Simulations of magneto-acoustic pulsations in atmospheres of rapidly oscillating Ap stars

TL;DR

This paper presents 2-D non-linear magneto-hydrodynamical simulations that replicate observed pulsational behaviors in rapidly oscillating Ap stars, revealing the influence of atmospheric structure and magnetic fields on wave propagation.

Contribution

It introduces the first comprehensive 2-D simulations of magneto-acoustic pulsations in roAp star atmospheres, incorporating realistic stratification and magnetic effects.

Findings

Simulations reproduce key observed pulsation features.

Density inversion at the photosphere significantly affects pulsation depth dependence.

Magnetic fields influence wave reflection and node formation.

Abstract

Rapidly oscillating Ap stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magneto-acoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out 2-D time-dependent, non-linear magneto-hydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base.