The cool hydrogen-deficient carbon stars and their atmospheres
Abstract
Photoelectric photometry of a large sample of R CrB and hydrogen-deficient carbon
stars was obtained over a period of five months in order to search for variability and
determine the period if variable. All the stars in the sample were found to be variable.
Only for the stars S Aps, U Aqr and V CrA were sufficient observations obtained to
enable periods to be identified. The determined periods were 39.7, 41.8 and 69.0 days
respectively. These periods are in agreement with the theoretical period-temperature
relationship.
Photoelectric photometry of the hot hydrogen-deficient star DY Cen was obtained
over a period of four weeks. DY Cen was confirmed to be variable and the dominate
period of 3.8 days determined. This period was consistent with the period-temperature
relationship.
Model atmospheres were calculated for hydrogen-deficient compositions with temperatures between 5000-8000K and surface gravities between 0.0 and 4.4. The models
included the effects of molecular formation, convection and line-blanketing. It was
shown that the temperature structure was strongly dependent on the composition, in
particular the ratios of C/He and H/He.
R CrB was re-analysed using these new models. The derived atmospheric parameters were T[subscript(eff)] = 7400 ± 500K, log g = 0.55 ± 0.25, ξ[subscript(t)] = 8 ± 2kms⁻¹
and C/He=0.005.
High resolution spectra were obtained of RY Sgr in order to do a similar analysis. The
derived parameters were T[subscript(eff)] = 7000 ± 500K, log g = 0.65 ± 0.25, ξ[subscript(t)] = 10 ± 2kms⁻¹
and
C/He=0.005. Both stars were found to have solar metallicities with no over-abundances
of s-process elements. The abundances of C, N and O were all enhanced relative to the
solar values.
Medium resolution spectra were obtained at the Isaac Newton telescope of suspected R CrB stars in order to correctly classify them. The stars were classified on
the basis of the strength of the hydrogen lines and the G band. BG Cep, LO Cep,
CC Cep, DZ And, RZ Vul, VZ Vul, V638 Her and V1405 Cyg were all classified as not
being R CrB stars. UV Cas, SU Tau and SV Sge were classified as R CrB stars.
Type
Thesis, PhD Doctor of Philosophy
Collections
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.