[Mesa-users] Thank you and questions

[Warrick Ball]

Hi Piet,

I'll try to answer the questions I'm most comfortable with.  Note that 
while I'm answering these questions for MESA specifically, the principles 
of how opacities are computed and used are roughly the same for all 
stellar evolution codes.

> 1. I use the Asplund09 metal fractions, both for initial_zfracs and
>    kappa_file_prefix.  I take it the first sets the initial conditions
>    and the second is for calculating the (initial?) opacities.  So could
>    one, to create an absurd scenario, use Asplund09 for one and, say,
>    GS98 for the other?

`initial_zfracs` determines the metal mixture in the stellar model. 
`kappa_file_prefix` determines which set of opacity tables is used to 
interpolate the opacity of material in the star mye answer to 2 to might 
help).

You could certainly choose different values for these parameters if you 
want.

> 2. Related to that, it is not clear to me what elements (as well as
>    isotopes and ionization states) are evolved and used for opacity
>    calculations by MESA.

MESA comes with tables of opacities, mostly as a function of density \F1, 
temperature T, hydrogen abundance X and metallicity Z.  (There are also 
tables that have the carbon and oxygen abundances as variables but I won't 
complicate the discussion with those now.)  These tables are precomputed 
by other teams and groups that make detailed calculations and then average 
the results into Rosseland mean opacities, which is what MESA.  The 
individual metal abundances are assumed to be distributed according to 
whichever solar mixture is specified in the filename. e.g. GS98 opacities 
means that the individual metal abundances follow the patter in GS98, 
scaled by the overall value of Z.

AFAIK the ionization states will depend on whatever equation of state was 
used by the team/code that computed the opacities, which may or may not be 
exactly the same one as used in MESA.

> 4. I take it that opacity_factor simply multiplies all calculated
>    opacities by that factor?  That to get around by the difference in
>    metallicity derived from spectroscopy and that indicated by
>    helioseismology?  (It worked for me to reconcile the observed metal
>    fractions with T_eff and R_sun for one solar mass, once element
>    diffusion is switched on as well).  Is it possible to tinker with the
>    opacity_factor for individual elements?

Yes, all the opacities are multiplied by `opacity_factor`.  I don't think 
it is intended for any specific purpose but yes, a user could perhaps use 
it to try to reconcile a solar model in the way you describe.

I'm not sure about whether or not its possible to tinker with the opacity 
factor of individual elements.  I suspect the answer is "yes but it's 
complicated"...

Cheers,
Warrick


------------
Warrick Ball
Postdoc, School of Physics and Astronomy
University of Birmingham, Edgbaston, Birmingham B15 2TT
W.H.Ball at bham.ac.uk
+44 (0)121 414 4552


On Fri, 12 Oct 2018, mesa-users at lists.mesastar.org wrote:

> Dear all,
>
> First a big "Thank You" to everyone who put MESA together and is maintaining
> it!\A0 I don't program or run big codes, so I was apprehensive, but it turned
> out that everything worked as described in the directions, and the directions
> were unambiguous. What a difference with, say, upgrading from MacOs El 
> Capitan
> to Sierra, which took me a long frustrating weekend.
>
> It is indeed also a lot of fun to follow a star as it is evolving, and 
> amazing
> to see how fast the run goes.\A0 My project is looking at solar evolution under
> different mass loss scenarios related to the Faint Young Sun Paradox.
>
> Here are my questions, and forgive me if they seem amateurish:
>
> 5. v_div_cs is the convective velocity as a fraction of the sound speed at
> \A0\A0 the surface cell, correct?\A0 Is that an observable, say by line broadening
> \A0\A0 in stars, or Doppler shifts and tracking on the Sun?\A0 Curiously I find
> \A0\A0 that that parameter is quite sensitive to the mass loss history.\A0 I do
> \A0\A0 not understand why, but if this is confirmed upon multiple checks,
> \A0\A0 perhaps it could be a diagnostic.\A0 Am I missing an obvious physical
> \A0\A0 explanation?
> 6. I sort of get the idea of "s%", but can you give me the exact definition?
> 7. This is a comment, not a question.\A0 I was confused initially by the line
> \A0\A0 "wind_scheme = ''".\A0 I left the space between single quotes empty, as I
> \A0\A0 thought was indicated, but that led to an error message.\A0 One really has
> \A0\A0 to comment out that line to make it work.\A0 Not totally clear (for me).
> 8. Upon compiling many error messages appear of the sort "ld: warning: object
> \A0\A0 file (/Applications/mesasdk/lib/libhdf5_fortran.a(H5_ff.o)) was built for
> \A0\A0 newer OSX version (10.13) than being linked (10.12)".\A0 That is meant to
> \A0\A0 say to use High Sierra instead of Sierra, I think.\A0 Do you know if that
> \A0\A0 leads to errors?\A0 Doesn't seem so, as MESA works fine on Sierra for me.
> \A0\A0 (I'd rather not upgrade my MacOs again....).
>
> Thanks again!
>
> -- 
> Piet Martens, Dept. of Physics & Astronomy
> Georgia State University
> 25 Park Place, 6th Floor, Room 624
> Atlanta, GA 30303
> Office Phone:  404-413-6612
> Dept. Fax:  404-413-5481
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>
>