Martin Beech

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


I have used a generalized Naur and Osterbrock (1953) criterion to investigate the manner in which a convective core develops. Stars more massive than {dollar}\sim{dollar}30 Mo can develop convective cores before the onset of central hydrogen burning. It is estimated that stars more massive than {dollar}\sim{dollar}95 Mo will have a convective core throughout the whole of their pre-main sequence phase.;Pre-main sequence evolution has been calculated under the accretion paradigm: beginning with a collapsed core, mass is accreted until a final stellar mass is reached. The calculations begin with a one solar mass core.;The initial 1 Mo model was fully convective and burning deuterium in its core. As the model mass increased from 1 Mo to 8.5 Mo four critical masses were found in the accretion history. Between 1 and 3 Mo, the models are fully convective. At 3.1 Mo a radiative region appears at a mass fraction of 0.25. For models in the mass range 3.1 to 5.4 Mo, the deuterium shell burning source gradually moves outward in mass fraction. For masses between 5.4 and 7.1 Mo our models have radiative interiors. Central hydrogen burning reactions become important at masses above 6.5 Mo, and a small convective core first breaks out at 7.1 Mo. The accretion track intersects the canonical ZAMS at a mass of 8.5 Mo.;Between 8.5 and {dollar}\sim{dollar}15 Mo the internal structure, luminosity and effective temperature of the accretion and canonical ZAMS models were essentially the same. Beyond {dollar}\sim{dollar}17 Mo, the accretion models become increasingly more luminous and cooler. For masses greater than {dollar}\sim{dollar}15 Mo, the accretion models have smaller convective cores than the same mass canonical models. By the time that the accretion model has reached a mass of 30 Mo, a comparison with the canonical 30 Mo ZAMS model reveals a luminosity and effective temperature difference of {dollar}\Delta{dollar}Log L/Lo = +0.022, and {dollar}\Delta{dollar}Log T{dollar}\sb{lcub}\rm c{rcub}{dollar} = {dollar}-{dollar}0.003.;The main sequence evolution of the accretion built 30 Mo model takes place with a lower luminosity (typically {dollar}\Delta{dollar}Log L/Lo {dollar}\approx{dollar} {dollar}-{dollar}0.015) but at the same effective temperature as the canonical model. The convective core mass fraction of the accretion built 30 Mo model during its main sequence evolution is found to be systematically 5 percent smaller than that of the canonical 30 Mo model.;I have extended the accretion sequence to a final mass of 45 Mo. From these models I determine the boundary beyond which massive stars are first predicted to become optically visible. (Abstract shortened by UMI.)



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