[mesa-users] overshoot_f_above_nonburn/burn_he/...

Tue Jun 25 07:06:55 EDT 2013

Roni,

I highly doubt if somebody will be able to tell you a nice set of values that are applicable for various scenarios. As mentioned by these people, the set of "f" values will change probably a lot depending on what you're specifically trying to do, and probably you need to be creative yourself in order to justify your choice. Let's say you're trying to do core He burning in massive stars, then one method coming to my mind is to figure out the amount that doesn't directly contradict with the observed RGB and BSG ratio...

People have been trying to measure the overshooting through a single parameter using eclipsing binaries, open clusters, apsidal motion studies etc and more recently with asteroseismology. The results from all these works look more like a scatter plot rather than a nice relation, even for the 'simpler' case of main sequence core overshooting.

Tuguldur

On Jun 25, 2013, at 3:39 AM, Roni Waldman <roni181066 at gmail.com> wrote:

> Hi Umberto,
> 
> 
> Yes, you are right. As I also said to Ilka the parametrization depends on the specific model to compute.
> Concerning the lower value of f=0.008 recommended for He convective zones: is recommended to correctly reproduce the surface abundance of H-deficent post-AGB stars, i.e. stars that have lost their envelope through intense winds and show as surface abundances what were before He-intershell abundances. Surface abundances of such stars have been observed and published in papers like Werner et al 2007 and Werner et al 2009.. where specific range of He4, C12 and O16 are given, thus giving another strong observational constraint. A lower value of f is necessary in reproduce such abundances.
> 
> Can you point me to any reference calibrating  f=0.008 to the observational results of Werner et al?
> And again, this result is only relevant for AGB He shells flashes, not for e.g. He core burning.
> 
> Cheers,
> Roni
> 
> From: Roni Waldman [roni181066 at gmail.com]
> Sent: Tuesday, June 25, 2013 11:14 AM
> To: Umberto Battino
> Subject: Re: [mesa-users] overshoot_f_above_nonburn/burn_he/...
> 
> Hi Umberto,
> 
> 
> overshooting parameters are also constrained by hydrodinamical 3D simulations (e.g. Freytag et al. 1996, Herwig 2007 etc...).. I can tell you something for low/intermediate mass stars.. where a classic setting is given by setting f=0.014 to all the boundaries, except for the helium ones, where a lower value is recommended (f=0.008). Consider these just as indicative values, then it depends much more on the specific model you want to compute.. for example during the so called "Third dredge up" events during the AGB phase the f value under the hydrogen convective envelope is generally multiplied by a factor of 9 using the inlist command "overshoot_below_noburn_factor".
> Check  also Herwig 2000 paper for more details! (Astron. Astrophys. 360, 952–968 (2000)). This will surely be useful.. :)
> 
> These are indeed very interesting papers regarding this subject.
> One has to keep in mind, however, that these are results valid for specific cases, e.g. the Herwig 2007 paper deals with He shell flashes in AGB stars, and shows different behavior in the upper and lower boundaries of the convective zone. This already points out the fact that there is no reason to assume the same overshoot parameter would apply for let's say He core burning. Also the authors point out there is a numerical uncertainty in the results of a factor of a few.
> 
> On what basis do you say that a lower value of f=0.008 is recommended for He convective zones?
> 
> Cheers,
> Roni
> 
> 
> 
> From: Roni Waldman [roni181066 at gmail.com]
> Sent: Tuesday, June 25, 2013 8:59 AM
> To: Ilka Petermann
> Cc: mesa-users
> Subject: Re: [mesa-users] overshoot_f_above_nonburn/burn_he/...
> 
> Hi Ilka and MESA users,
> As far as I know, the only observational evidence is that which you mention, and it is relevant for the main sequence, i.e. overshoot above the H burning core.
> People usually use the same parameters for all other convective regions just because there is no better thing to do.
> If anybody knows otherwise, I will be happy to be corrected.
> Cheers,
> Roni
> 
> 
> On Mon, Jun 24, 2013 at 10:44 PM, Ilka Petermann <m.ilka at gmx.de> wrote:
> Dear MESA users,
> I want to implement overshooting in my calculations and have a question about the MESA parameters describing it.
> If I understood correctly, a value for overshooting (e.g. overshoot_f_above_burn_h) is adjusted to match the width of the main sequence or to fit properties determined by asteroseismology,...
> However, there are also parameters for overshoot_f_above_nonburn/burn_he/burn_z? Is there a 'common recipe' to assign values to these?
> Thanks!
> Ilka
> 
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