Absence of two-flavor color-superconductivity in compact stars

The simplest pattern of color superconductivity involves BCS pairing betweenup and down quarks. We argue that this ``2SC'' phase will not arise within acompact star. A macroscopic volume of quark matter must be electrically neutraland must be a color singlet. Satisfying these requirements imposes asignificant free energy cost on the 2SC phase, but not on color-flavor locked(CFL) quark matter, in which up, down and strange quarks all pair. As afunction of increasing density, therefore, one may see a single phasetransition from hadronic matter directly to CFL quark matter. Alternatively,there may be an intervening phase in which the different flavors self-pair, orpair with each other in a non-BCS pattern, such as in a crystalline colorsuperconductor.Comment: Significant changes to correct an error in the derivation of the free energy for neutral CFL quark matter; result unchanged. 18 pages, 2 figure