Thermohaline mixing and gravitational settling in carbon-enhanced metal-poor stars

Stancliffe, Richard J.; Glebbeek, Evert

doi:10.1111/j.1365-2966.2008.13700.x

Cited by 103 publications

(155 citation statements)

References 42 publications

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“…-We assume efficient thermohaline mixing, that is, the accreted material is instantaneously mixed with the accreting star. The calculations of Stancliffe et al (2007) suggest this approximation is reasonable in many cases, even though Stancliffe & Glebbeek (2008) and Stancliffe (2009) show that other processes, such as gravitational settling, may in some circumstances reduce the effect of thermohaline mixing. To account for the possibility of inefficient thermohaline mixing, in model set D the accreted material remains on the stellar surface until mixed in by convection.…”

Section: Input Physicsmentioning

confidence: 99%

“…The most recent detailed models of AGB evolution and nucleosynthesis of Stancliffe & Glebbeek (2008), Karakas (2010) and Lugaro et al (2012) find third dredge-up in stars of mass down to 0.9 M at metallicity Z = 10 −4 . In the models of Karakas (2010) and Lugaro et al (2012), the nucleosynthesis products of stars are computed on a grid of 16 initial masses in the range [0.9, 6] M , taking into account 320 isotopes from 1 H up to 210 Po.…”

Section: Introductionmentioning

confidence: 99%

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Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

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Pols

Stancliffe³

et al. 2015

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The stellar population in the Galactic halo is characterised by a large fraction of carbon-enhanced metal-poor (CEMP) stars. Most CEMP stars have enhanced abundances of s-process elements (CEMP-s stars), and some of these are also enriched in r-process elements (CEMP-s/r stars). In one formation scenario proposed for CEMP stars, the observed carbon excess is explained by invoking wind mass transfer in the past from a more massive thermally-pulsing asymptotic giant branch (AGB) primary star in a binary system. In this work we generate synthetic populations of binary stars at metallicity Z = 0.0001 ([Fe/H] ≈ −2.3), with the aim of reproducing the observed fraction of CEMP stars in the halo. In addition, we aim to constrain our model of the wind masstransfer process, in particular the wind-accretion efficiency and angular-momentum loss, and investigate under which conditions our model populations reproduce observed distributions of element abundances. We compare the CEMP fractions determined from our synthetic populations and the abundance distributions of many elements with observations. Several physical parameters of the binary stellar population of the halo are uncertain, in particular the initial mass function, the mass-ratio distribution, the orbital-period distribution, and the binary fraction. We vary the assumptions in our model about these parameters, as well as the wind mass-transfer process, and study the consequent variations of our synthetic CEMP population. The CEMP fractions calculated in our synthetic populations vary between 7% and 17%, a range consistent with the CEMP fractions among very metal-poor stars recently derived from the SDSS/SEGUE data sample. The resulting fractions are more than a factor of three higher than those determined with default assumptions in previous population-synthesis studies, which typically underestimated the observed CEMP fraction. We find that most CEMP stars in our simulations are formed in binary systems with periods longer than 10 000 days. Few CEMP stars have measured orbital periods, but all that do have periods up to a few thousand days. Our results are consistent only if this small subpopulation represents the short-period tail of the underlying period distribution. The results of our comparison between the modelled and observed abundance distributions are significantly different for CEMP-s/r stars and for CEMP-s stars without strong enrichment in r-process elements. For the latter, our simulations qualitatively reproduce the observed distributions of carbon, sodium, and heavy elements such as strontium, barium, europium, and lead. Contrarily, for CEMP-s/r stars our model cannot reproduce the large abundances of neutron-rich elements such as barium, europium, and lead. This result is consistent with previous studies, and suggests that CEMP-s/r stars experienced a different nucleosynthesis history to CEMP-s stars.

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Section: Input Physicsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

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Pols

Stancliffe³

et al. 2015

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“…Subsequent dilution of the accreted material in the envelope of HE 1327-2326 via convection or thermohaline mixing would be expected (Stancliffe & Glebbeek 2008). Because radial velocity variations have so far not been detected for HE 1327-2326, the Roche-lobe overflow mass transfer scenario is not favoured.…”

Section: Surface Composition and Comparison To The Halo Star He 1327-mentioning

confidence: 99%

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

Campbell

Lugaro

Karakas

2010

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117

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Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M and a metallicity of [Fe/H] = −6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α, n) 16 O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ∼4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If this is true, it puts constraints on the primordial initial mass function.

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“…Attempts to model this effect have been made, but the degree to which the transferred material is diluted on the surface of the receiving star is still very poorly constrained (Stancliffe et al 2007;Stancliffe & Glebbeek 2008).…”

Section: Dilution Of the Accreted Materialsmentioning

confidence: 99%

The role of binaries in the enrichment of the early Galactic halo

Hansen

Andersen

Nordström

et al. 2016

A&A

138

146

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Context. Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high-and low-C bands, mostly populated by binary and single stars, respectively. Aims. Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Methods. We have systematically monitored the radial velocities of a sample of 22 CEMP-s stars for several years with ∼monthly, high-resolution, low S/N échelle spectra obtained at the Nordic Optical Telescope (NOT) at La Palma, Spain. From these spectra, radial velocities with an accuracy of ≈100 m s −1 were determined by cross-correlation with optimised templates. Results. Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82 ± 10%, while four stars appear to be single (18 ± 10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for eleven of the binaries and provisional orbits for six long-period systems (P > 3000 days), and orbital circularisation timescales are discussed. Conclusions. The conventional scenario of local mass transfer from a former asymptotic giant branch (AGB) binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their natal clouds by an external (distant) source. This finding has important implications for our understanding of carbon enrichment in the early Galactic halo and some high-redshift damped lyman alpha (DLA) systems, and of the mass loss from extremely metal-poor AGB stars.

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Thermohaline mixing and gravitational settling in carbon-enhanced metal-poor stars

Cited by 103 publications

References 42 publications

Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

The role of binaries in the enrichment of the early Galactic halo

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