Stochastic low-frequency variability of 50 massive stars in the Cygnus OB associations and the Small Magellanic Cloud

ABSTRACT In recent years, high-precision high-cadence space photometry has revealed that stochastic low-frequency (SLF) variability is common in the light curves of massive stars. We use the data from the Transiting Exoplanet Survey Satellite to study and characterize the SLF variability found in a sample of 49 O- and B-type main-sequence stars across six Cygnus OB associations and one low-metallicity Small Magellanic Cloud star AV 232. We compare these results to 53 previously studied SLF variables. We adopt two different methods for characterizing the signal. In the first, we follow earlier work and fit a Lorentzian-like profile to the power density spectrum of the residual light curve to derive the amplitude $\alpha _0$, characteristic frequency $\nu _{\rm char}$, and slope $\gamma$ of the variability. In our second model-independent method, we calculate the root mean square (RMS) of the photometric variability as well as the frequency at 50 per cent of the accumulated power spectral density, $\nu _{50~{{\ \rm per\ cent}}}$, and the width of the cumulative integrated power density, w. For the full sample of 103 SLF variables, we find that $\alpha _0$, $\gamma$, RMS, $\nu _{50~{{\ \rm per\ cent}}}$, and w correlate with the spectroscopic luminosity of the stars. Both $\alpha _0$ and RMS appear to increase for more evolved stars, whereas $\nu _{\rm char}$ and $\nu _{50~{{\ \rm per\ cent}}}$ both decrease. Finally, we compare our results to 2D and 3D simulations of subsurface convection, core-generated internal gravity waves, and surface stellar winds, and find good agreement between the observed $\nu _{\rm char}$ of our sample and predictions from subsurface convection.