Tomography of evolved star atmospheres

Cool giant and supergiant stars are among the largest and most luminous stars in the Universe and, therefore, dominate the integrated light of their host galaxies. These stars were extensively studied during last few decades, however their relevant properties like photometric variability and mass loss are still poorly constrained. Understanding of these properties is crucial in the context of a broad range of astrophysical questions including chemical enrichment of the Universe, supernova progenitors, and the extragalactic distance scale. The atmospheres of evolved stars are characterized by complex dynamics due to different interacting processes, such as convection, pulsation, formation of molecules and dust, and the development of mass loss. These dynamical processes impact the formation of spectral lines producing their asymmetries and Doppler shifts. Thus, by studying the line-profile variations on spatial and temporal scales it is possible to reconstruct atmospheric motions in stars and link them to the photometric variability and mass loss. The tomographic method, which is based on the cross-sectioning through the stellar atmosphere and recovering the velocity field for each atmospheric slice, is an ideal technique for this purpose. In this colloquium, I will present the tomographic method and its application to spectroscopic and spectro-interferometric observations of giant and supergiant stars as well as to state-of-the-art three-dimensional numerical simulations to constrain their atmospheric motions on spatial and temporal scales and better understand respective mechanisms responsible for their photometric variability and mass loss