The Impact of Carbon Enhancement on Extra Mixing in Metal-Poor Stars

Abstract

We critically examine the constraints imposed by carbon-enhanced metal-poor (CEMP) stars on the mixing mechanisms that operate in red giants. CEMP stars are created when the surface layers of a metal-poor dwarf are enriched with He-burning products via mass transfer from an evolved donor. The difference between main sequence (MS) and red giant CEMP abundances can be used as a diagnostic of the timescale for the mixing of the processed material into stellar interiors on the MS. Abundance trends with luminosity among red giant CEMP stars test theories of canonical extra mixing for low mass giants with a high bulk metallicity. We find a significant dilution in CN enrichment in giant CEMP stars relative to their MS precursors, and take this as evidence that thermohaline mixing induced by mean molecular weight inversions is ineffective in CEMP stars. This contradicts models that rely on efficient thermohaline mixing induced by small mu gradients in red giants, because such models would predict that MS CEMP stars with large mu inversions would be homogenized on a very short timescale. The data do not rule out slower MS thermohaline mixing comparable to previously published estimates. We also find that canonical extra mixing is strongly suppressed in CEMP giants relative to stars with the same iron abundance. A likely cause is the shift in the location of non-equilibrium CN processing relative to the steep mu gradient in the hydrogen burning shell, which also occurs in solar metallicity RGB stars. Implications for the mass accreted by CEMP stars and the mechanism for canonical extra mixing are discussed.

arXiv comment22 pages, 7 figures, submitted to ApJ

References in corpus (23)

• Nucleosynthetic signatures of the first stars
• Carbon Enhanced Metal-Poor Stars. I. Chemical Compositions of 26 Stars
• The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample
• First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements
• The Binary Frequency Among Carbon-Enhanced, s-Process Rich, Metal-Poor Stars
• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Cool bottom processes on the thermally-pulsing AGB and the isotopic composition of circumstellar dust grains
• Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
• The Frequency of Carbon-Enhanced Metal-Poor Stars in the Galaxy from the HERES sample
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements
• Evolution and Yields of Extremely Metal Poor Intermediate Mass Stars
• Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
• The Origin of Carbon-Enhancement and Initial Mass Function of Extremely Metal-Poor Stars in the Galactic Halo
• First stars X. The nature of three unevolved Carbon-Enhanced Metal-Poor stars
• CS 30322-023: an ultra metal-poor TP-AGB star?
• Rotation of Horizontal Branch Stars in Globular Clusters
• A Search for Nitrogen-Enhanced Metal-Poor Stars
• The Origins of Two Classes of Carbon-Enhanced, Metal-Poor Stars
• High resolution infrared spectroscopy of the old open cluster NGC6791
• Abundance Anomalies and Rotational Evolution of Low Mass Red Giants: A Maximal Mixing Approach
• Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe
• From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries

Cited by in corpus (24)

• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models
• Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor and Sextans
• Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
• s-Process in Low Metallicity Stars. I. Theoretical Predictions
• The Stellar Abundances for Galactic Archaeology (SAGA) Database II - Implications for Mixing and Nucleosynthesis in Extremely Metal-Poor Stars and Chemical Enrichment of the Galaxy
• Numerical Simulations of Thermohaline Convection: Implications for Extra-Mixing in Low-Mass RGB Stars
• Thermohaline mixing and gravitational settling in carbon-enhanced metal-poor stars
• Binary stars in the Galactic thick disc
• Magneto-Thermohaline Mixing in Red Giants
• The i-process yields of rapidly-accreting white dwarfs from multicycle He-shell flash stellar evolution models with mixing parameterizations from 3D hydrodynamics simulations
• The depletion of carbon by extra mixing in metal-poor giants
• The effects of thermohaline mixing on low-metallicity asymptotic giant branch stars
• The primordial and evolutionary abundance variations in globular-cluster stars: a problem with two unknowns
• The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
• A CEMP-no star in the ultra-faint dwarf galaxy Pisces II
• Light element abundances in carbon-enhanced metal-poor stars
• 3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
• Barium stars as tracers of s-process nucleosynthesis in AGB stars I. 28 stars with independently derived AGB mass
• [O/Fe] Estimates for Carbon-Enhanced Metal-Poor Stars from Near-IR Spectroscopy
• Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. II. Comparison of CEMP Star Frequencies with Binary Population Synthesis Models
• Modelling the nucleosynthetic properties of carbon-enhanced metal-poor RR Lyrae stars
• Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars
• Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment
• Optical and NIR spectroscopy of cool CEMP stars to probe the nucleosynthesis in low mass AGB binary system