CHE Logic Fix for Issue #1303

[veome22]

Amended the CHE logic so that the determination of CHE uses the OrbitalAngularVelocity() rather than m_Star->Omega(). This allows stars to change into CHE even if they don't have any spin, as can happen when TIDES_PRESCRIPTION is NONE.

For the binary shown in Issue #1303 ,

./COMPAS/src/COMPAS --random-seed 4907 -n 1  
--metallicity 0.0017724805683766772  
--initial-mass-1 41.997969179405416 --initial-mass-2 41.997969179405416  
--semi-major-axis 0.06926436655793679  
--chemically-homogeneous-evolution-mode PESSIMISTIC  
--enhance-CHE-lifetimes-luminosities TRUE  
--scale-CHE-mass-loss-with-surface-helium-abundance TRUE  
--enable-rotationally-enhanced-mass-loss TRUE  
--scale-terminal-wind-velocity-with-metallicity-power 0.0  
--mass-change-fraction 0.005 --radial-change-fraction 0.005  
--tides-prescription NONE  
--pulsational-pair-instability-prescription HENDRIKS  
--PPI-CO-Core-Shift-Hendriks 1.111111 --PPI-upper-limit 80.0 --PISN-lower-limit 80.0  
--common-envelope-formalism TWO_STAGE 
--wolf-rayet-multiplier 1.291550

the output becomes:

0: DCO formed: (Chemically_Homogeneous -> Black_Hole) + (Chemically_Homogeneous -> Black_Hole)

[veome22]

Also made the CHE treatment conserve angular momentum by moving all the spin-related code to ProcessTides(). I have used almost the same routine as PERFECT tides, but slightly modified to allow for only one star to be synchronized while no tides are applied to non-CHE stars.

[ilyamandel]

I had clearly made a mistake. I should have updated the angular frequency of the stars when setting them up (see around lines 190-198 of BaseBinaryStar.cpp) and I did not. Thank you for catching that.

I think it's important to realise and clearly state (I'd even add it to the changelog comments and to whatsnew, so we don't forget) that you are now making the assumption that all stars in binaries with sufficiently rapid initial angular frequencies are going to be set as CHE, whatever the tides option, and that this initialisation does not conserve total angular momentum. I am generally OK with that (presumably, tides were efficient pre-ZAMS, a part of the stellar evolution we don't consider), but I'd ask for two changes:

• Don't do this if the initial rotational frequency of stars is explicitly specified. Presumably, if the user actively chooses a value, they have a reason for doing so. In that case, we should only evolve those angular frequencies under the action of tides, and not by fiat.

• Update the comment on lines 302--310, it's no longer accurate.

Oh, and you'll need to merge in dev and update the changelog accordingly.

[ilyamandel]

P.S. One small possible update to tidal synchronisation of CHE stars (again, I like your general approach!):

// no (real) root found, synchronize CHE stars ignoring angular momentum conservation
if (m_Star1->StellarType() == STELLAR_TYPE::CHEMICALLY_HOMOGENEOUS){m_Star1->SetOmega(omega);}
if (m_Star2->StellarType() == STELLAR_TYPE::CHEMICALLY_HOMOGENEOUS){m_Star2->SetOmega(omega);}

In this case, you probably want the stars to be forced to have the orbital angular frequency. Right now, you are giving them a negative omega, which was just meant as an indication that no root was found.

[jeffriley]

Looks good to me. Thanks @veome22

[ilyamandel]

Fantastic -- thank you, @veome22 ! I'll now pull this into dev.