~ 7.5

Ar Si S

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!