institute for astronomy and astrophysics, university of tbingen, germany june 29, 2005planetary...

June 29, 2005 Planetary Nebulae as Astronomical Tools, Gdansk, Poland 1

Institute for Astronomy and Astrophysics, University of Tübingen, Germany

Light and heavy metal abundancesin hot central stars

Klaus WernerUniversity of Tübingen, Germany

Collaborators:A. Hoffmann, T. Rauch, E. Reiff, I. Traulsen (Tübingen)

J.W. Kruk (JHU, USA)



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) starsAbundance determinations of neon, fluorine, iron



Analysis of hottest H-rich CSPN

Observations: HST/STIS UV-spectra of 7 central starsNGC 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




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 > 100,000 K (no

unique model fit to all Balmer lines possible; higher Balmer series members require higher Teff)



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




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




Stellar masses: 0.55 – 0.65 M

Traulsen et al. (2005)



Summary of abundance analysis of hottest H-rich CSPN5 out of 7 stars have essentially solar CNO abundances (weak 3rd dredge-up

because of low mass? Mf=0.65 M Mi=3 M)Two exceptions: • LS V +4621 (=Sh2-216): CNO and He 1-2 dex subsolar

Teff=93,000K log g=6.9 → 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 20but: 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




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



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:

He=33%, C=48%, O=17% (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)



Prominent born-again stars: FG Sge and Sakurai’s star



AGB star structure

+CO core material (dredged up)

From Lattanzio (2003)

10-2M

10-4M



Wolf-Rayetcentral stars

PG1159 stars

non-DAwhite dwarfs



• 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)

H-deficient PG1159 (central) stars



s-process in AGB starsNeutron 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

dep

th

H-burning

He-burning

Lattanzio 1998

s-process in 13C pocket




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




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: Ne=2%



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 starNe 20 times solar(=2%)

H-rich central starNe solar

Ne VII 973.3 Å. For the very first time identified in astrophysical source



H-deficient PG1159 (central) starsFUSE 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



s-process in AGB starsNeutron 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

dep

th

H-burning

He-burning

Lattanzio 1998

19F production in 13C pocket



Fluorine production• Nucleosynthesis path :

14N(α,)18F(+)18O(p,α)15N(α,)19F• Protons are provided by 14N(n,p)14C with 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 p injection from H-envelope necessary: “partial mixing” (this also activates the usual s-process)



First discovery of fluorine inhot post-AGB stars:

F VI 1139.50 Å

fluorine abundance in PG1159 stars up to 200 times solar

institute for astronomy and astrophysics, university of tbingen, germany june 29, 2005planetary...

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