[Mesa-users] Question to an unusual behaviour at the end of hydrogen burning in massive star

[Etienne Kaiser]

Dear MESA community

I'm trying to calculate a 25 Msun star at pre-solar metalicity using MESA
revision 10108 and I stumbled over a rather unusual behaviour of the model
at the end of hydrogen burning.

At a point at the end of hydrogen burning, i.e. when the central hydrogen
mass fraction is <1d-4, there is a sudden increase of central hydrogen to
about 0.1 % in mass fraction within one step and the burning & convection
revive. Moreover, the code drastically decreases the timesteps. I attached
a Kippenhahn diagram (reddish are the burning regions and blueish the
convective regions), a Herzsprung-Russel (HRD) and a Tc-Rhoc plot, showing
the unusual region. After this unusual behaviour, the model calculates fine.

I tried different things to avoid this unusual behaviour, such as various
increased spatial and temporal resolution, including the Type2 opacity
tables, MLT++ or use a different reagion network (approx21 -> mesa206) but
nothing of these seems to help. I tried to use the hydro debugging tool as
well but didn't get a good clue.
Interestingly, if I increase the f-parameter from the exponential decaying
diffusion model for the convective boundary mixing and approximately double
the spatial resolution, the strange behaviour disappears. It also
disappears when going to smaller masses. I attached the inlist as well.

I wonder if anybody came across a similar problem and has found a solution.
I unsuccessfully tried to find something in the mailing archives.

Thanks in advance.
Cheers,
Etienne
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