Heavy elements in planetary nebulae: a theorist's gold mine

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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.

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.

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  • 1. Heavy elements in planetarynebulae: A theorists 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-processThe s process is responsible forthe production of about half theabundances of elements heavierthan 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 theWhere 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 theWhere 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 sourcesLow mass AGBs Intermediate mass AGBsLower temperature ~4 Msun Higher temperatureIn between pulses During thermal pulses proton diffusion 13C(α,n)16O 22Ne(α,n)25Mg Interpulse phase (t ~ 105 years)
  • 12. The neutron sourcesLow mass AGBs Intermediate mass AGBsLower temperature ~4 Msun Higher temperatureIn 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 Flog10 (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!