This dissertation presents a detailed study of many aspects of the Horizontal
Branch (HB) phase of stellar evolution. A classical technique of stellar structure
analysis is summarized, and applied to Zero-Age Horizontal Branch (ZAHB)
models. The chief conclusions from this work are firstly, that the total mass of
the envelope sensitively affects the luminosity of the hydrogen-burning shell and
the equilibrium of the helium-rich core. Secondly, the rapid progression of models
across the Hertzsprung-Russell diagram with decreasing mass is the result of important
changes in the hydrostatic structure of the stars. Thirdly, the luminosity-metallicity
relationship of the Zero Age models results from the change in the
core equilibrium luminosity with the CNO abundance of the shell region, together
with the decrease in stellar mass at fixed effective temperature. The change in
the mass-temperature relation with CNO is found to be the most important determining
factor in the Horizontal Branch stellar distribution, and therefore is the
most appropriate ‘first parameter’ for HB morphology. The evolution of the stars
is then considered, and the analysis of the interior structures provides a reclassification
of HB track morphology into three categories, depending on whether the
model contains an outer convection zone or a radiative outer envelope, and on the
luminosity of the hydrogen-burning shell. Lastly, the question of the formation
of red-giant stars is considered; the general conclusions of this part of the study
support the arguments presented by Yahil and van den Horn (1985).
Next, the evolution of the convective core of HB stars is reviewed, together
with a detailed account of the numerical techniques developed for modelling semi-convection. The problems associated with the late phase of HB evolution are also
discussed. A brief review of the physical inputs and numerical methods used in
the interior is presented, focussing on the calculation and implementation of the
Equation of State. The calculations performed for this study are then presented
in detail. The effects of oxygen enhancement on zero-age sequences are illustrated
for a range in metallicity, and theoretical relations between luminosity and metallicity
for the ZAHBs are demonstrated. The evolutionary tracks computed are
illustrated and summarized in extensive tabulations in the Appendices.
The final chapter reproduces previously published studies of globular clusters.
The first of these investigates the globular cluster NGC104 (47 Tucanae). By fitting
the theoretical models to recent CCD photometry of the cluster, it was found
that its initial helium content must have been close to 24% by mass. In addition,
the best fits show that models for [Fe/H] = -0.65 provide an excellent match to
the horizontal branch, if (m - M )v ≈ 13.44, and thereby yield consistency over
the entire color-magnitude diagram of the cluster. The second study presents an
investigation of the horizontal branch of M15. Detailed matches of our theoretical
sequences to the cluster observations indicate that high envelope helium abundances
are incompatible with the observed morphology. It is found that there is a
clear preference for values of 0.21 ≾ Y ≾ 0.25, independent of the value of [O/Fe].
The precision of the method is reduced by uncertainties in the observations and in
the available synthetic temperature-bolometric-correction relations. The oxygen enhanced
zero-age HB models are found to have a period-colour relationship which
is almost identical to that of their scaled-solar counterparts, but they reduce significantly
the predicted double-mode variable masses. Importantly, it is found that,
for reasonable assumptions about the reddening to M15, there is no discrepancy
between the predicted and observed periods for the RR Lyrae variables. However,
the period shift between M3 and M15 can be explained by canonical models only
if the helium abundance in both clusters is low (Yhb ~ 0.21), and the bulk of the
RR Lyrae star population in M15 is at late stages of evolution. These conclusions
are reconsidered in the light of the new calculations presented here. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9485 |
| Date | 20 June 2018 |
| Creators | Dorman, Benjamin |
| Contributors | VandenBerg, Don A. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
Page generated in 0.002 seconds