Thermohaline instability and rotation-induced mixing
Thermohaline mixing has recently been identified as the mechanism that governs the photospheric
composition of low-mass bright giants (Charbonnel &
Zahn 2007b). This double-diffusive instability is observed in salted water in the form of elongated fingers,
when the temperature is stably stratified, but salt is not, with fresh water at the bottom and salted at the top, the overall stratification
being dynamically stable (Stern 1960).
In stars, this instability is induced by the molecular weight inversion created by the 3
He(3He,2p)4He reaction in the external wing of the hydrogen burning shell. Indeed this peculiar
reaction converts two particles into three and thus decreases the mean molecular weight, as already pointed out by Ulrich (1971) although in a different stellar
context.
The thermohaline instability is expected to set in after the first dredge-up when the star reaches the RGB luminosity bump. In terms
of stellar structure, the RGB bump corresponds to the moment when the hydrogen-burning shell encounters the chemical discontinuity created inside the star by the
convective envelope at its maximum extent during the first dredge-up. When the source shell (which provides the stellar luminosity on the RGB) reaches the border
of the H-rich previously mixed zone, the corresponding decrease in molecular weight of the H-burning layers induces a drop in the total stellar luminosity, thereby
creating a bump in the luminosity function since stars spend a relatively long time at this location. Afterwards H-burning occurs in a region of uniform composition,
allowing for the molecular weight inversion due to 3He burning to show up and thus enabling the thermohaline instability to set in.
The aim of series of papers (Charbonnel & Lagarde (2010); (Lagarde et al. (2011); and Lagarde et al. (2012))
where we investigate the occurrence and the impact of thermohaline mixing in stars of various initial masses and metallicities with non-canonical models,
i.e., taking into account self-consistently this mechanism together with rotation-induced mixing.
We confirmed that thermohaline mixing is the main physical process governing the surface
abundances of 3He, 7Li, C, and N for stars more evolved than the
RGB bump in all the models with initial masses below 2.2 M⊙, although its efficiency is increasing with decreasing initial stellar mass.
In all cases 3He decreases by a large fraction in the stellar yields compared to the standard models, although
we find that low-mass stars remain net producers of 3He.
N.Lagarde, T.Decressin, C.Charbonnel, P.Eggenberger, S.Ekström, and A.Palacios
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