First Detection of the Van Hoof Effect in First-overtone RR Lyrae Stars: A New Kinematic Classification of Shock Waves
4 affiliations
- 1University of Cote d’Azur
- 2Chinese Academy of Sciences
- 3Tibet & Antarctica Network Ground Observatories
- 4Fuyang Normal University
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.
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BibTeX
@article{chadid2026first,
title = {{First Detection of the Van Hoof Effect in First-overtone RR Lyrae Stars: A New Kinematic Classification of Shock Waves}},
author = {Merieme Chadid and Liyong Liu and Jean Vernin and Peng Zong},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aeace1},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeace1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - First Detection of the Van Hoof Effect in First-overtone RR Lyrae Stars: A New Kinematic Classification of Shock Waves
AU - Merieme Chadid
AU - Liyong Liu
AU - Jean Vernin
AU - Peng Zong
PY - 2026
DA - 2026/10/14
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeace1
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeace1
AB - 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.
SN - 2041-8205
SN - 2041-8213
ER -