Indirect imaging of nonradial pulsations in a rapidly oscillating Ap star

Many types of stars show periodic variations of radius and brightness, which are commonly referred to as " stellar pulsations." Observed pulsational characteristics are determined by fundamental stellar parameters. Consequently, investigations of stellar pulsations provide a unique opportunity to verify and refine our understanding of the evolution and internal structure of stars. However, a key boundary condition for this analysis - precise information about the geometry of pulsations in the outer stellar envelopes - has been notoriously difficult to secure. Here we demonstrate that it is possible to solve this problem by constructing an " image" of the pulsation velocity field from time series observations of stellar spectra. This technique is applied to study the geometry of nonradial pulsations in a prototype magnetic rapidly oscillating Ap ( roAp) star, HR 3831. Our velocity map directly demonstrates an alignment of pulsations with the axis of the global magnetic field and reveals a significant magnetically induced distortion of pulsations. This observation constitutes a long- sought solution of the problem of the pulsation geometry of roAp stars and enables very stringent tests of the recent theories of stellar magnetoacoustic oscillations.