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 > 100 M(fisheye) (Z = 0.02), > 70 M(fisheye) (Z = 0.05), and > 15 M(fisheye) 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(fisheye) 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). The effective temperatures of very luminous stars (> 120 M(fisheye)) found in the LMC, the SMC, and the Galaxy are discussed in relation to this metal dependence of a curving upper main-sequence.