| dc.description.abstract | Post asymptotic giant branch (post-AGB) stars, central stars of planetary nebulae
(CSPNe) and planetary nebulae (PNe) are important phases of stellar evolution
as the material they feedback is the seed of subsequent star formation in a galaxy.
The majority of low and intermediate mass stars are expected to evolve through
these channels, however, it is uncertain how many actually do, and at what rate.
The Galactic halo, with its older population, provides a direct test of evolutionary
models for low mass stars.
Birthrate estimates of PNe are uncertain and worse still, are in contradiction with
accepted white dwarf (WD) birthrate estimates. Much of the uncertainty stems
from the lack of complete samples and poorly determined distance estimates. New
surveys such as the Sloan Digital Sky Survey (SDSS), Galaxy Evolutionary Ex-
plorer (GALEX) and the INT Photometric H® Survey (IPHAS) have discovered
many new PNe and have observed the far edges of the Galaxy. Improved methods
of determining distances to CSPNe are presented here, using model atmospheres,
evolutionary tracks and high resolution reddening maps utilising these revolution-
ary surveys.
Locating the CSPN is non-trivial particularly for evolved PNe, as they are ex-
tended with their central star often displaced from the centre of the nebula. There-
fore, photometric criteria are required to locate the CSPN in the nebula’s field.
Synthetic photometry of the CSPNe is derived from spectral energy distributions
(SEDs) computed from a grid of model atmospheres covering the parameter range
of CSPNe. The SEDs are convolved with filter transmission curves to compute synthetic magnitudes for a given photometric system which are then calibrated
with standard stars and WDs.
A further project borne out of a search for luminous central stars of faint PNe,
resulted in a systematic search for post-AGB stars in the Galactic halo. In this
work, new candidate halo post-AGB stars are discovered from a search through
the SDSS spectroscopic database. Combined with previously identified halo post-
AGB stars, including the results of a sub-sample from the Palomar-Green (PG)
survey, the number of observed and predicted populations are compared. The
number of observed post-AGB candidates shows a remarkable deficit to expecta-
tions. A survey within a subset of the photometric database of SDSS supports
the findings of the PG and SDSS spectroscopic surveys.
These findings provide strong evidence for a lack of post-AGB stars in the Galac-
tic halo and thick disc. A plausible explanation is that a large fraction of stars
in these old, metal-poor populations are evolving via alternative channels. The
implications of such a result are far reaching with knock on effects for stellar
evolutionary theory, galactic evolution and extragalactic redshift estimates. | en_US |
