Effects of main-sequence mass loss on stellar and galactic chemical evolution

Guzik, Joyce
Major Professor
Lee Anne Willson
Curtis Struck-Marcell
Committee Member
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Physics and Astronomy
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Physics and Astronomy

L. A. Willson, G. H. Bowen and C. Struck-Marcell have proposed that 1 to 3 solar mass stars may experience evolutionarily significant mass loss during the early part of their main-sequence phase. The suggested mass-loss mechanism is pulsation, facilitated by rapid rotation. Initial mass-loss rates may be as large as several times 10[superscript]-9M[subscript] o/yr, diminishing over several times 10[superscript]8 years. We attempted to test this hypothesis by comparing some theoretical implications with observations. Three areas are addressed: Solar models, cluster HR diagrams, and galactic chemical evolution;Mass-losing solar models were evolved that match the Sun's luminosity and radius at its present age. The most extreme viable models have initial mass 2.0 M[subscript] o, and mass-loss rates decreasing exponentially over 2-3 x 10[superscript]8 years. Compared to a constant-mass model, these models require a reduced initial [superscript]4He abundance, have deeper envelope convection zones and higher [superscript]8B neutrino fluxes. Early processing of present surface layers at higher interior temperatures increases the surface [superscript]3He abundance, destroys Li, Be and B, and decreases the surface C/N ratio following first dredge-up;Evolution calculations incorporating main-sequence mass loss were completed for a grid of models with initial masses 1.25 to 2.0 M[subscript] o and mass loss timescales 0.2 to 2.0 Gyr. Cluster HR diagrams synthesized with these models confirm the potential for the hypothesis to explain observed spreads or bifurcations in the upper main sequence, blue stragglers, anomalous giants, and poor fits of main-sequence turnoffs by standard isochrones;Simple closed galactic chemical evolution models were used to test the effects of main-sequence mass loss on the F and G dwarf distribution. Stars between 3.0 M[subscript] o and a metallicity-dependent lower mass are assumed to lose mass. The models produce a 30 to 60% increase in the stars to stars-plus-remnants ratio, with fewer early-F dwarfs and many more late-F dwarfs remaining on the main sequence to the present. The ratio of stars to stellar remnants and the white dwarf age distribution may prove valuable in distinguishing between explanations for the observed bimodal present-day stellar mass function.