Asymptotic giant-branch (AGB) stars are identified and classified in five 7'x7' fields
spaced along M31's SW semi-major axis using a four band photometric system. An
investigation of the AGB luminosity functions and red giant-branch widths reveals significant
differences between the star forming histories of the five fields. The distance
modulus of M31 is derived using carbon stars (C-stars) and found to be consistent with
both a value obtained from Cepheids and with values in the literature.
The ratio of AGB C- to M-stars (C/M ratio) in the five fields is found to increase with
galactocentric distance and it is shown that photometric incompleteness is not responsible
for this effect. This is the first clear demonstration of a varying C/M ratio in an external
galaxy. The C/M ratios appear to be insensitive to star-forming history differences but
sensitive to metallicity differences between the fields. Previous observations are used to
define a relationship between the C/M ratio and metallicity, and this is used to obtain
estimates of the field metallicities. These estimates are found to be consistent with a
previous measurement of M31's metallicity gradient. The C /M ratios measured in M31
indicate that the composition of M31's interstellar medium may be position dependent,
and evidence is cited in favour of this.
Follow up spectroscopy was obtained in two of the five fields, and is used to show
that the photometric system did an excellent job of discriminating between M-, S- and
C-stars. Of the 48 C-stars for which spectra were obtained, 7 have strongly enhanced
¹³C bands (J-stars), 2 have strong H⍺ emission, while 3 are found to exhibit enhanced
Li absorption (Li-stars). Both the J- and Li-stars are fainter than predicted by current
theoretical models, while the colours of the H⍺ stars suggest they may be in the terminal phases of their evolution. The C₂ and CN bandstrengths of the C-stars are measured,
and no correlation between these bandstrengths and either M[sub bol] or (V — I) is found. It
is suggested that this lack of correlation is due to an age spread. The spectra of the first
confirmed S-star in M31 is presented, and two evolutionary pathways are suggested to
account for this star's high luminosity. / Science, Faculty of / Physics and Astronomy, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/6283 |
| Date | 05 1900 |
| Creators | Brewer, James Philip |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Format | 10192414 bytes, application/pdf |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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