2023MNRAS.519..902S

The angular momentum distribution of classical Cepheids in the outer Milky Way disc is bimodal with a gap at $L_\mathrm{gap}=2950\, \rm{km\,s^{-1}}$ kpc, corresponding to $R=13\,$ kpc, while no similar feature has been found in the general population of disc stars. We show that star formation in multiple spiral arm segments at the same azimuth leads to such multimodality that quickly dissolves and only shows in young stars. Unlike other explanations, such as a 1:1 orbital resonance with the Galactic bar, this also accounts for the observed steepening of the stellar warp at L_gap, since the adjacent spiral arms represent different parts of the warped gas disc, and for the predominance of the gap at negative Galactic longitude (ℓ < 0°), since for complex spiral structure this mechanism is limited in azimuth. In this scenario, the gap is clearly present only in young stars, as observed, while most purely stellar dynamical origins would affect all disc populations, including older disc stars.