Thermohaline mixing: a physical mechanism governing the photospheric composition of low-mass giants

By C. Zahn. 2007

277 citations recorded by the source corpus.

Aims. Numerous spectroscopic observations provide compelling evidence for a non-canonical mixing process that modifies the surface abundances of Li, C and N of low-mass red giants when they reach the bump in the luminosity function. Eggleton and collaborators have proposed that a molecular weight inversion created by the 3 He( 3 He, 2p) 4 He reaction may be at the origin of this mixing, and relate it to the Rayleigh-Taylor instability. We argue that one is actually dealing with a double diffusive instability referred to as thermohaline convection and we discuss its influence on the red giant branch. Methods. We compute stellar models of various initial metallicities that include thermohaline mixing, which is treated as a diffusive process based on the prescription given originally by Ulrich for the turbulent diffusivity produced by the thermohaline instability in stellar radiation zones. Results. Thermohaline mixing simultaneously accounts for the observed behaviour of the carbon isotopic ratio and of the abundances of Li, C and N in the upper part of the red giant branch. It significantly reduces the 3 He production with respect to canonical evolution models as required by measurements of 3 He/H in galactic HII regions. Conclusions. Thermohaline mixing is a fundamental physical process that must be included in stellar evolution modeling.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/68023f9b5c640f8744d8d122e31b5388d197a8d5

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In Astronomy and Astrophysics

1. A. Karakas
2. M. Lugaro
3. C. Conroy
4. J. Lattanzio
5. Aaron Dotter
6. E. Carretta
7. Roberto Gallino
8. R. Gratton

In Instrumentation

1. A. Karakas
2. E. Carretta
3. A. Bragaglia
4. R. Gratton
5. M. Lugaro
6. C. Conroy
7. Benjamin D. Johnson
8. Aaron Dotter

Citations in the corpus

• ANGULAR MOMENTUM TRANSPORT WITHIN EVOLVED LOW-MASS STARS (2014, background): “The nature of this mixing is currently debated (See e.g. Palacios et al. 2006; Charbonnel & Zahn 2007; Nordhaus et al. 2008; Cantiello & Langer 2010; Traxler et al. 2011; Denissenkov & Merryfield 2011; Brown et al. 2013).”
• Intrinsic iron spread and a new metallicity scale for globular clusters (2009, background): “…from a few alterations presently well understood in term of an extra-mixing episode after the red giant branch (RGB) bump (Charbonnel 1994, 1995; Charbonnel & Zahn 2007; Eggleton et al. 2007) the abundance variations are inherited by currently observed, long lived GC stars from a previous…”
• MODULES FOR EXPERIMENTS IN STELLAR ASTROPHYSICS (MESA): PLANETS, OSCILLATIONS, ROTATION, AND MASSIVE STARS (2013, methodology): “Figure 12 shows a calculation including the effects of thermohaline mixing during the RGB phase of a 1M star after the luminosity bump (e.g. Charbonnel & Zahn 2007; Cantiello & Langer 2010).”
• Stellar Models and Yields of Asymptotic Giant Branch Stars (2007, background): “While the physical mechanism for the extramixing is unknown3 it is commonly thought to be rotation (e.g. Palacios et al. 2006), although we also refer to the mixing recently discovered by Eggleton et al. (2006) and investigated further by Eggleton et al. (2007) and Charbonnel & Zahn (2007).”
• Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 , (2009, background): “Charbonnel & Zahn (2007) describe how this second Li abundance drop can be explained by a mixing process called thermohaline convection, which becomes efficient when the hydrogenburning shell crosses the chemical discontinuity left behind by the first dredge-up, and which rapidly transports surface…”
• STELLAR YIELDS FROM METAL-RICH ASYMPTOTIC GIANT BRANCH MODELS (2016, background): “2M ) models are higher than measured in RGB stars (see discussions in Charbonnel 1994; Charbonnel & Zahn 2007; Eggleton et al. 2008; Karakas & Lattanzio 2014).”
• MESA ISOCHRONES AND STELLAR TRACKS (MIST). I. SOLAR-SCALED MODELS (2016, methodology): “Note however that in the literature αth spans the range 1–1000 (Kippenhahn et al. 1980; Charbonnel & Zahn 2007; Cantiello & Langer 2010; Stancliffe 2010; Wachlin et al. 2011).”
• CONVECTIVE–REACTIVE PROTON–12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS (2010, background): “Another process, that could effect the evolution of 3He during the progenitor evolution of Sakurai’s object is extra-mixing below the convective envelope during either the RGB or AGB (e.g. Wasserburg et al. 1995; Charbonnel & Zahn 2007; Denissenkov 2010).”
• EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, background): “…estimate of its efficiency during both the giant branches can be obtained only by comparing parameterized models with observational constraints (Charbonnel & Zahn 2007; Eggleton et al. 2008; Nordhaus et al. 2008; Denissenkov et al. 2009; Busso et al. 2010; Denissenkov 2010; Palmerini et al.…”
• Thermohaline instability and rotation-induced mixing - III. Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars of various metallicities (2012, methodology): “– Thermohaline mixing induced by 3He-burning becomes efficient only on the RGB at the bump luminosity (Charbonnel & Zahn 2007b; Charbonnel & Lagarde 2010).”
• THE AGE AND MASS OF THE α HERCULIS TRIPLE-STAR SYSTEM FROM A MESA GRID OF ROTATING STARS WITH 1.3 ⩽ M/M☉ ⩽ 8.0 (2013, background): “Different proposed extra mixing mechanisms are: rotation and atomic diffusion (Maeder & Meynet 2012), internal gravity waves (Talon & Charbonnel 2005), magnetic dynamo (Busso et al. 2007), and thermohaline mixing after the sub-giant phase (Charbonnel & Zahn 2007; Cantiello & Langer 2010).”
• Thermohaline instability and rotation-induced mixing I. Low- and intermediate-mass solar metallicity stars up to the end of the AGB (2010, background): “Thermohaline mixing has recently been identified as the mechanism that governs the photospheric composition of low-mass bright2 giants (Charbonnel & Zahn 2007b, hereafter CZ07).”
• s‐Process in low‐metallicity stars – I. Theoretical predictions (2010, background): “In this case, thermohaline mixing or gravitational settling could dilute the original AGB material (Vauclair 2004; Eggleton, Dearborn & Lattanzio 2006; Charbonnel & Zahn 2007; Denissenkov & Pinsonneault 2008; Stancliffe & Glebbeek 2008; Denissenkov, Pinsonneault & MacGregor 2009).”
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “The resulting efficiency of all these mixing is very difficult to estimate (Vauclair 2004; Eggleton et al. 2006; Charbonnel & Zahn 2007; Charbonnel & Lagarde 2010; Stancliffe & Glebbeek 2008; Stancliffe 2010; Denissenkov & Pinsonneault 2008; Denissenkov et al. 2009; Denissenkov 2010; Cantiello &…”
• Red giant masses and ages derived from carbon and nitrogen abundances (2015, unknown): “Possible sources of deep mixing could be rotation (Charbonnel 1995; Chanam´e, Pinsonneault, & Terndrup 2005) or thermohaline instabilities (Charbonnel & Zahn 2007), although the importance of this process is debated (Angelou et al. 2012).”
• CARBON-ENHANCED METAL-POOR STAR FREQUENCIES IN THE GALAXY: CORRECTIONS FOR THE EFFECT OF EVOLUTIONARY STATUS ON CARBON ABUNDANCES (2014, background): “7 The free parameter is related to the aspect ratio of the salt fingers responsible for the mixing (see Charbonnel & Zahn 2007 for further details).”