How should we deal with conflicting mass transfer efficiency prescriptions? #1393
Description
General: We sometimes have conflicting demands for just how efficient we want mass transfer to be. For example, a user might ask for a fixed accretion efficiency fraction of 0.5 with --mass-transfer-accretion-efficiency-prescription FIXED and --mass-transfer-fa 0.5. But then, by default, some of our accretors have their own ideas about accretion efficiency -- for example, HeWDs have an accretion efficiency of either 0 or 1, depending on the mass transfer rate from the donor and whether the donor is He rich; or accreting black hols may have Eddington-limited accretion. Which choice should we take in this case?
Specific: I realised while working on the text of Methods Paper II and therefore reading the code (this is really proving to be a very useful exercise for me!) that the accretion efficiency for white dwarf accretors is currently being computed using the thermal timescale mass transfer rate from the donor, not the dynamical timescale mass transfer rate. It's actually not completely trivial how to do this self-consistently, since the nuclear timescale mass transfer rate is determined by the condition that the donor loses just enough mass in the timestep to have its radius equal to the Roche lobe size -- but we have to know the mass transfer efficiency in advance to determine the change in the Roche lobe size on mass transfer... Anyway, happy to think about how to do this right, but first, I'd like to hear people's thoughts on the general question, since they'll inform what we do here.