Surface Manifestation of Stochastically Excited Internal Gravity Waves

TL;DR

This study compares theoretical predictions and numerical simulations of internal gravity waves in massive stars, revealing surface g-mode peaks and demonstrating how shorter simulations can predict the spectrum effectively.

Contribution

It validates and reconciles different theoretical models of IGWs with numerical simulations, improving understanding of surface wave manifestations in stars.

Findings

Simulations show g-mode peaks consistent with Lecoanet et al (2019).

Corrected amplitude estimates align with simulation results.

Shorter simulations can predict the final wave spectrum effectively.

Abstract

Recent photometric observations of massive stars show ubiquitous low-frequency "red-noise" variability, which has been interpreted as internal gravity waves (IGWs). Simulations of IGWs generated by convection show smooth surface wave spectra, qualitatively matching the observed red-noise. On the other hand, theoretical calculations by Shiode et al (2013) and Lecoanet et al (2019) predict IGWs should manifest at the surface as regularly-spaced peaks associated with standing g-modes. In this work, we compare these theoretical approaches to simplified 2D numerical simulations. The simulations show g-mode peaks at their surface, and are in good agreement with Lecoanet et al (2019). The amplitude estimates of Shiode et al (2013) did not take into account the finite width of the g-mode peaks; after correcting for this finite width, we find good agreement with simulations. However, simulations need to be run for hundreds of convection turnover times for the peaks to become visible; this is a long time to run a simulation, but a short time in the life of a star. The final spectrum can be predicted by calculating the wave energy flux spectrum in much shorter simulations, and then either applying the theory of Shiode et al (2013) or Lecoanet et al (2019).