Evolution (ctd)
€Transfer
of material from a lower mass star to a higher mass star (as in a CV)
causes the orbital separation to increase, lengthening the period and causing the star
to fall away from contact with its Roche Lobe and accretion to stop
€Hence
some other mechanism has to been in place for stable mass transfer
€Main
candidate for more than two decades has been the magnetic
braking caused by wind material from the secondary being threaded by field
lines to large distances (Zwaan & Verbunt)
€This
magnetic braking robs the binary of angular momentum, so the two stars
move closer together, maintaining contact with the Roche Lobe and quasi-stable mass
transfer
€From this an expected population of CVs can be generated
as a
function of orbital period, given ages and mass transfer rates