Core–Halo Structure of a Chemically Homogeneous Massive Star and Bending of the Zero–Age Main–Sequence

We have recalculated the interior structure of very massive stars of uniform chemical composition with the OPAL opacity. Very massive stars are found to develop a core–halo structure with an extended radiative–envelope. With the core–halo structure, because a more massive star has a more extended envelope, the track of the upper zero–age main-sequence (ZAMS) curves redward in the H–R diagram at > 100M⊙ (Z = 0.02), > 70M⊙ (Z = 0.05), and > 15M⊙ for helium ZAMS (X = 0., Z=0.02). Therefore, the effective temperatures of very massive ZAMS stars are rather low: e.g., for a 200M⊙ star, logTeff = 4.75 (Z = 0.004), 4.60 (Z = 0.02), 4.46 (Z = 0.05), and 4.32 (Z = 0.10). The effective temperatures of very luminous stars (> 120M⊙) found in the LMC, the SMC, and the Galaxy are discussed in relation to this metal dependence of a curving upper main-sequence.