Kepler sheds new and unprecedented light on the variability of a blue supergiant: Gravity waves in the O9.5Iab star HD 188209

Abstract

Stellar evolution models are most uncertain for evolved massive stars. Asteroseismology based on high-precision uninterrupted space photometry has become a new way to test the outcome of stellar evolution theory and was recently applied to a multitude of stars, but not yet to massive evolved supergiants.Our aim is to detect, analyse and interpret the photospheric and wind variability of the O9.5 Iab star HD 188209 from Kepler space photometry and long-term high-resolution spectroscopy. We used Kepler scattered-light photometry obtained by the nominal mission during 1460 d to deduce the photometric variability of this O-type supergiant. In addition, we assembled and analysed high-resolution high signal-to-noise spectroscopy taken with four spectrographs during some 1800 d to interpret the temporal spectroscopic variability of the star. The variability of this blue supergiant derived from the scattered-light space photometry is in full in agreement with the one found in the ground-based spectroscopy. We find significant low-frequency variability that is consistently detected in all spectral lines of HD 188209. The photospheric variability propagates into the wind, where it has similar frequencies but slightly higher amplitudes. The morphology of the frequency spectra derived from the long-term photometry and spectroscopy points towards a spectrum of travelling waves with frequency values in the range expected for an evolved O-type star. Convectively-driven internal gravity waves excited in the stellar interior offer the most plausible explanation of the detected variability.

Onset of non-linear internal gravity waves in intermediate-mass stars

“…Clearly, wave breaking and the ensuing turbulence that is generated would lead to more efficient mixing and angular momentum transport than simple radiative dissipation. The strong observational evidence for enhanced coupling between convective and radiative regions (Beck et al 2012;Deheuvels et al 2012;Tayar & Pinsonneault 2013) and the possible observational detections of IGW (Aerts & Rogers 2015;Aerts et al 2017Aerts et al , 2018Ramiaramanantsoa et al 2018) both point to breaking IGW, which in turn, points to a flatter frequency generation spectrum from convection than what theoretical models predict or higher convective velocities than those predicted by MLT. It is therefore vital that more theoretical and numerical investigations into the convective generation spectrum of IGW, and the physics that drives it, are undertaken.…”

Magnetic characterization of the SPB/β Cep hybrid pulsator HD 43317

“…Such stochastic IGWs have only recently been observed for massive stars (e.g. Blomme et al 2011;Neiner et al 2012b;Aerts et al 2017). In addition, it has become clear from Kepler data that gravitoinertial modes in B-and F-type pulsators with period spacings are dipole modes (Van Reeth et al 2015;Pápics et al 2017).…”

Characterizing the observational properties of δ Sct stars in the era of space photometry from the Kepler mission

“…The δ Sct stars are a diverse group of pulsating stars found at the intersection of the main sequence and the classical instability strip in the Hertzsprung-Russell (HR) diagram, which corresponds to spectral types between A2 V and F2 V for main sequence Population I stars, and effective temperatures approximately between 6400 ≤ T eff ≤ 8600 (Breger 2000b;Rodríguez & Breger 2001;Aerts et al 2010;Uytterhoeven et al 2011). This places δ Sct stars within an interesting transition region in the HR diagram between low-mass stars with radiative cores and thick convective envelopes (M 1 M ) and high-mass stars with large convective cores and radiative envelopes (M 2 M ) -see Bowman (2017) for a recent review of δ Sct stars.…”

“…This makes asteroseismology unique, with its strong observational constraints of stellar physics used for improving theoretical models of stellar structure and evolution. The dominant pulsations in δ Sct stars are self excited by the opacity (κ) mechanism operating in the He ii ionisation zone (Cox 1963;Breger 2000b;Aerts et al 2010), which produces pressure (p) modes with periods as long as eight hours and short as 15 min (Uytterhoeven et al 2011;Holdsworth et al 2014). These p modes typically have low radial orders and are most sensitive to the surface layers of a star.…”

“…Significant progress in understanding the physics of δ Sct stars has been made since the space photometry revolution facilitated by the CoRoT (Auvergne et al 2009) and Kepler (Borucki et al 2010) space missions. The rich pulsation spectra of δ Sct stars offer the potential to probe physics at different depths using asteroseismology, thus these stars can provide useful constraints of stellar structure and evolution theory in an important transition region on the main sequence (Breger 2000b;Aerts et al 2010). The full scientific potential of studying pulsation modes in δ Sct stars with asteroseismology in the era of space photometry has yet to be exploited.…”