Question: How find stability by polar?

in this equation i solved without polar which is not shown well without polar specially when a=0 without polar if we solve this  and the other point are complex which not shown like a point in plot, except point (0,0) other not fine, but i want to solve by polar for finding  limit cycle the polar way are so usefull and also for some nonlinear system how i can  change the system to polar and find r* and theta* and plot them ?

polar.mw

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