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.