  We discuss theoretical scenarios on crossover between nuclear matter (NM) and
quark matter (QM). We classify various possibilities into three major scenarios
according to the onset of diquark degrees of freedom that characterizes
color-superconducting (CSC) states. In the conventional scenario NM occurs at
the liquid-gas (or liquid-vacuum at zero temperature) phase transition and QM
occurs next, after which CSC eventually appears. With the effect of strong
correlation, the BEC-BCS scenario implies that CSC occurs next to NM and QM
comes last in the BCS regime. We adopt the quarkyonic scenario in which NM, QM,
and CSC are theoretically indistinguishable and thus these names refer to not
distinct states but relevant descriptions of the same physical system. Based on
this idea we propose a natural scheme to interpolate NM near normal nuclear
density and CSC with vector coupling at high baryon density. We finally discuss
the mass-radius relation of the neutron star and constraints on parameters in
the proposed scheme.
