Leading Wave as a Component of the Spiral Pattern of the Galaxy
  The spiral pattern of the Galaxy identified by analyzing the kinematics of young stars within 3 kpc of the Sun is Fourier decomposed into spiral harmonics. The spiral pattern of the Galaxy is shown to be representable as a superposition of trailing and leading waves with interarm distances of 1.8(+/-0.4) kpc and 4(+/-2) kpc, respectively. Shock waves are probably present only in the portions of the trailing spiral pattern where it crosses the crest of the leading wave. The small interarm distance of the trailing spiral wave (1.8 kpc) can be explained by its evolution - by the decrease in the interarm distance as the wave is displaced toward the inner Lindblad resonance. The Carina arm may be part of this resonance ring. 
A. M. Mel'nik