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 (TESS) 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 SMC 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\% of the accumulated power spectral density, $\nu_{50\%}$, 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, $\nu_{50\%}$, 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 $\nu_{50\%}$ both decrease. Finally, we compare our results to 2-D and 3-D 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 sub-surface convection.