Radiative Transfer Modeling of the Winds & Circumstellar Environments of Hot And Cool Massive Stars

Abstract

We present modeling research work of the winds and circumstellar environments of prototypical hot and cool massive stars using advanced radiative transfer (RT) calculations. This research aims at unraveling the detailed physics of various mass-loss mechanisms of luminous stars in the upper H-R diagram. Very recent 3-D RT calculations, combined 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. The mass-loss rates computed from matching DACs in HD 64760 (B Ib) do not reveal appreciable changes from the rates of smooth wind models. Intermediate yellow supergiants (such as Rho Cas, F-G Ia0), on the other hand, show prominent spectroscopic signatures of strongly increased mass-loss rates during episodic outbursts. Long-term spectroscopic monitoring of hypergiants near the Yellow Evolutionary Void reveals that their mass-loss rates and wind-structure are dominated by photospheric eruptions and large-amplitude pulsations that impart mechanical momentum to the circumstellar environment by propagating shock waves. In massive red supergiants, however, clear evidence for mechanical wave propagation from the sub-photospheric convection zones is lacking. Recent spatially resolved HST-STIS observations inside Betelgeuse's (M Iab) very extended chromosphere and dust envelope show evidence of warm chromospheric gas far beyond the dust condensation radius of RT models. Models for these long-term spectroscopic observations demonstrate that the chromospheric pulsations are not spherically symmetric. The STIS observations point to the importance of mechanical wave propagation for heating and sustaining chromospheric conditions in the extended winds of red supergiants.