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 > 100 M® (Z=0.02), >70 Af® (Z=0.05), and > 15 M® for helium ZAMS (X=0, Z=0.02). Therefore, the effective temperatures of very massive ZAMS stars are rather low: e.g., for a 200 M® star, log Teff=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 (> 120 M® ) found in the LMC, the SMC, and the Galaxy are discussed in relation to this metal dependence of a curving upper main-sequence.
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Ishii et al. (1999) studied this question.