[mesa-users] Solid-body rotation in convective zones?
cantiel at kitp.ucsb.edu
Wed Dec 4 01:42:35 EST 2013
On Dec 3, 2013, at 10:17 PM, David Deschatelets <david.deschatelets.1 at ulaval.ca> wrote:
> Hello MESA users,
> I've been analyzing for a while the effects of a Tayler-Spruit dynamo taking place within the radiative zone of a star.
> The difference between a non-magnetic and magnetic simulation is easily observable and a near solid-body rotation can be seen in the radiative region due to angular momentum transport.
> What took my attention on the evolution results is the angular velocity distribution in convective zones of the star (mostly when the star evolves away from the MS phase). I thought differential rotation was present in these regions (according to many papers I read) but what I'm observing is a constant angular velocity through the whole region.
> This is well illustrated in Heger 2005 fig 1: hhttp://m.iopscience.iop.org/0004-637X/626/1/350/fulltext/?providedHtml=61318.text.html
> An explanation on this figure is: "In those regions large diffusion coefficients for angular momentum lead to nearly rigid rotation in all but the latest stages of the evolution", but I thought diffusion coefficients due to magnetic instability weren't present in such regions. Maybe are we talking here of another diffusion coefficient?
The assumption in MESA (as well as in other stellar evolution codes, see e.g. the Heger et al. 2005 paper you refer to) is that in convection zones angular momentum is transported by turbulent viscosity.
The value of the resulting diffusivity is usually quite high (~10^15 cm^2/s) and therefore leads to rigid rotation in the calculations.
> Is there a dynamo in the convection layer that is responsible for this?
No dynamo action is present in convective regions in MESA.
> Same phenomenon can be seen in the second MESA instrument paper fig 30: http://arxiv.org/pdf/1301.0319v2.pdf
> This constant velocity in convective regions is present whether magnetic field is on or off.
> Also, what is the reason for such rotation braking when going from radiative region to convective region
The reason is that, as explained above, in the calculations angular momentum is generally transported much more rapidly in convection zones than in radiative zones.
> (and then of course, why is the angular velocity being constant once the convective region is reached).
Again, because turbulent diffusivity enforces rigid rotation, i.e. constant angular velocity, in convective regions.
Note that this is an assumption and other approaches are possible.
For example some hydro calculations (I think mostly by David Arnett and Casey Meakin) might suggest that convection
leads to a status of constant specific angular momentum (as opposite to the constant angular velocity mentioned above).
Hope this helps,
Matteo Cantiello | http://matteocantiello.com/
Postdoctoral Fellow, Kavli Institute for Theoretical Physics
Scientific Advisor, Authorea | http://authorea.com/
> Thank you all for your answers.
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