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Increasing main sequence stellar luminosity with stellar mass leads to theeventual dominance of radiation pressure in stellar envelope hydrostaticbalance. As the luminosity approaches the Eddington limit, additionalinstabilities (beyond conventional convection) can occur. These instabilitiesreadily manifest in the outer envelopes of OB stars, where the opacity increaseassociated with iron yields density and gas pressure inversions in 1D models.Additionally, recent photometric surveys (e.g. TESS) have detected excessbroadband low frequency variability in power spectra of OB star lightcurves,called stochastic low frequency variability (SLFV). This motivates our novel 3DAthena++ radiation hydrodynamical (RHD) simulations of two 35$\,$M$_\odot$ starenvelopes (the outer $\approx$15$\%$ of the stellar radial extent), one on thezero-age main sequence and the other in the middle of the main sequence. Bothmodels exhibit turbulent motion far above and below the conventional ironopacity peak convection zone (FeCZ), obliterating any ``quiet" part of thenear-surface region and leading to velocities at the photosphere of10-100$\,$km$\,$s$^{-1}$, directly agreeing with spectroscopic data. Surfaceturbulence also produces SLFV in model lightcurves with amplitudes andpower-law slopes that are strikingly similar to those of observed stars. Thecharacteristic frequencies associated with SLFV in our models are comparable tothe 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 mainsequence O star envelopes exhibit SLFV originating from the FeCZ.

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