Stochastic Low Frequency Variability in 3-Dimensional Radiation Hydrodynamical Models of Massive Star Envelopes

TL;DR

This study uses 3D radiation hydrodynamical simulations to explore stochastic low frequency variability in massive star envelopes, revealing turbulence and surface motions consistent with observations and linking SLFV to the iron convection zone.

Contribution

First 3D RHD models of massive star envelopes demonstrating turbulence and SLFV consistent with observations, without free parameters.

Findings

Surface velocities of 10-100 km/s match spectroscopic data.

SLFV amplitudes and slopes resemble observed stellar lightcurves.

Characteristic frequencies align with the thermal timescale of the FeCZ.

Abstract

Increasing main sequence stellar luminosity with stellar mass leads to the eventual dominance of radiation pressure in stellar envelope hydrostatic balance. As the luminosity approaches the Eddington limit, additional instabilities (beyond conventional convection) can occur. These instabilities readily manifest in the outer envelopes of OB stars, where the opacity increase associated with iron yields density and gas pressure inversions in 1D models. Additionally, recent photometric surveys (e.g. TESS) have detected excess broadband low frequency variability in power spectra of OB star lightcurves, called stochastic low frequency variability (SLFV). This motivates our novel 3D Athena++ radiation hydrodynamical (RHD) simulations of two 35$\,$M$_\odot$ star envelopes (the outer $\approx$15$\%$ of the stellar radial extent), one on the zero-age main sequence and the other in the middle of the main sequence. Both models exhibit turbulent motion far above and below the conventional iron opacity peak convection zone (FeCZ), obliterating any ``quiet" part of the near-surface region and leading to velocities at the photosphere of 10-100$\,$km$\,$s$^{-1}$, directly agreeing with spectroscopic data. Surface turbulence also produces SLFV in model lightcurves with amplitudes and power-law slopes that are strikingly similar to those of observed stars. The characteristic frequencies associated with SLFV in our models are comparable to the thermal time in the FeCZ ($\approx$3-7$\,$days$^{-1}$). These simulations, which have no free parameters, are directly validated by observations and, though more models are needed, we remain optimistic that 3D RHD models of main sequence O star envelopes exhibit SLFV originating from the FeCZ.