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

1. 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

2. Evolutionary tracks for a 2 M star. Born-again track offset for clarity. (Werner & Herwig 2006)

3. AGB star structure +CO core material (dredged up) from Lattanzio (2003)

4. s-process in AGB stars • Neutron sources are 2 reactions starting from 12Cand22Ne nuclei (from 3α-burning shell): • 12C(p,)13N(+)13C(α,n)16Oprotons mixed down from H envelope • 22Ne(α,n)25Mg H-burning He-burning depth Lattanzio 1998

5. 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

6. 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

7. 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)

8. 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

9. Non-LTE modeling

10. 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

11. Late He-shell flash 10-4 M 10-2 M +CO core material (dredged up)

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

13. 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

14. 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.

15. 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?

16. s-process in AGB stars • Neutron sources are 2 reactions starting from 12Cand22Ne nuclei (from 3α-burning shell): • 12C(p,)13N(+)13C(α,n)16Oprotons mixed down from H envelope • 22Ne(α,n)25Mg H-burning He-burning Tiefe Lattanzio 1998

19. 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!

20. Fluorine 19F

21. Nucleosynthesis path for F production in He-burning environments: • 14N(α,)18F(+)18O(p,α)15N(α,)19F • Protons provided by14N(n,p)14C , neutrons liberated from 13C(α,n)O16 • 14N and13C 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,α)16O • - He converts 19F into Ne: 19F(α,p)22Ne

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

23. 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

24. 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

25. Silicon • Si IV resonance doublet in HST/STIS spectrum of PG1159-035 (Jahn 2005)

26. Sulfur • S VI resonance doublet in FUSE spectrum of PG1159-035 model: S=3% solar (Jahn 2005)

27. 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