Discovery of resonating integration modes in triple-mode high-amplitude $\delta$ Scuti stars: A new evolutionary phase indicator

TL;DR

This study identifies a new resonating integration mode in triple-mode high-amplitude δ Scuti stars, revealing their post-main sequence phase and providing insights into rapid stellar evolution during the Hertzsprung gap.

Contribution

It uncovers a previously misidentified non-radial mode as a resonating integration mode, demonstrating its prevalence and linking it to stellar evolution in the Hertzsprung gap.

Findings

Resonating integration mode is widespread among triple-mode HADS.

All studied stars are in the post-main sequence crossing the Hertzsprung gap.

The RI mode shows significant amplitude and frequency variations over ~20 days.

Abstract

High-amplitude $\delta$ Scuti stars (HADS) that pulsate in their first three radial modes are rare in current samples. Here, we analyse five such triple-mode HADS observed by the Transiting Exoplanet Survey Satellite (TESS) and {report that the previously considered second overtone mode ($f_2$) is actually the non-radial component of a resonating integration mode (RI mode, resulting from resonance between a radial p-mode and a non-radial p-g mixed mode), which shows significant amplitude and frequency variations over short timescales (approximately 20 days).} This RI mode appears to be widespread among these stars. Notably, all five stars are in the post-main sequence evolutionary phase, actively crossing the Hertzsprung gap. These stars offer valuable insights into stellar evolution during the Hertzsprung gap, which is one of the most rapid evolutionary stages in a star's life.