Semiconvection in halo stars and primordial helium abundance

• Main Author: Tarbell, Theodore Dean
• Format: Others
• Published: 1976
• Online Access: https://thesis.library.caltech.edu/4289/1/Tarbell_td_1976.pdf; Tarbell, Theodore Dean (1976) Semiconvection in halo stars and primordial helium abundance. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/J7AW-9943. https://resolver.caltech.edu/CaltechETD:etd-10282008-143946 <https://resolver.caltech.edu/CaltechETD:etd-10282008-143946>

Convective overshooting and semiconvection in core-helium burning stars are studied with emphasis on laboratory experiments and terrestrial observations. A necessary condition for the onset of semiconvection is derived, and the Schwarzschild neutrality criterion for the semiconvective zone is justified. Six evolutionary sequences for horizontal branch stars in the globular cluster M3 are computed. They illustrate the effects of different treatments of overshooting and semiconvection, helium-burning nuclear reaction rates, and the primordial helium abundance Y[subscript 0]. By comparing horizontal branch and red giant lifetimes with observations of the relative numbers of stars, the result Y[subscript 0] = 0.20 ± 0.04 is derived for M3. This agrees well with results of standard Big Bang nucleosynthesis, but it is definitely smaller than the "normal" Population I helium abundance. Evolutionary models for subdwarf B stars are computed which show excellent agreement with observed gravities and effective temperatures; subsequent evolution will probably match observations of subdwarf O stars. These models are burning helium at their centers with thin, inert hydrogen envelopes. The hypothesis of mass loss at the helium flash can explain the small envelope masses and the observed gaps in the color distribution of blue halo stars. An upper limit to the initial helium abundance of sdB stars is derived from their light-to-mass ratios. The limit is Y[subscript 0] ≤ 0.25 - 0.05 (log Z + 2), and this limit is uncertain by at least ± 0.10 because of the small sample size and the possibility of systematic observational errors.