[mesa-users] inquiry about changing alpha enhancement in MESA

[Francis Timmes]

hi sarah (and mesa community since the topic may be of wider interest),


> Dear all,
> 
> I want to evolve a low mass star with alpha-enhanced chemical composition using MESA. I have tried these options:
> 
> file_for_uniform_xa = ''
> set_uniform_initial_xa_from_file = .false.
> set_uniform_xa_from_file = .false.

this is the way to go for fine grained control of the initial composition.


> Dose anybody know the relation between [alpha/Fe] and mass fraction of each alpha element!? actually I want to evolve my model with different metallicity and non-zero [alpha/Fe], but I need the mass fraction of each element that related to the specific [alpha/Fe].


this paper 

http://adsabs.harvard.edu/abs/2013ApJ...774...75W

is arguably one of the best ways to go as it is
a) isotopic, 
b) "observationally informed", and
c) galactic chemical evolution model indpendent.

chris west, lead author of the aforementioned paper, is interested in 
getting the tool described in the paper out into the mesa community. 
with his permission i am providing the link below to the idl program that 
generates an isotopic composition vector at any metallicity. 

chris is also working on porting the idl to python, and we'll make that 
available to the mesa community through several online venues once he 
is satisfied with the translation.

in case idl is an immediate rate limiting step, the zip file link below also 
contains the output of chris' tool for [z] = 0.0, -1.0, -2.0, and -3.0. 
these composition vectors may help get you started on alpha-enhanced models.

http://mesastar.org/tools-utilities/idl/chris-west-abundance-vector-generator


> Is there any option in MESA we could import alpha enhancement ([alpha/Fe]) and work with it directly (instead of preparing a list of alpha elements abundance)? Do I have any other choice for evolving my model with MESA?

as noted above, file_for_uniform_xa is the easiest and most direct way
to specify an arbitrary initial composition. 


> and Do you have any suggestion about the "net" of this evolution ?

depends on the science goal. small nets are useful for quick results 
and parameter surveys but will lack the nucleosynthesis detail 
of large nets (which consume more compute resources).

fxt


> I appreciate your help and time in advance.
> 
> Best
> 
> Sarah