Element abundances of bare planetary nebula central stars and the shell burning in AGB stars

Klaus WernerInstitut für Astronomie

und Astrophysik Universität Tübingen

Dorothee Jahn U. TübingenThomas Rauch U. TübingenElke Reiff U. TübingenFalk Herwig Los Alamos NLJeff Kruk JHU Baltimore

Astrophysics and Nuclear Structure, Hirschegg, Jan. 16, 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 22Ne nuclei (from 3α-burning shell):

12C(p,)13N(+)13C(α,n)16O protons mixed down from H envelope

22Ne(α,n)25Mg

de

pth

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)

• 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)

10-2 M

10-4 M

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

late He-shell flashcauses return to AGB

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 22Ne nuclei (from 3α-burning shell):

12C(p,)13N(+)13C(α,n)16O protons mixed down from H envelope

22Ne(α,n)25Mg

Tie

fe

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:

- H splits 19F into O and He: 19F(p,α)16O

- He converts 19F into Ne: 19F(α,p)22Ne

First discovery of fluorine inhot 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

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

(Jahn 2005)

SulfurS VI resonance doublet in FUSE spectrum of PG1159-035

(Jahn 2005)

model: S=3% solar

SummaryHydrogen-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