Transcript The Impact of Carbon Enhancement on Extra Mixing in Metal

3D Numerical Simulations of Thermohaline Mixing in
Low-Mass Red Giants
Pavel Denisenkov, UVic
3D Numerical Simulations of Thermohaline Mixing in
Low-Mass Red Giants
A story about how important and difficult it is to take account of mixing in
stars when studying stellar nucleosynthesis
Pavel Denisenkov, UVic
The abundance of 3He should have increased by an order of magnitude since
the Big Bang Nucleosynthesis, but it has been remaining nearly constant
(From Bania, T. M., Rood, R. T., and Balser, D. S. 2007, Space Science Review, 130, 53)
The incomplete pp-chain reactions produce a lot of 3He far from the centre of
a low-mass star on the main sequence, and low-mass stars are the most abundant
species of stellar populations
Left:
Evolution of a low-mass star in the HR diagram (M = 0.83M , Z = 0.0015)
Right: Changes of abundance profiles inside the star that drive its evolution
Data points are field stars with M = 0.8 – 0.9M, Z = 0.0005 – 0.002 from Gratton et al. (2000)
Arrow in the right panel shows the bottom of convective envelope
Left:
Evolution of a low-mass star in the HR diagram (M = 0.83M , Z = 0.0015)
Right: Changes of abundance profiles inside the star that drive its evolution
• on the MS, 3He is produced in a wide pocket (blue curve in the right panel)
• after the MS, convection spreads 3He all over the envelope (the 1st dredge-up)
• mass-loss on the RGB and AGB will deliver this 3He into ISM
Left:
Evolution of a low-mass star in the HR diagram (M = 0.83M , Z = 0.0015)
Right: Changes of abundance profiles inside the star that drive its evolution
• on the MS, 3He is produced in a wide pocket (blue curve in the right panel)
• after the MS, convection spreads 3He all over the envelope (the 1st dredge-up)
• mass-loss on the RGB and AGB will deliver this 3He into ISM
How to explain the fact that the abundance of 3He in the ISM has not
changed since the BBN?
Low-mass red-giant-branch (RGB) stars show evidence of extra-mixing operating
in their radiative zones between the hydrogen-burning shell and convective
envelope that starts above the so-called “bump luminosity”.
This RGB extra-mixing is most likely to reduce the envelope 3He abundance back
to its BBN value.
What happens at the bump luminosity?
Left:
Evolution in the HRD around the bump luminosity (M = 0.83M , Z = 0.0015)
Right: H-burning shell erases abundance discontinuities
What happens at the bump luminosity?
Left:
Evolution in the HRD around the bump luminosity (M = 0.83M , Z = 0.0015)
Right: H-burning shell erases abundance discontinuities
What happens at the bump luminosity?
Left:
Evolution in the HRD around the bump luminosity (M = 0.83M , Z = 0.0015)
Right: H-burning shell erases abundance discontinuities
On upper RGB, above the bump luminosity, the radiative zone has
uniform chemical composition, which facilitates fluid buoyancy of
any origin
Thermohaline convection as a mechanism for extra mixing in RGB stars
Thermohaline convection is driven by a small difference in salinity S between a fluid element and its
surrounding medium when the atomic diffusivity is much lower than the heat diffusivity K. In the
ocean, it develops when both S and T decrease with depth and it takes a form of salt fingers, hence
“salt-fingering convection” is another term for it.
P ( µ*, ρ1*,T )
Reaction 3He(3He,2p)4He decreases µ
locally byΔµ ≈ −0.0001
Double-diffusive (
K) instability leads to growing salt fingers in water
Thermohaline convection as a mechanism for extra mixing in RGB stars
The RGB thermohaline convection is driven by a small difference in µ between a fluid parcel and its
surrounding medium because the atomic diffusivity is much lower than the heat diffusivity K. It
develops when µ decreases with depth (usually, µ increases with depth in stars)
Reaction 3He(3He,2p)4He decreases µ
locally byΔµ ≈ −0.0001
Thermohaline convection as a mechanism for extra mixing in RGB stars
The RGB thermohaline convection is driven by a small difference in µ between a fluid parcel and its
surrounding medium because the atomic diffusivity is much lower than the heat diffusivity K. It
develops when µ decreases with depth (usually, µ increases with depth in stars)
Thermohaline convection as a mechanism for extra mixing in RGB stars
The RGB thermohaline convection is driven by a small difference in µ between a fluid parcel and its
surrounding medium because the atomic diffusivity is much lower than the heat diffusivity K. It
develops when µ decreases with depth (usually, µ increases with depth in stars)
Reaction 3He(3He,2p)4He decreases µ locally
by |Δµ | ‹ 0.0001 (Eggleton et al. 2006)
2D numerical simulations of thermohaline convection in the oceanic and RGB cases
(a change of color from red to blue corresponds to an increase of S and µ)
Salinity field
Mean molecular weight field
3D numerical simulations of the oceanic and RGB thermohaline convection
Ocean (S)
RGB (µ)
• The empirically constrained rate of RGB extra-mixing is Dµ ≈ 0.01K which is a factor
of 50 higher than the value estimated in our 2D and 3D numerical simulations
• Therefore, the physical mechanism of RGB extra-mixing is most likely to be
different from thermohaline convection driven by 3He burning
• A promising alternative mechanism is the buoyant rise of magnetic flux rings
(Parker’s instability)
The End