[mesa-users] Initial default composition

[Jan-Torge Schindler]

Hi everyone,

I am not sure if this has been discovered before. If so, good!

I am working with the MESA version 4906 and always create PMS models.
In the controls.default the following lines are written:
initial_z = 0.02d0 ! initial metallicity

             ! initial_z can be any value from 0 to 0.04 when you are 
creating a pre-main sequence model.

             ! however, if you are loading a zams model, then initial_z 
must match one of the prebuilt values.
             ! look in the 'data/star_data/zams_models' directory to see 
what prebuilt zams Z's are available.
             ! at time of writing, only 0.02 was included in the 
standard version of star.

          initial_y = -1 ! initial helium mass fraction (< 0 means use 
default)

             ! initial_y can be any value you'd like when you are 
creating a pre-main sequence model.
             ! for pre-main sequence models, the default for *initial_y 
is 0.24 + 3*initial_z.*

I realized by not specifying the initial composition in my inlist, the 
following initial composition is chosen:
Z = 0.02 -> like it should be
Y = 0.28 -> according to the equation above it should be 0.24+3*0.02 = 
0.3 !!!
Z = 1-X-Y = 0.7 -> should have been 0.68 respectively.

Running a second inlist with Z=0.01 I realized that the implemented 
relationship is *
*initial_y is 0.24 + _*2*_*initial_z.

Maybe I am wrong somewhere and this issue resolves itself. Maybe this is 
known.
I am just writing this in case no one stumbled across this before.
Cheers,
Jan-Torge
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