We present modeling research work of the winds and circumstellar environments of a variety of prototypical hot and cool massive stars using advanced radiative-transfer calculations. This research aims at unraveling the detailed physics of various mass-loss mechanisms of luminous stars in the upper portion of the H-R diagram. Very recent 3D radiative-transfer calculations, com-bined with hydrodynamic simulations, show that radiatively-driven winds of OB supergiants are structured due to large-scale density and velocity fields caused by rotating bright spots at the stellar equator. The mass-loss rates computed from matching Discrete Absorption Components (DACs) in WE observations of HD 64760 (B Ib) do not reveal appreciable changes from the rates of unstructured (smooth) wind models. Intermediate yellow supergiants (such as the yellow hypergiant p Cas, F—G Ia0), on the other hand, show prominent spectroscopic signatures of strongly increased mass-loss rates during episodic outbursts that cause dramatic changes of the stellar photospheric conditions. Long-term high-resolution spectroscopic monitoring of cool hypergiants near the Yellow Evolutionary Void reveals that their mass-loss rates and wind-structure are dominated by photospheric erup-tions and large-amplitude pulsations that impart mechanical momentum to the circumstellar environment by propagating acoustic (shock) waves. In massive red supergiants, however, clear evidence for mechanical wave propagation from the sub-photospheric convection zones is lacking, despite their frequently observed spectroscopic and photometric variability. Recent spatially resolved HST-STIS observations inside Betelgeuse's (M Tab) very extended chro-mosphere and dust envelope show evidence of warm chromospheric gas far be-yond the dust-condensation radius of radiative-transfer models. Models for these long-term spectroscopic observations demonstrate that the chromospheric pulsa-tions are not spherically symmetric. The STIS observations point to the impor-tance of mechanical wave propagation for heating and sustaining chromospheric conditions in the extended winds of red supergiants.
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