Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry

Abstract

Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of stellar interiors accumulate with stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing stellar interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe.Stars born with masses larger than approximately eight times the mass of the Sun play a significant role in the evolution of galaxies. They are the chemical factories that produce and expel heavy elements through their wind and when they end their lives as supernovae and form a black hole or neutron star 1-3 . However, the chemical yields that enrich the interstellar medium and the remnant mass strongly depend on the progenitor star's interior properties 4 . The detectable progenitors of supernovae include blue supergiant stars, which are hot massive stars in a shell-hydrogen or core-helium burning stage of stellar evolution. Stellar evolution models of these post-main sequence stars contain by far the largest uncertainties in stellar astrophysics, as observational constraints on interior mixing, rotation and angular momentum transport are missing. These phenomena are further compounded when coupled with mass loss, binarity and magnetic fields 1-3 . Across astrophysics, from star formation to galactic evolution, it is imperative to calibrate theoretical models of massive stars using observations because they determine the evolution of the cosmos.A unique methodology for probing stellar interiors is asteroseismology 5 , which -similarly to seismology of earthquakes -uses oscillations to derive constraints on the structure of stars.The study of stellar interiors of low-mass stars like the Sun has undergone a revolution in

TESS light curves of γ Cas stars

“…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.…”

“…• Red noise: A gradual increase in periodogram amplitudes can be seen towards the lowest frequencies, especially in HD 45314, once the lightcurve has been detrended and cleaned from its main periods. This so-called "red noise" actually is stellar in origin, rather than instrumental, as its presence has been reported in several massive stars (Blomme et al 2011;Rauw et al 2019;Bowman et al 2019). Subsection 3.1.1 provides further details on the properties of the red noise in our targets.…”

Is HR 6819 a triple system containing a black hole?

“…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).…”

Relative importance of convective uncertainties in massive stars

“…Saio, Georgy & Meynet (2013) suggest to distinguish between the two BSG types with radial pulsations (e.g. Bowman et al 2019) in addition to the CNO surface enrichment, i.e. BSG type II exhibits radial pulsations.…”