The use of white dwarf cooling times is now established as one of the most important
methods of determining the age of our Galaxy. Estimates of the age of the Galactic Disk
from the study of cool white dwarf samples have remained at approximately 9 Gyr since
the technique was first applied over a decade ago, in contrast to globular clusters ages
and cosmological timescales, which have been the subject of much revision and debate.
Much of the white dwarf work has of necessity relied on all -sky photographic proper
motion surveys to provide sufficiently large numbers of white dwarfs in conjunction with
quantifiable survey limits, unfortunately resulting in demonstrably incomplete samples.
This thesis describes a survey specifically undertaken to obtain a sample of cool white
dwarfs directly from original Schmidt plate material. Concerns regarding the subjectivity of `eye -blinked' proper motion measures are avoided by using digitised data obtained
using the COSMOS and SuperCOSMOS microdensitometers. The observational material is restricted to a single 6° field in which an exceptionally large database of over 300
Schmidt plates exists. As a brief aside, the issue of stacking digitised plate data is examined. In particular, recommendations for the correct weighting algorithm and bad pixel
rejection algorithm to adopt are made, following detailed experimentation with a variety
of techniques. The plate stacking technique is used for the white dwarf survey material
to extend the photometric survey limits, while the optimum proper motion limit to adopt
is investigated in detail, with the principal concern being elimination of contaminants
while retaining as many genuine proper motion objects as possible. Application of the
reduced proper motion population discrimination technique to the resultant proper motion catalogue results in a final sample comprising 56 white dwarf candidates. Follow up
spectroscopic observations show the survey has been successful in identifying cool white
dwarfs, gives no indication of contamination in the white dwarf sample by other stellar
types, and also provides two further cool white dwarfs to include in the sample which were
marginally excluded by the reduced proper motion survey criteria.
Comparison of photographic colours with model predictions allow effective temperatures
and bolometric luminosities to be estimated for the sample members. The luminosity
function constructed from the sample members is in good agreement with previous work,
although the slightly higher space density found for the coolest white dwarfs leads to a
modest increase in the age estimate for the Galactic Disc when the luminosity function is
compared to theoretical models. The Disc age is found to be 10±1/3 Gyr.
Recurring themes in this work are the related questions of sample completeness and
sample contamination. These have been addressed in several ways, notably via number
counts, the V /Vmax completeness test and sample distributions on the reduced proper motion diagram. The results of these analyses may be summarised by stating that the sample
is consistent with being drawn from a complete sample, shows no statistical evidence of
incompleteness, and is unlikely to be contaminated by non -degenerates. Probably the
most serious source of error here is the omission of very cool white dwarfs due to the conservative reduced proper motion survey criteria adopted. This concern, along with other
considerations regarding the atmospheric constituents of the sample members, imply that
a 9 Gyr Disc age may be regarded as a quite robust lower limit from this work.