2023ApJ...951..127C

We provide the first detection of the Van Hoof effect over a complete Blazhko cycle in the variations of H β, neutral, and ionized metallic lines, measurable on 2782 high-resolution line profiles obtained during the course of a 41 day continuous and unprecedented survey of RR ab star, RR Lyr. We detect phase-lag variations in the velocity-velocity diagrams during the Blazhko period. The Van Hoof effect is maximum at the Blazhko modulation maximum and minimum at the Blazhko modulation minimum. Both turbulent velocities and equivalent widths are correlated with the Van Hoof and Blazhko effects. We investigate the differential motion between the upper and lower photosphere and between the photosphere and high atmosphere in the RR Lyr star. The differential radial velocity and differential acceleration curves show bulges and dents, respectively, during the inward moving atmosphere, but at the pulsation amplitude maximum, the differential acceleration curve is double peaked with a high differential radial-velocity gradient. Correlations between differential radial velocity, separating distance, and differential acceleration are monotonous during the Blazhko cycle, but the double-peak structure, left peak and right peak, is strongly Blazhko antiphased and phased, respectively. We explain the physical mechanisms driving the modulation of these quantities by the Sh_PM1, Sh_PM, Sh_PM2, and Sh_PM3 receding shocks, and the Sh_H+He outwarding main shock. The 3D interplay between the Van Hoof, Blazhko, and hypersonic-hydrodynamical effects provides a powerful tool for stellar pulsation and evolution challenges.