Thermohaline mixing in low mass giants

1
Thermohaline mixing in low mass giants

• Matteo Cantiello
• N. Langer and H. Hoekstra
• Astronomical Institute Utrecht

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
2
Outline of the talk

• The physics of thermohaline mixing
• Thermohaline mixing in stars
• Methods
• Results of our models
• Discussion

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
3
Thermohaline instability

• Double diffusive instability
• Plays an important role in the oceans (salt
  fingers thermohaline circulation)
• Mixing occurs on a thermal timescale

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
4
Thermohaline mixing in the kitchen
t 15 m
t 0
t 5 m
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
5
Thermohaline mixing in stars

• Accreting binary system (Wellstein et al. 2001,
  Stancliffe et al. 2007)
• Accreting star during planetary formation
  (Vauclair 2004)
• Off-center ignition in semi-degenerate cores
• Low mass giants (Eggleton et al. 2006, Charbonnel
  Zahn 2007a)

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
6
Thermohaline mixing in red giants
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
7
Stellar models
Z 0.02
1, 1.5, 2, 3 MSun _at_ ZSun (Suijs et al. 2008)

• 1D-Hydrodynamic code
• Mixing treated as a diffusive process
• Rotational mixing (Heger et al. 2000)
• Magnetic mixing (Spruit, 2002)
• Thermohaline mixing (Wellstein et al. 2001,
  Kippenhahn et al. 1980)

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
8
Results mixing during the RGB and beyond - 1MSun
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
9
Surface abundances evolution
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
10
Results mixing during the RGB and beyond - 2MSun
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
11
Halting Thermohaline mixing
(7/6 - 4/3)
3He 4He 7Be 7Be -e 7Li 7Li 1H
2 4He
1 2 3
Controlled by ratio 3He/ 4He after 1Dup, which
decreases with M (e.g. Boothroyd Sackmann 1999)
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
12
Thermally-pulsingAGB (TP-AGB)
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
13
Thermal pulse
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
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Li surface abundace
Th. Mixing
3DUP
Li-rich, low mass AGB stars (Uttenthaler et al.
2007)
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
15
Thermohaline mixing VsRotational mixing and B
fields
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
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Rotational Mixing and B fields
1 MSun Vi 10 km/s
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
17
A mass threshold for Th.Mixing

• Th. mixing working in M lt Mth 1.5MSun
• Mth sensitive to mixing efficiency parameter
• 4 of low mass stars avoid thermohaline mixing
  (Charbonnel Do Nascimento 1998)
• Mass threshold ?
• Fossil B fields ? (Charbonnel Zahn 2007b)

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
18
Efficiency of thermohaline mix

• Controversy Ulrich vs Kippenhahn
• Theory (Kippenhahn 1980) and simulations (e.g.
  Merryfield 1995) suggest no fingers in stars
• Ulrich (long fingers) needed to explain RGB
  abundances (Charbonnel Zahn 2007a)
• Interaction of th.mix and shear maybe important
• Need further study / realistic hydro simulations!

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
19
Conclusions

• Thermohaline mixing can efficiently burn 3He
  during the RGB in stars below 1.5MSun
• Th.mix can occur also during core He burning and
  on the AGB and TP-AGB
• Rotational instabilities and magnetic mixing have
  lower diffusion coefficients than Th.mix
• The interaction of rotation and B fields with
  thermohaline mixing need to be investigated to
  determine the efficiency parameter of th.mix

Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008
20
Thank you !
Matteo Cantiello
Thermohaline Mixing in low-mass giants
IAU252 Sanya - April 2008