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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 themolecular weight inversion created above the hydrogen shell during firstdredge-up when helium-3 burns via 3He(3He,2p)4He. The standard 1D diffusivemixing scheme cannot simultaneously match carbon and lithium abundances to NGC6397 red giants. We investigate two modifications to the standard scheme: (1)an advective two stream mixing algorithm, and (2) modifications to the standard1D thermohaline mixing formalism. We cannot simultaneously match carbon andlithium abundances using our two stream mixing approach. However we develop amodified diffusive scheme with an explicit temperature dependence that cansimultaneously fit carbon and lithium abundances to NGC 6397 stars. Ourmodified diffusive scheme induces mixing that is faster than the standardtheory predicts in the hotter part of the thermohaline region and mixing thatis slower in the cooler part. Our results infer that the extra mixing mechanismneeds further investigation and more observations are required, particularlyfor stars in different clusters spanning a range in metallicity.

A phenomenological modification of thermohaline mixing in globular cluster red giants