Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars - I. Stellar yield tables and the CEMPs

By S. Campbell, J. R. G. I. O. Astronomy, Astrophysics, Taipei, Taiwan. Cspa, M. University, Australia.. 2008

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Context. The growing body of spectral observations of the extremely metal-poor (EMP) stars in the Galactic Halo provides constraints on theoretical studies of the chemical and stellar evolution of the early Universe. Aims. To calculate yields for EMP stars for use in chemical evolution calculations and to test whether such models can account for some of the recent abundance observations of EMP stars, in particular the highly C-rich EMP (CEMP) halo stars. Methods. We modify an existing 1D stellar structure code to include time-dependent mixing in a diffusion approximation. Using this code and a post-processing nucleosynthesis code we calculate the structural evolution and nucleosynthesis of a grid of models covering the metallicity range: -6.5 ≤ [Fe/H] ≤ -3.0 (plus Z = 0), and mass range: 0.85 ≤ M ≤ 3.0 $M_\odot$, amounting to 20 stars in total. Results. Many of the models experience violent nuclear burning episodes not seen at higher metallicities. We refer to these events as “Dual Flashes” since they are characterised by nearly simultaneous peaks in both hydrogen and helium burning. These events have been reported by previous studies. Some of the material processed by the Dual Flashes is dredged up causing significant surface pollution with a distinct chemical composition. We have calculated the entire evolution of the $Z=0$ and EMP models, from the ZAMS to the end of the TPAGB, including extensive nucleosynthesis. In this paper, the first of a series describing and analysing this large data set, we present the resulting stellar yields. Although subject to many uncertainties these are, as far as we are aware, the only yields currently available in this mass and metallicity range. We also analyse the yields in terms of C and N, comparing them to the observed CEMP abundances. At the lowest metallicities ([Fe/H] ≲ -4.0) we find the yields to contain ~ 1 to 2 dex too much carbon, in agreement with all previous studies. At higher metallicities ([Fe/H] ~ -3.0), where the observed data set is much larger, all our models produce yields with [C/Fe] values consistent with those observed in the most C-rich CEMPs. However it is only the low-mass models that undergo the Dual Shell Flash (which occurs at the start of the TPAGB) that can best reproduce the C and N observations. Normal Third Dredge-Up can not reproduce the observations because at these metallicities intermediate mass models ( M ≳ 2 $M_\odot$) suffer HBB which converts the C to N thus lowering [C/N] well below the observations, whilst if TDU were to occur in the low-mass ( M ≤ 1 $M_\odot$) models (we do not find it to occur in our models), the yields would be expected to be C-rich only, which is at odds with the “dual pollution” of C and N generally observed in the CEMPs. Interestingly events similar to the EMP Dual Flashes have been proposed to explain objects similarly containing a dual pollution of C and N – the “Blue Hook” stars and the “Born Again AGB” stars. We also find that the proportion of CEMP stars should continue to increase at lower metallicities, based on the results that some of the low mass EMP models already have polluted surfaces by the HB phase, and that there are more C-producing evolutionary episodes at these metallicities. Finally we note that there is a need for multidimensional fluid dynamics calculations of the Dual Flash events, to ascertain whether the overproduction of C and N at ultra-low metallicities found by all studies is an artifact of the 1D treatment.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

View Paper: https://www.semanticscholar.org/paper/90b60ddc3fd5f4ab7f2e3460c7746659d1a30a25

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Citations in the corpus

The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, methodology): “Low mass stars with [Fe/H] < −2.5 may experience a huge thermal pulse (Hollowell et al. 1990; Iwamoto et al. 2004; Cristallo et al. 2009; Campbell & Lattanzio 2008; Lau et al. 2009; Suda & Fujimoto 2010; Straniero et al. 2010).”
The transition from population III to population II-I star formation (2010, methodology): “The existence of popIII star with masses below∼ 102 M⊙ is supported by numerical simulations (e.g. Yoshida 2006; Yoshida et al. 2007; Campbell & Lattanzio 2008; Suda & Fujimoto 2010), which find that it is possible to fragment primordial clouds down to∼ 10 M⊙.”
The s process: Nuclear physics, stellar models, and observations (2010, unknown): “This complex feature has been found by many authors despite the different physics adopted in the various works (Campbell and Lattanzio, 2008; Cristallo et al., 2007; Fujimoto et al., 2000; Hollowell et al., 1990; Iwamoto et al., 2004; Lau et al., 2009; Straniero et al., 2004; Suda et al., 2008;…”
s‐Process in low‐metallicity stars – I. Theoretical predictions (2010, methodology): “…and nuclear burning phenomena not treated here are find to occur in IMS, as hot bottom burning (Sugimoto 1971; Iben 1973; Karakas & Lattanzio 2003; Ventura & D’Antona 2005), or, for [Fe/H] −2.3, hot TDU (Goriely & Siess 2004; Herwig 2004; Campbell & Lattanzio 2008; Lau, Stancliffe & Tout 2009).”
Nucleosynthesis in Stars and the Chemical Enrichment of Galaxies (2013, methodology): “We take the yields and remnant masses from Campbell & Lattanzio (2008) for M = 0.85, 1.0, 2.0, and 3.0 M .”
EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, background): “A characteristic feature of our 3 M⊙ model at Z = 10 −3 is represented by the occurrence of the so-called Hot-TDU (Goriely & Siess 2004; Herwig 2004; Campbell & Lattanzio 2008; Lau et al. 2009).”
CONVECTIVE–REACTIVE PROTON–12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS (2010, background): “…2000; Suda et al. 2004; Iwamoto et al. 2004; Cristallo et al. 2009), metallicity low-mass stars (e.g. Hollowell et al. 1990; Schlattl et al. 2002; Campbell & Lattanzio 2008), young white dwarfs of solar metallicity (e.g. Iben et al. 1983; Herwig et al. 1999; Lawlor & MacDonald 2003), both…”
Updated stellar yields from asymptotic giant branch models (2009, background): “We also limit our discussion to AGB models with [Fe/H] & −2.3; see Campbell & Lattanzio (2008) for yields from very lowmetallicity AGB models.”
The evolution of isotope ratios in the Milky Way Galaxy (2011, methodology): “We take the yields and remnant masses from Campbell & Lattanzio (2008) for M = 0.85, 1.0, 2.0, and 3.0M⊙.”
THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS (2011, background): “…can be qualitatively explained by H and He burning occurring together when protons are ingested in the He-burning convective regions of AGB stars (Campbell & Lattanzio 2008; Iwamoto et al. 2004), or by the operation of some form of deep mixing of the envelope material of the AGB star companion…”