The nature of the low mass supergiants : RV Tauri and R Coronae Borealis variables
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A programme of photometric and spectroscopic observations has been undertaken in order to investigate the physical and chemical characteristics of the RV Tauri and R Coronae Borealis variables. These characteristics have, in turn, been used to examine a variety of theoretical models that attempt to describe the pulsation mechanisms, the underlying physical nature and the evolutionary status of these low mass supergiant variables. Long time baseline BVRI photometry was obtained at the Mt John University Observatory (MJUO) over intervals of between 750 and 1300 days for eleven of the RV Tauri variables. RV Tauri stars from both the RVa (constant mean magnitude) and RVb (varying mean magnitude) photometric subclasses and from all three spectroscopic Preston subtypes (A, B and C types) were included in the programme. Fourier and least-squares analyses of the light and colour variations have revealed the dominant periodicities and the stability of the pulsation in these stars. A harmonic (f + 2f) fit was found to be a good representation of the "deep-shallow" nature of both the light and colour curves in most stars. Contemporaneous high-resolution spectroscopy of these objects was acquired at MJUO and at the Mt Stromlo Observatory. This enabled us to link the photometric variations and the radial motions in these stars' atmospheres and to relate any photometric and spectral peculiarities. The appearance and phase-dependent behaviour of the Hα line profiles and the radial velocities were examined. Shock-related features, such as line doubling and recombination emission lines, were observed in all the programme stars. The observed Hα profiles were consistent with two enhancements of Hα emission following the passage of the primary and secondary shock waves through the photosphere around phases 0.2 and 0.6. In addition, the metallic lines showed profiles characteristic of an atmospheric shock wave at these phases. The derived spectroscopic and the photometric characteristics have been used to examine a variety of theoretical models that attempt to describe the pulsation mechanisms and the underlying physical nature of the RV Tauri stars. The nature of the RVb subclass and the interpretation that the RVa and RVb stars actually represent a continuum of properties is discussed in detail. A model of a binary system which undergoes periodic eclipses by dust or which interacts at certain orbital phases is presented. Two scenarios were proposed to explain the observed properties: an 'obscuration' model where the star is eclipsed by dust in the binary system; or an 'interaction' scenario where the two stars interact in some manner at a particular orbital phase. The interaction model generally has better agreement with the observed properties although it is quite likely, in a system where circumstellar and/or circumbinary material is known to exist, that a combination of obscuration and interaction is possible. A detailed analysis of the chemical composition of a number of RCB stars in the Galaxy and the Large Magellanic Cloud is presented. This study made use of high-resolution echelle spectra obtained at the Anglo-Australian Telescope. The results for the galactic star, SU Tau, revealed very similar properties to a number of other warm galactic RCB stars, most notably R CrB. This was the first abundance analysis completed for any extragalactic RCB stars and showed a similar abundance distribution to the galactic RCB stars. All these objects (galactic and extragalactic) have a reasonably consistent set of properties, providing evidence that it is possible for this phenomenon to exist in different nucleosynthetic environments.