Observational Consequences of Turbulent Pressure in the Envelopes of Massive Stars

Grassitelli, L.; Fossati, L.; Simón-Diáz, S.; Langer, N.; Castro, N.; Sanyal, D.
Bibliographical reference

The Astrophysical Journal Letters, Volume 808, Issue 1, article id. L31, 6 pp. (2015).

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The major mass fraction of the envelope of hot luminous stars is radiatively stable. However, the partial ionization of hydrogen, helium, and iron gives rise to extended sub-surface convection zones in all of them. In this work, we investigate the effect of the pressure induced by the turbulent motion in these zones based on the mixing-length theory, and we search for observable consequences. We find that the turbulent pressure fraction can amount up to ∼ 5% in OB supergiants and up to ∼ 30% in cooler supergiants. The resulting structural changes are, however, not significantly affecting the evolutionary tracks compared to previous calculations. Instead, a comparison of macroturbulent velocities derived from high-quality spectra of OB stars with the turbulent pressure fraction obtained in corresponding stellar models reveals a strong correlation between these two quantities. We discuss a possible physical connection and conclude that turbulent pressure fluctuations may drive high-order oscillations, which—as conjectured earlier—manifest themselves as macroturbulence in the photospheres of hot luminous stars.
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