Fermion-fermion stars

The fermion-fermion stars, i.e., the dark matter self-gravitating systems m ade from two kinds of fermions with different masses, are considered. We re view the stability of the systems, present a comparison between the maxima of gravitational redshift for fermion stars, compact stars, Bondi stars, bo nson stars and fermion-fermion stars, and then investigate rotation curves of fermion-fermion stars (two-component concentric spheres) which might be polytropic dark matter halos of galaxies. Results show that the fermion-fer mion stars would give rotation curves with hat part at large radii. This pr esents a striking contrast with the rotation curve of a single component fe rmion star which has no flat parts.