A BRITE view on the massive O-type supergiant V973 Scorpii: Hints towards internal gravity waves or subsurface convection zones

TL;DR

This study reports the detection of low-frequency brightness variations in the supergiant V973 Scorpii, suggesting internal gravity waves or subsurface convection zones as potential causes, based on high-precision photometry and simulations.

Contribution

It provides observational evidence of internal gravity waves or subsurface convection in a massive supergiant, linking photometric variability to theoretical models.

Findings

Detected broad low-frequency amplitude spectrum with prominent frequencies.

Mode lifetimes of 5-10 days observed.

Spectrum matches hydrodynamical simulations of internal gravity waves.

Abstract

Stochastically-triggered photospheric light variations reaching $\sim$$40$ mmag peak-to-valley amplitudes have been detected in the O8Iaf supergiant V973 Scorpii as the outcome of two months of high-precision time-resolved photometric observations with the BRIght Target Explorer (BRITE) nanosatellites. The amplitude spectrum of the time series photometry exhibits a pronounced broad bump in the low-frequency regime ($\lesssim$$0.9$ d$^{-1}$) where several prominent frequencies are detected. A time-frequency analysis of the observations reveals typical mode lifetimes of the order of $5-10$ days. The overall features of the observed brightness amplitude spectrum of V973 Sco match well with those extrapolated from two-dimensional hydrodynamical simulations of convectively-driven internal gravity waves randomly excited from deep in the convective cores of massive stars. An alternative or additional possible source of excitation from a subsurface convection zone needs to be explored in future theoretical investigations.