Compulsory Deep Mixing of³He and CNO Isotopes in the Envelopes of Low‐Mass Red Giants

Abstract

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3 He( 3 He,2p) 4 He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3 He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism, which we call 'δµ-mixing', operates rapidly (relative to the nuclear timescale of overall evolution, ∼ 10 8 yrs) once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M ⊙ develop 12 C/ 13 C ratios of 14.5 ± 1.5, while Pop II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M ⊙ , this mechanism also destroys 90% to 95% of the 3 He produced on the main sequence.Subject headings: stars: red giants; abundance anomalies has the unusual characteristic (among fusion reactions in stars) of lowering the mean molecular weight µ, creating a localised µ-inversion. This has already been noted by Ulrich (1972)

The early days of the Sculptor dwarf spheroidal galaxy

“…It has been predicted that the onset position along the RGB of an extra mixing between the bottom of the stellar convective envelope and the outermost layers of the advancing hydrogenshell is located at log L L ∼ 2.3 for a metal-poor 0.8 M star (Charbonnel 1994;Gratton et al 2000;Spite et al 2005;Eggleton et al 2008). Our sample of EMPS in Sculptor falls above this limit and therefore is expected to have converted C into N by the CNO cycle.…”

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

“…The onset of the extra mixing (δμ) lies at log L L = 1.2 for a 0.8 M star. Eggleton et al (2008) also show a metallicity dependence of the luminosity at the onset of the mixing, increasing from log L L = 1.4 to 2.4 for metallicity passing from 0.02 (solar) to 0.0001 (−2.3). An independent evidence confirming that our sample stars have experienced thermohaline convection comes from the low ratio 12 C/ 13 C = 6 +2 −1 , that we measure in Sex24-72, the most C-rich star of our sample, in which this can be done .…”

A super lithium-rich red-clump star in the open cluster Trumpler 5

“…They argue that removing the mean molecular weight discontinuity left over by the first dredge-up occurring at the RGB-bump led to the onset of the extra mixing between the hydrogen-burning shell (HBS) and the envelope, allowing the CF71 mechanism to become an effective route for producing lithium-rich giants. Indeed, it is now accepted that the onset of thermohaline mixing at the RGB-bump will induce extra mixing between the convective envelope and the stellar interior (see Eggleton et al 2008;Charbonnel & Lagarde 2010, and references therein). The more robust sample studied by Kumar et al (2011, hereafter KRL11), on the other hand, supports an association with the red-clump region and lithium production associated with the He-flash at the RGB tip.…”