The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars

By Klaus Werner, Falk Herwig. 2005

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We review the observed properties of extremely hot, hydrogen‐deficient post–asymptotic giant branch (AGB) stars of spectral type [WC] and PG1159. Their H deficiency is probably caused by a (very) late helium‐shell flash or an AGB final thermal pulse, laying bare interior stellar regions that are usually kept hidden below the hydrogen envelope. Thus, the photospheric elemental abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We summarize the state of the art of stellar evolution models that simulate AGB evolution and the occurrence of a late He‐shell flash. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non‐LTE model atmospheres. Good qualitative and quantitative agreement is found. Future work can contribute to an even more complete picture of the nuclear processes in AGB stars.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1451b17fdc3374391a0efa825b6fb9c23c8ee3c3

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NUGRID STELLAR DATA SET. I. STELLAR YIELDS FROM H TO BI FOR STARS WITH METALLICITIES Z = 0.02 and Z = 0.01 (2013, methodology): “…fDUP = 0.126 was used to generate a 13C-pocket for the s-process according to Herwig et al. (2003a), and fPDCZ = 0.008 (where PDCZ stands for pulse-driven convective zone) was used at the bottom of the He-shell flash convection zone (Herwig et al. 2007; Werner & Herwig 2006; Karakas et al. 2010).”
A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS (2011, background): “According to Werner & Herwig (2006), the mechanism by which hydrogen is destroyed — during the post-asymptotic giant branch (AGB) phase — to the level required to account for the almost pure helium-atmosphere DB stars discussed here, or even the DBA stars in our sample, is probably caused by a very…”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, background): “Spectroscopic observations of the PG 1159 central stars reveal O mass fractions as high as 20% (e.g., Werner & Herwig 2006) clearly at odds with standard stellar models.”
The Pulsating White Dwarf Stars (2008, methodology): “Note that Werner & Herwig (2006) have explicitly stressed that their determinations of the atmospheric parameters of seven objects must be considered preliminary.”
Evolutionary and pulsational properties of white dwarf stars (2010, background): “…of PG 1159 stars (Dreizler & Werner 1996; Unglaub & Bues 2000; Althaus et al. 2005a), hot and luminous stars with H-deficient and He-, carbon- and oxygen-rich surface layers that constitute a transition stage between post-AGB stars and most of the H-deficient white dwarfs (Werner & Herwig 2006).”
EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, background): “…& Barbuy 2006; Smiljanic et al. 2007; Liu et al. 2009), post-AGB stars (van Winckel & Reyniers 2000; Reddy et al. 2002; Reyniers et al. 2004), planetary nebulae (Sharpee et al. 2007; Sterling & Dinerstein 2008) and born-again AGB stars (PG 1159 stars) (Werner & Herwig 2006; Werner et al. 2009).”