[mesa-users] surface abund.

Sun Feb 14 16:48:04 EST 2016

Hi,

You may be asking for more from the current element diffusion implementation than it can deliver.  Recall that it is based on technology developed (by Anne Thoul and others) for solar conditions with main emphasis on diffusion in the solar core and no special concern for diffusion in the outer envelope (thanks to the large outer convective zone).  Turns out that the numerical challenge is much greater out near the surface since drift velocities get very large and the advection/diffusion equation is hard to solve numerically (even if we go to diffusion solver timesteps that are millions of times smaller than the stellar timesteps and are willing to wait a very long time for an answer that I wouldn't trust anyway).   If the surface has a substantial convective region, we can take advantage of that and lump the convective cells into a single large cell for element diffusion -- that let's us get around the numerical problems.  But if the surface is radiative, you need to be careful about element diffusion near the surface.   It isn't impossible to get useful results, but it is hard.  One example is the radiative levitation test suite case (based on work reported in Hu et al 2011 MNRAS).   This plot shows that diffusion can produce interesting abundance effects very near the surface --- the x axis is the dq depth that you are asking about.  

There is no envelope convection other than a small region of max 56Fe.

The plot makes it clear that for this particular case, the reported surface Fe will depend strongly on what dq you use to define the surface depth.   I can say in general what would constitute a "sensible" value.    The default is just a placeholder -- it is really up to the user to decide what is sensible for the particular problem.

Concerning your periodic jumps in surface abundance, I'll turn you over to mesa-users to help you track down the problem.

Good luck,
Bill

On Feb 14, 2016, at 1:19 PM, Earl Bellinger wrote:

> What is a sensible range of values for surface_avg_abundance_dq? 
> 
> I'm running into a problem right now where I occasionally encounter abundance discontinuities at the surface when I use an Eddington atmosphere with atomic diffusion enabled. First the metals will drain continuously from the surface, giving surface_h1 = 1. Then randomly, in a single time step, 25% of the surface will become mixed with other elements again. I've tried decreasing mesh_delta_coeff down to 0.1 and the time stepping down to 1d5 but it doesn't help any. Any suggestions would be appreciated. 
> 
> Best regards, 
> Earl Bellinger 
> Stellar Ages & Galactic Evolution Group 
> Max-Planck-Institut für Sonnensystemforschung 
> 
> On Sun, Feb 14, 2016 at 9:45 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
> 
> On Feb 14, 2016, at 12:31 PM, Bruno Lustosa wrote:
> 
> > Thank you Bill. then it means that I define the "size / length" of my surface in relation to star as a whole? I include it then Control?
> 
> correct.  the 'surface_avg_abundance_dq' is the fraction of mass considered surface to the total mass of the model.
> 
> searching the webpage http://mesa.sourceforge.net/controls_defaults.html for 'surface_avg_abundance_dq' will tell you that the default is set to 1d-8.   You can revise that in your inlist if you wish.
> 
> if you'd like to specify the thickness of the surface in other units, then you will need to add a few lines to your run_star_extras to define a new history entry.   not as trivial as using something that is built in, but it gives you the ability to get exactly what you want at a relatively small cost once you learn how to do it -- e.g., see http://mesa.sourceforge.net/run_star_extras.html#toc-1-2.
> 
> b
> 
> 
> 
> >
> > Bruno L. Moura
> > Instituto Federal do Rio Grande do Norte - IFRN
> > +55(84)99991-9550
> >
> >> Em 14 de fev de 2016, às 14:12, Bill Paxton <paxton at kitp.ucsb.edu> escreveu:
> >>
> >> Hi,
> >>
> >> Yes, to include surface be9 in the history, just add it to the list as follows
> >>
> >> surface be9
> >>
> >> Or, if you prefer,
> >>
> >> log_surface be9
> >>
> >> If at some step along the way, be9 is not in the current net, it will have abundance 0 as expected.
> >>
> >> The definition of "surface" is set by the control "surface_avg_abundance_dq" -- i.e., it isn't limited to the outermost cell.
> >>
> >> Cheers,
> >> Bill
> >>
> >>
> >>
> >>
> >>> On Feb 14, 2016, at 6:50 AM, Bruno Lustosa wrote:
> >>>
> >>> Hello everyone
> >>>
> >>> I wonder if the history_columns.list outputs I can include any surface abundance? For example, if I want plenty of any other element is just simply I put surface be9?
> >>>
> >>> Cheers
> >>>
> >>> Bruno
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