Subphotospheric fluctuations in magnetized radiative envelopes: contribution from unstable magnetosonic waves

Abstract

We examine the excitation of unstable magnetosonic waves in the radiative envelopes of intermediate-and high-mass stars with a magnetic field of ∼ kG strength. Wind clumping close to the star and microturbulence can often be accounted for when including small-scale, sub-photospheric density or velocity perturbations. Compressional waves -with wavelengths comparable to or shorter than the gas pressure scale height -can be destabilized by the radiative flux in optically-thick media when a magnetic field is present, in a process called the Radiation-Driven Magneto-Acoustic Instability (RMI). The instability does not require radiation or magnetic pressure to dominate over gas pressure, and acts independently of sub-surface convection zones. Here we evaluate the conditions for the RMI to operate on a grid of stellar models covering a mass range 3 − 40M at solar metallicity. For a uniform 1 kG magnetic field, fast magnetosonic modes are unstable down to an optical depth of a few tens, while unstable slow modes extend beyond the depth of the iron convection zone. The qualitative behavior is robust to magnetic field strength variations by a factor of a few. When combining our findings with previous results for the saturation amplitude of the RMI, we predict velocity fluctuations in the range ∼ 0.1 − 10 km s −1 . These amplitudes are a monotonically increasing function of the ratio of radiation to gas pressure, or alternatively, of the zero-age main sequence mass.

Testing the reliability of X-rays as a tool for constraining mass-loss rates of hot stars

“…For a realistic case of an Eddington-limb-darkened source, Sundqvist & Owocki (2015) find that this nodal type topology causes strong variabilities in the wind that extend down to the photosphere. Moreover, any variability at the inner boundary (e.g., bright spots (Ramiaramanantsoa et al 2018), g-modes, or radiation-driven magneto-acoustic waves (Fernández & Socrates 2013;Sen et al 2018)) would add to this variability and potentially cause shock structures to appear quickly after the wind leaves the surface.…”