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

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 μ gradients in red giants, because such models would predict that MS CEMP stars with large μ 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 nonequilibrium CN processing relative to the steep μ 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.