The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547

TL;DR

This paper demonstrates that turbulent pressure perturbations, rather than the opacity mechanism, can intrinsically excite coherent pulsations in the delta Scuti star HD 187547, challenging previous explanations of its oscillation spectrum.

Contribution

It introduces a new theoretical model showing turbulent pressure as a key pulsational driving mechanism in delta Scuti stars.

Findings

Turbulent pressure perturbations drive coherent pulsations.

Oscillations are excited mainly in the hydrogen ionization zone.

Opacity mechanism alone cannot explain the observed oscillations.

Abstract

HD 187547 was the first candidate that led to the suggestion that solar-like oscillations are present in delta Scuti stars. Longer observations, however, show that the modes interpreted as solar-like oscillations have either very long mode lifetimes, longer than 960 days, or are coherent. These results are incompatible with the nature of `pure' stochastic excitation as observed in solar-like stars. Nonetheless, one point is certain: the opacity mechanism alone cannot explain the oscillation spectrum of HD 187547. Here we present new theoretical investigations showing that convection dynamics can intrinsically excite coherent pulsations in the chemically peculiar delta Scuti star HD 187547. More precisely, it is the perturbations of the mean Reynold stresses (turbulent pressure) that drives the pulsations and the excitation takes place predominantly in the hydrogen ionization zone.