IvS Seminar: Cassandra Van der Sijpt (13/11/2025)

Cassandra Van der Sijpt presents her research on sub-surface turbulence in the envelopes of massive stars, focusing on its origin and statistical properties. She begins by explaining the importance of massive stars and the need for multi-dimensional modeling to understand their atmospheres and winds. The talk highlights that 1D models require ad-hoc parameters like micro-turbulence to match observations, indicating missing multi-dimensional physics. Using 2D and 3D radiation-hydrodynamical simulations, she shows that the envelopes of O-type stars are turbulent, with turbulent velocities up to 100 km/s. The turbulence broadens spectral lines and provides additional pressure support, affecting stellar structure. To understand its origin, she performs a linear perturbation analysis, identifying two instabilities: convective and strange-mode. However, the growth rates from simulations do not match the expected convective behavior, suggesting an opacity-driven mechanism. She also discusses ongoing work on characterizing turbulence statistically using autocorrelation functions, aiming to provide a physically motivated description for 1D models. The talk concludes with the importance of these findings for improving stellar atmosphere and evolution models.