Work

First Detection of the Van Hoof Effect in First-overtone RR Lyrae Stars: A New Kinematic Classification of Shock Waves

Year: 2026

Type: article

Abstract: We report the first detection of the Van Hoof effect in RRc stars using phase-resolved high-resolution spectroscopy. H α and cross-correlation metallic velocities trace phase-ordered hysteresis loops, revealing the loss and recovery of dynamical coupling between the photosphere and upper atmosphere. Y Crv, AU Vir, and MT Tel show closed, counterclockwise loops with areas of 82– 171 ( km s − 1 ) 2 and significances of 3.3–9.5 σ ; YZ Cap shows branch separation, but its phase gap prevents a closed area measurement. The velocity hysteresis is accompanied by phase-dependent changes in acceleration, line width, and equivalent width, linking the kinematic desynchronization to the spectroscopic response associated with shock propagation. The H α acceleration response follows Fe II at both dynamical events: the primary lag spans 0.019–0.056 cycles, while the more robust secondary lag reaches 0.079–0.228 cycles and exceeds the sampling scale in all four stars. The hysteresis morphology resolves the shock cycle into three kinematic regimes. Regime I is associated with the main outward shock, Sh He+H , where the atmospheric velocity separation grows near the primary acceleration. Regime II is associated with ballistic readjustment, Sh Ball , where dynamical desynchronization persists without H α emission or line doubling. Regime III is associated with gravitational infall and recompression, Sh Gravity , where the velocity separation decreases or reverses as the atmosphere approaches cycle closure. The Van Hoof effect therefore reveals a three-regime kinematic architecture of shock propagation and atmospheric readjustment in first-overtone RR Lyrae stars, extending to RRc pulsators the connection between velocity hysteresis and shock-driven atmospheric dynamics established for RRab stars.

Source: The Astrophysical Journal Letters

DOI: 10.3847/2041-8213/aeace1

Authors: Merieme Chadid, Liyong Liu, Jean Vernin, Peng Zong

Institutions: Université Côte d'Azur, Chinese Academy of Sciences, Fuyang Normal University

Language: English

Topic: Stellar, planetary, and galactic studies

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $2,400

Funders: National Natural Science Foundation of China, Chinese Academy of Sciences, European Southern Observatory