AGB star intershell abundances inferred from analyses of extremely hot H-deficient post-AGB stars

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AGB star intershell abundances inferred from
analyses of extremely hot H-deficient post-AGB
stars

• Klaus Werner
• Institut für Astronomie und Astrophysik
  Universität Tübingen Germany

Dorothee Jahn U. Tübingen Thomas Rauch U.
Tübingen Elke Reiff U. Tübingen Falk Herwig Los
Alamos NL Jeff Kruk JHU Baltimore
Constraints on AGB Nucleosynthesis from
Observations, Granada, Feb. 10, 2006
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Evolutionary tracks for a 2 M? star. Born-again
track offset for clarity. (Werner Herwig 2006)
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AGB star structure
CO core material (dredged up)
from Lattanzio (2003)
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s-process in AGB stars

• Neutron sources are 2 reactions starting from 12C
  and 22Ne nuclei (from 3a-burning shell)
• 12C(p,?)13N(??)13C(a,n)16O protons mixed down
  from H envelope
• 22Ne(a,n)25Mg

H-burning He-burning
?depth
Lattanzio 1998
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• Yields of s-process in intershell layer not
  directly accessible
• Intershell matter is hidden below massive,
  10-4M?, convective hydrogen envelope
• Dredge-up of s-processed matter to the surface of
  AGB stars, spectroscopically seen
• In principle Analysis of metal abundances on
  stellar surface allows to draw conclusions about
  many unknown burning and mixing processes in the
  interior, but difficult interpretation because
  of additional burning and mixing (hot bottom
  burning) in convective H-rich envelope
• Fortunately, nature sometimes provides us with a
  direct view onto intershell matter
  hydrogen-deficient post AGB stars (hottest
  pre-white dwarfs PG1159 stars) have lost their
  H-envelope

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Low-mass stars M lt 8 M?

• After AGB phase, the star shrinks and its surface
  temperature increases (Teff gt100,000K).
• Nuclear fusion shuts down, the star is now a hot
  white dwarf, and a central star of a planetary
  nebula
• Interior structure
• - C/O core contains 99 of total stellar mass
  (0.6 M?)
• - 10-2 M? helium layer (former intershell)
• - 10-4 M? hydrogen envelope
• WD radius Earth radius
• Usually, low-mass stars end as
• hydrogen-rich WD central stars

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• The PG1159 spectroscopic class, a group of ?35
  stars
• Very hot hydrogen-deficient post-AGB stars
• Teff 75,000 200,000 K
• log g 5.5 8
• M/M? 0.52 0.86 (mean 0.6)
• log L/L? 1.1 4.2
• Atmospheres dominated by C, He, O, and Ne, e.g.
  prototype PG1159-035
• He33, C48, O17, Ne2 (mass fractions)
• chemistry of material between H and He burning
  shells in AGB-stars (intershell abundances)

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Computation of model atmospheres and synthetic
spectra

• Model assumptions
• Plane-parallel geometry, hydrostatic and
  radiative equilibrium
• Non-local thermodynamic equilibrium (NLTE i.e.
  solution of rate eqs. instead of Saha-Boltzmann
  eqs.)
• Opacities
• Arbitrary chemical composition, all species from
  H to Ni
• Full NLTE metal line blanketing (opacity
  sampling)
• Atomic data from Kurucz and Opacity/Iron Projects
• Solution method for radiation transfer eqs.
  constraints
• Accelerated Lambda Iteration , ALI (Werner
  Husfeld 1985, Werner 1986)
• Tübingen model atmosphere package (TMAP), public
  access via http//astro.uni-tuebingen.de/rauch

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Non-LTE modeling
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• Loss of H-rich envelope probably consequence of
  late He-shell flash during post-AGB phase or even
  WD cooling phase (like Sakurais object and FG
  Sge) strong support by stellar evolution models
  (Herwig 2001)
• Hydrogen envelope (thickness 10-4 M?) is ingested
  and burned in He-rich intershell (thickness 10-2
  M?)
• Composition of He/C/O-rich intershell region
  dominates complete envelope on top of stellar C/O
  core

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Late He-shell flash
10-4 M?
10-2 M?
CO core material (dredged up)
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late He-shell flash causes return to AGB
Evolutionary tracks for a 2 M? star. Born-again
track offset for clarity. (Werner Herwig 2006)
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Herwig et al. (1999)
before flash
after flash
surface ?
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HST FUSE spectroscopy of PG1159 stars

• FUSE Far Ultraviolet Spectroscopic Explorer,
  912-1180 Å
• HST gt 1150 Å
• Photospheric spectra characterized by few, broad
  and shallow, absorption lines from highly ionized
  species.
• Mostly from He II, C IV, O VI, Ne VII, S VI, P V
• Here results of non-LTE model atmosphere
  abundance analyses for Ne, Fe, F, Si, S, P

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Neon

• Neon is synthesized in He-burning shell starting
  from 14N (from previous CNO cycling) via
  14N(a,n)18F(e?)18O(a,?)22Ne
• Intershell abundance of order 2 (20 times
  solar) expected on surface of PG1159 stars
• Confirmed by newly discovered NeVII line at 973.3
  Å.

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Iron

• FUSE spectral range covers strongest Fe VII
  lines.
• Up to now, FUSE spectra from three PG1159 stars
  with sufficiently high S/N analyzed
• What is expected? Reduced (sub-solar)
  intershell Fe abundance, by n-captures. Reduced
  to what extent?

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s-process in AGB stars

• Neutron sources are 2 reactions starting from 12C
  and 22Ne nuclei (from 3a-burning shell)
• 12C(p,?)13N(??)13C(a,n)16O protons mixed down
  from H envelope
• 22Ne(a,n)25Mg

H-burning He-burning
?Tiefe
Lattanzio 1998
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Iron

• No iron lines detectable in FUSE spectra of all
  three examined PG1159 stars Fe deficiency of 1-2
  dex.
• Very strong Fe depletion in intershell!

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Fluorine
19F
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• Nucleosynthesis path for F production in
  He-burning environments
• 14N(a,?)18F(??)18O(p,a)15N(a,?)19F
• Protons provided by 14N(n,p)14C , neutrons
  liberated from 13C(a,n)O16
• 14N and 13C can result from H-burning by CNO
  cycling, but not enough to produce significant
  amounts of F
• Additional proton injection from H-envelope
  necessary partial mixing (this also activates
  the usual s-process)
• General problem 19F, the only stable F isotope,
  is fragile and readily destroyed in hot stellar
  interiors by H and He
• - H splits 19F into O and He 19F(p,a)16O
• - He converts 19F into Ne 19F(a,p)22Ne

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First discovery of fluorine in hot post-AGB
stars F VI 1139.50 Å F abundance in PG1159
stars up to 200 times solar
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• We derive F overabundances up to 10-4 (200
  solar) in some PG1159 stars (Werner, Rauch Kruk
  2005)
• F abundance in intershell of Lugaro et al. (2004)
  evolution models is right
• In order to explain Jorissen et al.s (1992)
  observed F abundances in AGB stars, dredge-up
  must be more efficient than predicted by current
  models

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Silicon, phosphorus, sulfur

• Silicon abundance hardly affected in intershell.
  Expect essentially solar abundance in PG1159
  stars. Confirmed by analyses of several objects
  (Reiff et al. 2005, Jahn et al. 2005)
• Phosphorus evolutionary models predict
  overabundances in intershell (factor 4-25, still
  uncertain). Not confirmed by spectroscopy. P
  about solar.
• Sulfur models predict slight depletion (0.6
  solar, still uncertain). Not confirmed by
  observations Wide spread observed, S down to 1
  solar

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Silicon

• Si IV resonance doublet in HST/STIS spectrum of
  PG1159-035

(Jahn 2005)
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Sulfur

• S VI resonance doublet in FUSE spectrum of
  PG1159-035

model S3 solar
(Jahn 2005)
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Summary

• Hydrogen-deficient post-AGB stars exhibit
  intershell matter on their surface. Consequence
  of a late He-shell flash.
• Results of abundance determinations in PG1159
  stars
• He, C, N, O, Ne, F, Si are in line with
  predictions from evolutionary models
• Fe depletion is surprisingly large (up to 2 dex
  sub-solar)
• P is roughly solar, but models predict strong
  enhancement
• S is expected to stay solar, but large depletions
  (up to 2 dex) are found
• Direct view on intershell matter allows to
  conclude on details in nuclear processes and
  mixing processes in AGB stars
• ? Testing stellar evolution models