The majority of stars in the Galaxy are low mass stars, with M 
5M 
. Unlike the higher mass stars, some aspects of which will
be considered later, significant mass loss only occurs in the late stages of
stellar evolution, serving to enrich the interstellar medium with the products
of nucleosynthesis (He, C, N and O). Because low mass stars constitute such a
large majority, knowledge of the last stages of their evolution is of great
importance for the study of the evolution of the Galaxy and the interstellar
medium (Trams et al. 1989).
On the exhaustion of core hydrogen in such a star, the core contracts and the
temperature and pressure just outside the core are sufficiently high for shell
hydrogen burning to start. Except for the least massive stars (M 2M ) (Shu 1981) this takes place partly via the CNO cycle:
C+ 
H 
N+ 
N+ 
C+ 
C+ H 
N+ N+ H 
O+ O+ N+ N+ H 
He+ C
The star moves off the main sequence and on to the red giant branch (RGB). The stellar radius increases to typically 100 solar radii and the products of stellar nucleosynthesis (primarily He and N, as products of the CNO cycle) begin to appear at the stellar surface. This is the result of the extension of the star setting up a strong convection zone in the outer stellar envelope, which reaches down to the hydrogen burning shell. The surface abundances are modified as follows (Becker & Iben 1979, Trams et al. 1991);
He remains almost constant, as the surface abundance is
already large.
N is enhanced. The conversion of N to O is the slowest step in the CNO cycle, and processed
material therefore appears mostly in the form of N.
C is slightly depleted. As the processed material is
mostly in the form of N, and the total abundance of C, N and O remains
constant, the C abundance will decrease as the cyclic reaction
proceeds.
O abundance remains almost constant as it is not
directly involved in the CNO cycle, although some will be processed into N (Becker & Iben 1979). As the core mass increases, the core temperature rises until the onset of core helium burning. In the lower mass stars, this can occur in a violent process known as the helium flash (Zeilik et al. 1992). The star now leaves the RGB and settles on the horizontal branch (HB), in a relatively stable He-burning phase.
When the core helium is exhausted, the core contracts and the star becomes
cooler, once again moving to the right on the HR diagram. The star is now on the
asymptotic giant branch (AGB), and the outer envelope is strongly convective,
leading to the mixing of He and N to the surface in the second dredge-up. Shell
hydrogen burning persists and shell helium burning commences. The energy
production of this shell is large, causing renewed expansion (Becker & Iben
1979). The helium shell is wholly converted to carbon, and shell hydrogen
burning almost stops. The surface abundance changes are similar to those
produced in the first dredge-up, with a further enhancement of N and depletion of C. Hence stars that have only passed the first dredge-up
are not easily distinguished from those that have also undergone the second
dredge-up (Trams et al. 1991). However, very low mass stars (M 2M ) will not pass through a second dredge-up phase.
Hydrogen shell burning once again takes over the energy production of a star
on the AGB, as the conditions at the outer boundary of the carbon/oxygen core
are not extreme enough for efficient helium shell burning (Trams et al. 1991).
Helium will burn sporadically (described as thermal pulsing) via the triple-
process (3 He C+ ). During these pulses, the star will expand and hydrogen
shell burning ceases, while a strong convection zone is again produced bringing
further products of nucleosynthesis to the stellar surface in the third
dredge-up. The surface abundances are modified as follows:
C is processed in the hydrogen burning shell.
C or N in the helium burning shell.
-captures. These reactions are strongly temperature
dependent and occur only in the more massive stars (Iben & Renzini 1983).
Hence a star which has passed the third dredge-up should exhibit relative
overabundances, with respect to the original chemical composition, of C, N and
O, with carbon more abundant than oxygen as it is a direct product of helium
burning. For heavier elements, one would expect a decreasing enrichment from
Ne to 
S, as the -capture reaction rates decrease (Lang 1980). Stars will
now evolve from the AGB towards higher effective temperatures and are described
as post-asymptotic giant branch (PAGB) stars. The majority of low and
intermediate mass stars (M 
1-8M ) are believed to pass through this post-asymptotic giant
branch stage on their way to becoming planetary nebulae (PNe). However, this
brief transition phase (typically 10 yr (Wood & Faulkner 1986; Schönberner 1983,
1987)) is one of the least well understood stages of stellar evolution and
recently there has been renewed interest in spectroscopic studies of stars
believed to be at this evolutionary stage.
Recent work has concentrated on cooler PAGB stars of spectral type A, F and
G, which have been observed for some years, often in globular clusters (Renzini
1985, Luck 1993). However, the work in this thesis deals with hotter analogues
to these stars, which are likely to be the immediate precursors of planetary
nebulae (McCausland et al. 1992). These PAGB stars are of spectral type B, and
typically show very large carbon depletions. In the most massive and luminous
AGB stars, a fourth process, hot bottom burning (HBB) may occur (Becker &
Iben 1980; Renzini & Voli 1981; Meatheringham et al. 1990), and may remove
carbon produced via the triple- process. During the interpulse phase, temperatures may be
sufficiently high to cause CNO processing at the base of the convective
envelope. Thus the entire stellar envelope may be cycled through CNO processing,
with the conversion of C to N, which is convected to the surface. Alternatively, it
is possible that such stars arrived at the AGB with carbon already depleted, as
descendants of the rare weak G-band stars, giants which are thought to be carbon
deficient owing to large scale diffusion in Ap stars during the main sequence
phase (Lambert & Sawyer 1984). This highlights the difficulty that although
PAGB stars will have had their surface abundances modified via convective
processes, the observed abundances are also dependent on the original stellar
metallicity and other processes. Therefore their chemical compositions may also
mimic those of Population I objects. For example, many of the cool A, F and G
PAGB stars reveal near solar abundances for several elements (Venn & Lambert
1990; Bond 1991).
The hotter B-type PAGB stars have only recently been identified. As mentioned above, high resolution spectroscopy is required to differentiate them from main sequence stars, on the basis of their modified chemical compositions. However, determination of the surface gravity is sufficient to distinguish them from subluminous stars such as subdwarfs, which are characterised by higher gravities (Lamontagne et al. 1985; Heber et al. 1986; Moehler et al. 1990). In addition, some PAGB stars emit a detectable excess of radiation in the infrared, which may indicate the presence of a dust shell surrounding the star (Kwok et al. 1987). In this thesis, several projects related to PAGB stars are discussed, and are introduced below.
Tim kendall