Fig. 20.

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Hertzsprung–Russel diagram with the positions of the Aa1, Aa2, and Ab components of δ Ori and evolutionary tracks from Paxton et al. (2015). Numbers next to the main sequence indicate the theoretical masses, small numbers the theoretical gravitational acceleration log g. According to the 3-body model, the masses are 17.1, 8.5, 8.7 M⊙. For the tertiary (Ab), it is in disagreement with the evolutionary track, but in agreement with the value of log g3 inferred from normalised spectra (Hγ). The instability region of pulsations (β Cep type, order ℓ = 0) is indicated as grey area. Other modes (ℓ = 1, 2, 3) can be found in a very similar region (Paxton et al. 2015). Both Aa1, Ab components are located here, and they can exhibit photometric variability attributed to pulsations.
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