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
Thomas Rauch
Elke Reiff
Falk Herwig
Jeff Kruk
U. Tübingen
U. Tübingen
U. Tübingen
Los Alamos NL
JHU Baltimore
Constraints on AGB Nucleosynthesis from Observations, Granada, Feb. 10, 2006
Evolutionary tracks for a 2 M star. Born-again track offset for clarity.
(Werner & Herwig 2006)
AGB star structure
+CO core material
(dredged up)
from Lattanzio (2003)
s-process in AGB stars
Neutron sources are 2 reactions starting from 12C and
nuclei (from 3α-burning shell):
12C(p,)13N(+)13C(α,n)16O
22Ne
protons mixed down from H envelope
22Ne(α,n)25Mg
depth
H-burning
He-burning
Lattanzio 1998
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
Low-mass stars
M < 8 M
After AGB phase, the star shrinks and its surface temperature
increases (Teff >100,000K).
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
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:
He=33%, C=48%, O=17%, Ne=2% (mass fractions)
• = chemistry of material between H and He burning
shells in AGB-stars (intershell abundances)
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
Non-LTE modeling
• Loss of H-rich envelope probably consequence of late
He-shell flash during post-AGB phase or even WD
cooling phase (like Sakurai’s 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
Late He-shell flash
+CO core material
(dredged up)
late He-shell flash
causes return to AGB
Evolutionary tracks for a 2 M star. Born-again track offset for clarity.
(Werner & Herwig 2006)
HST & FUSE spectroscopy of PG1159 stars
• FUSE: Far Ultraviolet Spectroscopic Explorer, 912-1180 Å
• HST: > 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
Neon
• Neon is synthesized in He-burning shell starting from 14N
(from previous CNO cycling) via 14N(α,n)18F(e+)18O(α,)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.
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?
s-process in AGB stars
Neutron sources are 2 reactions starting from 12C and
nuclei (from 3α-burning shell):
12C(p,)13N(+)13C(α,n)16O
22Ne
protons mixed down from H envelope
22Ne(α,n)25Mg
Tiefe
H-burning
He-burning
Lattanzio 1998
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!
Fluorine
19F
• Nucleosynthesis path for F production in He-burning
environments:
14N(α,)18F(+)18O(p,α)15N(α,)19F
• Protons provided by 14N(n,p)14C , neutrons liberated from
13C(α,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:
19F(p,α)16O
- H splits 19F into O and He:
19F(α,p)22Ne
- He converts 19F into Ne:
First discovery of fluorine in
hot post-AGB stars:
F VI 1139.50 Å
F abundance in
PG1159 stars up to
200 times solar
•
•
•
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
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
Silicon
Si IV resonance doublet in HST/STIS spectrum of PG1159-035
(Jahn 2005)
Sulfur
S VI resonance doublet in FUSE spectrum of PG1159-035
model: S=3% solar
(Jahn 2005)
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