[Mesa-users] Issues Evolving AM CVn Binary

[Joshua Aiken]

Hi everyone,

I am currently trying to model the evolution of a system that starts as a (lowish mass) ZAMS donor star and a white dwarf companion (a cataclysmic variable (CV) system), that through mass transfer erodes the H content of the donor star, turning the system into an AM CVn system (a H-deficient/He-rich donor onto a white dwarf accretor, with an orbital period less than 1 hour). For the most part, the evolution works as expected, however, during the AM CVn phase, MESA struggles to converge and I haven't been able to figure out why (and how to fix the issue).

I am using MESA r211211 and I have provided the inlists that I am using for a specific case (M1=1.10Msun, M2=1.2Msun, P=2.88days) and also included the history file and plot of the period-donor mass evolution for that case. The convergence issues start when mass transfer begins during the AM CVn phase (when the period is below ~1Hr). The provided case specifically ends due to the redo limit, but in other similar cases the timestep becomes really small and the evolution still doesn't go anywhere. The mass transfer rate is only ~10^-7 Msun/yr which is completely expected for an AM CVn.  Has anyone experienced similar issues when trying to transfer mass from a donor composed of almost entirely helium? or have any ideas on what the problem might be/things to try?

Any help on getting these evolutions to progress further through the AM CVn phase (ideally until the donor mass is about 0.02 Msun) would be greatly appreciated.

Thanks,
Josh
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