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

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 12 C and 22 Ne nuclei (from 3α-burning shell): 12 C(p,  ) 13 N(  + ) 13 C(α,n) 16 O protons mixed down from H envelope 22 Ne(α,n) 25 Mg  depth H-burning He-burning Lattanzio 1998

Yields of s-process in intershell layer not directly accessible Intershell matter is hidden below massive, M , 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 (T eff >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  ) M  helium layer (former intershell) 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 T eff = 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 PG : 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 ~ 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 M  ) is ingested and burned in He-rich intershell (thickness 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) M  M 

Evolutionary tracks for a 2 M  star. Born-again track offset for clarity. (Werner & Herwig 2006) late He-shell flash causes return to AGB

before flash after flash Herwig et al. (1999) surface →

HST & FUSE spectroscopy of PG1159 stars FUSE: Far Ultraviolet Spectroscopic Explorer, 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 14 N (from previous CNO cycling) via 14 N(α,n) 18 F(e + ) 18 O(α,  ) 22 Ne Intershell abundance of order 2% (20 times solar); expected on surface of PG1159 stars Confirmed by newly discovered NeVII line at 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 12 C and 22 Ne nuclei (from 3α-burning shell): 12 C(p,  ) 13 N(  + ) 13 C(α,n) 16 O protons mixed down from H envelope 22 Ne(α,n) 25 Mg  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 19 F

Nucleosynthesis path for F production in He-burning environments: 14 N(α,  ) 18 F(  + ) 18 O(p,α) 15 N(α,  ) 19 F Protons provided by 14 N(n,p) 14 C, neutrons liberated from 13 C(α,n)O N and 13 C 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: 19 F, the only stable F isotope, is fragile and readily destroyed in hot stellar interiors by H and He: - H splits 19 F into O and He: 19 F(p,α) 16 O - He converts 19 F into Ne: 19 F(α,p) 22 Ne

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

We derive F overabundances up to (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 PG (Jahn 2005)

Sulfur S VI resonance doublet in FUSE spectrum of PG (Jahn 2005) model: S=3% solar

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