The r -mode instability in rotating compact stars is used to constrain the phase of matter at high density .
The color-flavor-locked phase with kaon condensation ( CFL-K0 ) and without ( CFL ) is considered in the temperature range 10 ^ { 8 } K \lesssim T \lesssim 10 ^ { 11 } K. While the bulk viscosity in either phase is only effective at damping the r -mode at temperatures T \gtrsim 10 ^ { 11 } K , the shear viscosity in the CFL-K0 phase is the only effective damping agent all the way down to temperatures T \gtrsim 10 ^ { 8 } K characteristic of cooling neutron stars .
However , it can not keep the star from becoming unstable to gravitational wave emission for rotation frequencies \nu \approx 56 - 11 Hz at T \approx 10 ^ { 8 } -10 ^ { 9 } K. Stars composed almost entirely of CFL or CFL-K0 matter are ruled out by observation of rapidly rotating neutron stars , indicating that dissipation at the quark-hadron interface or nuclear crust interface must play a key role in damping the instability .