Initial period from which the USP planets started tidal migration if only asynchronous equilibrium tides raised on the star were operating and P_spin ≫ 1 day. We adopt ﹩{Q}_{s}/{k}_{L,s}={10}^{7}﹩, and the lines indicate the range ﹩{Q}_{s}/{k}_{L,s}=[{10}^{6},{10}^{8}]﹩. The lifetime of each system is assumed to be 5 Gyr, and the color coding indicates the effective temperature of the host stars.