A Study of Stochastic Low-Frequency Variability for Galactic O-type Stars

TL;DR

This study investigates the stochastic low-frequency variability in O-type stars, revealing correlations with stellar properties and suggesting multiple physical processes, including FeCZ, IGWs, wind instability, and granulation, as underlying the observed variability.

Contribution

It provides the first comprehensive analysis linking SLF variability to stellar parameters in a large sample of O-type stars, highlighting potential physical mechanisms.

Findings

Significant correlations between SLF amplitude and stellar mass, radius, age, and mass-loss rate.

Decreased characteristic frequency and steepness with increasing radius for stars with M ≥ 30 M_sun.

Evidence suggesting multiple physical processes, such as FeCZ, IGWs, wind instability, and granulation, contribute to SLF variability.

Abstract

In order to explore how the ubiquitous stochastic low-frequency (SLF) variability of O-type stars is related to various stellar characteristics, we compiled a sample of 150 O-type stars observed via ground-based spectroscopic surveys, alongside photometric data obtained from the Transiting Exoplanet Survey Satellite (TESS). We analyzed 298 light curves obtained from TESS sectors 1-65 for the stars in our sample. Leveraging the spectroscopic parameters, we used Bonnsai to determine masses, radii, fractional main sequence ages, and mass-loss rates for stars of our sample. Subsequently, we identified possible correlations between the fitted parameters of SLF variability and stellar properties. Our analysis unveiled four significant correlations between the amplitude and stellar parameters, including mass, radius, fractional main sequence ages, and mass-loss rate. For stars with $M \gtrsim 30 M_{\odot}$, we observed a decrease in characteristic frequency and steepness with increasing radius. Finally, we compared various physical processes that may account for the SLF variability with our results. The observed SLF variability may arise from the combined effects of FeCZ and IGWs, with IGWs potentially more dominant in the early stages of stellar evolution, and the contribution of FeCZ becoming more significant as stars evolve. Meanwhile, our results indicate that the SLF variability of O-type stars bears certain signatures of the line-driven wind instability and granulation.