2019ApJ...883...89O

The mechanism of mass loss in late evolutionary stages of low- and intermediate-mass stars is not yet well understood. Therefore, it is crucial to study the dynamics of the region within a few R_*, where the wind acceleration is considered to take place. We present a three-dimensional diagnosis of the atmospheric dynamics of the closest asymptotic giant branch star R Dor from the low photospheric layers to the extended outer atmosphere, for the first time for a star other than the Sun. The images reconstructed with a spatial resolution of 6.8 mas-seven times finer than the star's angular diameter of 51.2 mas in the continuum-using the AMBER instrument at the Very Large Telescope Interferometer show a large, bright region over the surface of the star and an extended atmosphere. The velocity-field maps over the star's surface and atmosphere obtained from the Mg and H₂O lines near 2.3 µm forming at atmospheric heights below ∼1.5 R_* show little systematic motion beyond the measurement uncertainty of 1.7 km s^–1. In marked contrast, the velocity-field map obtained from the CO first overtone lines reveals systematic outward motion at 7-15 km s^–1 in the extended outer atmosphere at a height of ∼1.8 R_*. Given the detection of dust formation at ∼1.5 R_*, the strong acceleration of material between ∼1.5 and 1.8 R_* may be caused by the radiation pressure on dust grains. However, we cannot yet exclude the possibility that the outward motion may be intermittent, caused by ballistic motion due to convection and/or pulsation.