Other
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........37s&link_type=abstract
Ph.D. Thesis, Queen's University Belfast, Belfast
Other
3
Spectroscopy, Post-Agb, Hot Post-Agb, Evolution, Photometry
Scientific paper
In this thesis, we present an LTE model atmosphere analyses of a group of early B-type postasymptotic giant branch (pAGB) stars. With initial masses ≤ 9M&sun;, post-AGB stars form an important group of evolved stars and provide a unique opportunity to study stellar evolution almost on a human time-scale. Post-AGB stars have spectral types ranging from K to B and luminosities between 103 and 104L&sun;. These objects ended their asymptotic giant branch (AGB) evolution phase with a period of strong mass loss (10-7 - 10-4M&sun; yr-1) and have been evolving from cooler to hotter temperatures at almost constant luminosity on a timescale of ˜ 104yr. B-type pAGB stars span a wide range in effective temperature (10 000 - 30 000K). Their expected surface gravities (log g ) and effective temperatures ( Teff ) coincide with those of B stars evolving from the main sequence. Therefore systematic observational analyses are required to distinguish these two groups. Furthermore, p!
ost-AGB stars may be divided into four distinct groups based on their chemical composition. In this thesis, groups I and II represent post-AGB stars which are very metal deficient with C/O ≈ 1 and metal poor with C/O<1, when compared with the Sun, respectively. The question is whether hot pAGB stars belong to either of these four groups. Three further objectives included:
1. to discover whether post-AGB star have helium-normal or helium-rich photospheres. 2. the detection and measurement of s-process element abundances (e.g. Sr, Y, Ba, Hf). 3. to determine whether they show any anomaly in phosphorus abundance such as that seen in the extreme helium stars (EHes). High-resolution ´echelle spectra of several post-AGB stars were obtained at the AAT in 1999 and 2005 in order to study chemical composition, rotation velocities and other fundamental properties. Echelle spectra present many difficulties for data reduction, including the problems of order rectification and merging. To address these problems we developed an ´echelle spectrum reduction package, known as TIGER. These spectra were analyzed using model atmospheres and synthetic spectra computed with the Armagh LTE stellar atmospheres software. The semiautomated spectral fitting package SFIT was used to measure the stellar surface parameters and composition. The results show that Teff of the programme stars are in the range 15 000 - 25 000 K and log g are in the range 2.5 - 3.0. In addition to being metal-poor stars, they show mostly C/O<1. Several of our programme stars, namely HD119608, LSS4331, LSS5112, and LB3116 confirm this. The majority of hot post-AGB stars can be identified with the group II, metal-poor and C-deficient post-AGB stars. The model atmosphere parameters, LTE element abundances and estimated distance obtained here support the idea that programme stars are in true post-AGB stars. We detected helium enrichment in the post-AGB stars Hen3-1428 and LSS4331. We did not detect any evidence of s-process elements, primarily because of the high Teff of our targets. Our results do not show overabundance in phosphorus for any hot pAGB stars. Since we used the same atomic data and methods, we conclude that the enhancement of phosphorus previously found in some EHe stars is real. We studied stellar wind signatures for the post-AGB star LSIV-12 111. Emission line equivalent widths for Balmer lines show changes between two different epochs. Hen3-1428 and LSIV-12 111 show blue shifted absorption lines. A stellar wind is clearly present in both stars. We compared variability of a group of post-AGB and a group EHe stars using archival photometry. We did not detect variability in EHe stars. We detected variability in five post-AGB stars. Large variations in HR4049, HD213985, and HD52961 appear to be related to the binary period.
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