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Core-halo Structure of Chemically Homogeneous Massive Star and Bending of Zero-Age Main-Sequence

We have recalculated 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 a extended radiative-envelope. With the core-halo structure, a more massive star has a more extended envelope, then the track of upper zero-age main-sequence (ZAMS) curves redward in the H-R diagram at > 100 M_sol (Z=0.02), >70 M_sol (Z=0.05), and > 15 M_sol for helium ZAMS (X=0., Z=0.02). Therefore, effective temperature of a very massive ZAMS star is rather low: e.g., for the 200 M_sol star, log T_eff=4.75 (Z=0.004), 4.60 (Z=0.02), 4.46 (Z=0.05), and 4.32 (Z=0.10). Effective temperatures of very luminous stars (> 120 M_sol) found in the LMC, the SMC, and the Galaxy are discussed in relation with this metal dependence of curving upper main-sequence.