A phenomenological modification of thermohaline mixing in globular cluster red giants

Abstract

Thermohaline mixing is a favoured mechanism for the so-called "extra mixing" on the red giant branch of low-mass stars. The mixing is triggered by the molecular weight inversion created above the hydrogen shell during first dredge-up when helium-3 burns via 3He(3He,2p)4He. The standard 1D diffusive mixing scheme cannot simultaneously match carbon and lithium abundances to NGC 6397 red giants. We investigate two modifications to the standard scheme: (1) an advective two stream mixing algorithm, and (2) modifications to the standard 1D thermohaline mixing formalism. We cannot simultaneously match carbon and lithium abundances using our two stream mixing approach. However we develop a modified diffusive scheme with an explicit temperature dependence that can simultaneously fit carbon and lithium abundances to NGC 6397 stars. Our modified diffusive scheme induces mixing that is faster than the standard theory predicts in the hotter part of the thermohaline region and mixing that is slower in the cooler part. Our results infer that the extra mixing mechanism needs further investigation and more observations are required, particularly for stars in different clusters spanning a range in metallicity.

15 pages, 15 figures

References in corpus (14)

• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• 3D Hydrodynamical Simulations of Surface Convection in Red Giant Stars. Impact on spectral line formation and abundance analysis
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• Chemical Transport and Spontaneous Layer Formation in Fingering Convection in Astrophysics
• Low temperature Rosseland opacities with varied abundances of carbon and nitrogen
• On the Numerical Treatment and Dependence of Thermohaline Mixing in Red Giants
• Deep Mixing and Metallicity: Carbon Depletion in Globular Cluster Giants
• Constructing stable 3D hydrodynamical models of giant stars
• Models for extremely metal-poor halo stars
• Diagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters
• Double-diffusive mixing in stellar interiors in the presence of horizontal gradients

Cited by in corpus (13)

• The lowest detected stellar Fe abundance: The halo star SMSS J160540.18-144323.1
• Lithium in red giant stars: Constraining non-standard mixing with large surveys in the Gaia era
• Is Thermohaline Mixing the Full Story? Evidence for Separate Mixing Events near the Red Giant Branch Bump
• The most metal-rich stars in the universe: chemical contributions of low and intermediate mass asymptotic giant branch stars with metallicities between $0.04\le Z\le \mathrm{0.10.}$
• Bridging the Gap between Intermediate and Massive Stars I: Validation of MESA against the State-of-the-Art Monash Stellar Evolution Program for a 2$M_{\odot }$ AGB Star
• Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models
• Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars
• Identifying Multiple Populations in M71 Using CN
• A combined study of thermohaline mixing and envelope overshooting with PARSEC: Calibration to NGC 6397 and M4
• Carbon Isotope Ratios in M10 Giants
• On the maximum helium content of multiple populations in the globular cluster NGC6752
• Carbon Abundance Inhomogeneities and Deep Mixing Rates in Galactic Globular Clusters
• Light Element Abundances and Multiple Populations in M53