Light and heavy metal abundances in hot central stars

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Light and heavy metal abundancesin hot central
stars

• Klaus Werner
• University of Tübingen, Germany
• Collaborators
• A. Hoffmann, T. Rauch, E. Reiff, I. Traulsen
  (Tübingen)
• J.W. Kruk (JHU, USA)

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Outline

• Results from UV spectral analysis of
• Some of the hottest known hydrogen-rich central
  stars
• - New Teff and log g determinations
• - Abundance determinations of CNO and iron
• Hydrogen-deficient PG1159 (central) stars
• Abundance determinations of neon, fluorine, iron

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Analysis of hottest H-rich CSPN

• Observations HST/STIS UV-spectra of 7 central
  stars
• NGC 1360, NGC 4361, NGC 6853, NGC 7293,
• Abell 36, LSS 1362, LS V 4621 ( Sh2-216)
• Selection criteria
• Extremely hot (Teff around 100,000 K)
• UV-bright (aimed at high resolution and high-S/N)
• Further observations for some of these objects
• FUSE far-UV spectra
• new optical spectra taken at CA 3.5m, SSO 2.3m,
  HET 9.2m

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Analysis of hottest H-rich CSPN

• Why UV spectroscopy?
• The only way to determine metal abundances.
  Metals are highly ionized, most metals have no
  spectral lines in the optical
• The only reliable way for precise Teff
  determination. Many metals show lines from at
  least 2 ionisation stages. Problems in the
  optical
• - He I / He II ionisation balance not
  available (no He I lines)
• - Balmer line problem still unsolved for Teff
  gt 100,000 K (no unique model fit to all
  Balmer lines possible higher Balmer
  series members require higher Teff)

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• Example Fixing Teff of NGC 7293 by using the
  lines from O IV, O V, O VI

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Analysis of hottest H-rich CSPN

• In this way, using several CNO ions, we revised
  Teff previously determined from optical spectra
  alone.
• Largest correction found for NGC 4361. Evolved
  from coolest to hottest object in our sample
• Teff 82,000 ? 126,000 K

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Analysis of hottest H-rich CSPN

• Stellar masses
• 0.55 0.65 M?

Traulsen et al. (2005)
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• Summary of abundance analysis of hottest H-rich
  CSPN
• 5 out of 7 stars have essentially solar CNO
  abundances (weak 3rd dredge-up because of low
  mass? Mf0.65 M? ? Mi3 M?)
• Two exceptions
• LS V 4621 (Sh2-216) CNO and He 1-2 dex
  subsolar
• Teff93,000K log g6.9 ? gravitational
  settling
• NGC 4361 This is a halo PN (Torres-Peimbert
  1990)
• Fe lines very weak, N is subsolar by factor 10,
  Si by factor 20
• but O is solar and very surprising C is 20
  oversolar
• Similar to K 648, the CSPN in the globular
  cluster M15 (Rauch et al. 2002)
• Possible 12C dredged up from C/O core

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Analysis of hottest H-rich CSPN

• Analysis of iron (group) lines is still on-going
  (Fe, Ni, Cr, Mn)
• Many objects display Fe V and/or Fe VI and Fe VII
  lines ? further check of Teff possible
  abundances. Example

Fit to Fe VI lines in LS V 4621
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New results on H-deficient PG1159 (central) stars

• Recall
• PG1159 stars represent the transition phase from
  Wolf-Rayet type central stars to non-DA white
  dwarfs
• They are extremely hot Teff 75,000 200,000 K
• Their atmospheres are dominated by He, C, and O
• e.g. prototype PG1159-035
• He33, C48, O17 (mass fractions)
• H-envelope ingested and burnt after a late
  He-shell flash
• Surface chemistry material between H and He
  burning shells in precursor AGB-star (intershell
  abundances)

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Prominent born-again stars FG Sge and Sakurais
star
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AGB star structure
10-4M?
10-2M?
CO core material (dredged up)
From Lattanzio (2003)
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Wolf-Rayet central stars
PG1159 stars
non-DA white dwarfs
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H-deficient PG1159 (central) stars

• FUSE spectroscopy, immediate aim identification
  abundance determination of trace metals
• PG1159 stars enable to study composition of
  intershell matter usually hidden under thick
  H-mantle
• Abundances reveal nuclear reaction chains and
  mixing processes in stellar interior
• ? testing stellar evolution theory
• Important intershell chemistry also affects
  efficiency of s-process (e.g. through 12C
  abundance dredged up from C/O core)

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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
s-process in 13C pocket
Lattanzio 1998
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H-deficient PG1159 (central) stars

• FUSE spectra reveal an underabundance of iron in
  PG1159 stars (1-2 dex) Miksa et al. (2002)
• Explanation
• Neutron captures completely destroy iron in the
  13C pocket
• Accumulation of Fe-deficient matter in the
  intershell after each thermal pulse (pulse-driven
  convection)
• Exhibition of this matter on surface after late
  He-flash

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H-deficient PG1159 (central) stars

• FUSE spectra reveal a overabundance of neon in
  PG1159 stars, 2 by mass 20 times solar
  (Werner et al. 2004)
• Explanation
• 22Ne is produced in He-burning shell by alpha
  captures on (CNO-cycled) 14N
• 22Ne is accumulated in intershell during thermal
  pulses
• Exhibition of Ne-enriched matter on surface after
  late He-flash. Model predictions Ne2

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• Ne VII 973.3 Å line in FUSE spectra detectable
  even at solar neon abundance level Only
  possibility to identify neon in hot hydrogen-rich
  (i.e. normal) central stars.

PG1159 central star Ne 20 times
solar (2) H-rich central star Ne solar
Ne VII 973.3 Å. For the very first time
identified in astrophysical source
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H-deficient PG1159 (central) stars

• FUSE spectra allow for the first identification
  of fluorine in post-AGB stars,
• F is solar in some PG1159 stars, but we find a
  strong overabundance of fluorine in other PG1159
  stars, up to 200 times solar! (Werner et al.
  2005)
• Explanation
• 19F is produced in s-processing 13C pocket and
  can be accumulated in intershell during thermal
  pulses
• Exhibition of F-enriched matter on surface after
  late He-flash

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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
19F production in 13C pocket
Lattanzio 1998
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Fluorine production

• Nucleosynthesis path
• 14N(a,?)18F(??)18O(p,a)15N(a,?)19F
• Protons are provided by 14N(n,p)14C with 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 p injection from H-envelope necessary
  partial mixing (this also activates the usual
  s-process)

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First discovery of fluorine in hot post-AGB
stars F VI 1139.50 Å fluorine abundance in
PG1159 stars up to 200 times solar