a) Density, velocity, Rosseland optical depth, and formation regions of Hα, He ii λ5411, and N iv λ7123 (the area under the curve is proportional to the normalized equivalent width) of SN 2013cu based on our CMFGEN model at 15.5 h after the explosion. b) Predicted ionization structure at 15.5 h after the explosion. c) Predicted ionization structure of a possible LBV progenitor before the SN explosion.