[Mesa-users] Close binaries radius spiking

[Alex Ferraro]

Greetings everyone

I am currently running MESA 12115 on Linux (OpenSUSE) and am running model
grids for massive binary interactions.

I have come across a perplexing issue regarding the behaviour of the donor
star for certain configurations of initial orbital period and mass ratio:
specifically when 0.5<q<0.7 and initial period in days is 3.

For example, a 20M primary and 14M secondary with initial period of 3
days: what occurs is a surface radial oscillation that grows, initially
quite small, but then 'spikes' --> the radius of the donor star expands
from logR_sun of approx. 0.9 to 1.2 and then contracts again. This happens
all within about 10 models. This occurs after case A mass transfer (using
the Kolb scheme) from the donor to the accretor. The major issue is that
this massive 'spike' subsequently leads to erratic binary evolution that
seems unphysical.

After delving into the parameters affecting the radius, I concluded that
there is no corresponding 'spike' in nuclear energy generation, opacities,
or even temperature. The luminosity 'spikes' in lock-step with the radius,
but further examination showed it was in fact the convective luminosity
that was 'spiking'. The convective velocity also shows this 'spike'
behaviour but I cannot ascertain what is the route cause (the calculations
between the differences of the thermodynamics gradients, dlnT/dlnP, i.e.
the radiative and adiabatic nablas, is my best guess).

Just to note I am not investigating radial stellar pulsations and have not
tinkered with any of the RSP parameters in MESA. I have attached my
inlists as well as my run_star_extras and run_binary_extras. I have also
attached some figures highlighting what I have stated.

Any ideas as to the cause of this are appreciated, thank you in advance.

Kind regards
Alex

USM Bogenhausen
LMU Munich, Germany
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