Evolution of a 30-solar-mass star with mass loss. II - Core He burning phase

Astronomy and Astrophysics – Astronomy

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Main Sequence Stars, Nuclear Fusion, Stellar Evolution, Stellar Mass Ejection, Helium, Hertzsprung-Russell Diagram, Mass To Light Ratios, Supergiant Stars

Scientific paper

Five tracks for a 30 M0. star with mass loss given by the algorithm Mdot = -kLR/M have been computed to core He exhaustion with initial ZAMS rates of (0.0, 1.0, 2.5, 5.0, 10.0) × l0-7 Msun yr-1. The main-sequence phase has been described in an earlier paper and the present paper continues the detailed comparison of the tracks and the internal structure of the evolving stars to clarify further the effects of mass loss on stellar evolution. Two of the sequences showed reversals in their direction of evolution across the HR diagram. These reversals have been demonstrated to occur when mass loss dominates over the effects of the shell source. The highest mass loss rate track reached the giant branch before burning any He, while the next highest rate produced a star which spent, due to a reversal, the major portion of the He burning as a hot blue supergiant and a WR type star -25 per cent of this time was at a temperature hotter than the ZAMS. As in the earlier paper it is shown that the mass-luminosity law can be used to explain features of the evolution and internal structure such as the changes of stellar luminosity, He core luminosity and He burning age with central He abundance. The sizes and positions of the convective shell are also discussed. The effect of mass loss on the time the stars spend in the various parts of the HR diagram during their evolution is examined. Most evolutionary sequences predict a gap in the HR diagram between the MS band and core He burning supergiants, in conflict with observations; only the track with a full reversal to the blue during core He burning filled this gap.

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