Photometric detection of internal gravity waves in upper main-sequence stars

Abstract

Context. Massive stars are predicted to excite internal gravity waves (IGWs) by turbulent core convection and from turbulent pressure fluctuations in their near-surface layers. These IGWs are extremely efficient at transporting angular momentum and chemical species within stellar interiors, but they remain largely unconstrained observationally. Aims. We aim to characterise the photometric detection of IGWs across a large number of O and early-B stars in the Hertzsprung–Russell diagram, and explain the ubiquitous detection of stochastic variability in the photospheres of massive stars. Methods. We combined high-precision time-series photometry from the NASA Transiting Exoplanet Survey Satellite with high-resolution ground-based spectroscopy of 70 stars with spectral types O and B to probe the relationship between the photometric signatures of IGWs and parameters such as spectroscopic mass, luminosity, and macroturbulence. Results. A relationship is found between the location of a star in the spectroscopic Hertzsprung–Russell diagram and the amplitudes and frequencies of stochastic photometric variability in the light curves of massive stars. Furthermore, the properties of the stochastic variability are statistically correlated with macroturbulent velocity broadening in the spectral lines of massive stars. Conclusions. The common ensemble morphology for the stochastic low-frequency variability detected in space photometry and its relationship to macroturbulence is strong evidence for IGWs in massive stars, since these types of waves are unique in providing the dominant tangential velocity field required to explain the observed spectroscopy.

TESS light curves of γ Cas stars

“…Only the parameters fitted to the remaining cases, for which good and reliable fits could be achieved, are presented in Table 2. It is important to note that these values are fully compatible with those observed for massive OB stars (Bowman et al 2020).…”

“…With the advent of high-precision space-borne photometry, it has also been found that a number of massive OB stars display elevated signal levels at low frequencies, aka "red noise", in their photometric times series (e.g. Blomme et al 2011;Ramiaramanantsoa et al 2018;Rauw et al 2019;Bowman et al 2019Bowman et al , 2020. Our γ Cas stars are no exception, showing that these stars fit the typical behaviour seen for stars of similar spectral types.…”

Is HR 6819 a triple system containing a black hole?

“…The presence of pulsations is also related to the relatively large macroturbulence derived (see Sect. 5.1.2 Simón-Díaz et al 2017), since stars with larger pulsation amplitudes typically show larger macroturbulence (Bowman et al 2020). We also note that Rivinius et al (2020) demonstrate variability caused by nonradial gravity-mode pulsations using SMEI and TESS photometry, with such low-frequency photometric variability being common among massive stars (Bowman et al 2019).…”

“…The line profiles of the narrow-lined primary are dominated by a "triangular" shape from a radial-tangential broadening profile (Gray 1973;Simón-Díaz & Herrero 2014). This implies that the dominant broadening mechanism is not rotation, but macroturbulent velocity (caused by non-radial pulsations, see Aerts et al 2009;Bowman et al 2020;and Sect. 3.4).…”

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…The insensitivity of the stochastic variability to a star's metallicity together with the fact that evolutionary timescales predicted most stars were likely on the main sequence was concluded as strong evidence that the observed stochastic variability is likely caused by IGWs excited by core convection (Bowman et al, 2019b). More recently, Bowman et al (2020) demonstrated that the morphology of the IGWs in the frequency spectrum probes the evolutionary properties of the star, such as mass and radius. Furthermore, the amplitudes of IGWs in photometry were found to correlate with the macroturbulent broadening in the spectral lines of dozens of massive stars observed by the TESS mission (Bowman et al, 2020), with macroturbulence also having been associated with pulsations (Aerts et al, 2009;Simón-Díaz et al, 2010, 2017.…”