Studies of pulsating carbon stars a thesis submitted for the degree of Doctor of Philosophy in the faculty of science, Bangalore University, Bangalore A. V. Raveendran [Ph.D Thesis]

Thesis Supervisor N. K. Rao

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1989

The present work was begun with the goal of understanding the formation and properties of circumstellar dust envelopes around carbon-rich objects, especially those belonging to the RV Tauri group of variables. A study of dust envelopes would provide valuable information not only on the mass loss from the objects but also on the nature of interstellar grains because of their probable origin in circumstellar envelopes. Both multiwavelength polarimetry and infrared photometry serve as the most useful observational tools for such an investigation.
The thesis essentially contains, ( i) broadband multiwavelength (UBVRI) polarimetry of a sample of carbon-rich objects belonging to the different variability types cepheids, RV Tauri stars and red variables - and its analysis, (ii) extensive two color (BV) photometry of the RV Tauri star AR Pup, (iii) an interpretation of the Infra Red Astronomy Satellite data of RV Tauri stars, and (iv) a discussion of some theoretical aspects of polarization produced by circumstellar dust envelopes. It has been divided into six chapters and each into several sections and subsections.
In the first chapter observational characteristics brief of the descriptions of the different variability classes are given. Short accounts of the mechanisms which cause the instabilities and the mass loss observed in these objects are also presented. The instrumentation used for the observations, and the method of observations and data reduction are discussed in the second and third chapters, respectively.
The data, analysis and results are presented in the fourth chapter which has two parts; part A deals with the cepheids and RV Tauri stars, and part B with the red carbon stars. The list of programme stars for polarimetry contained 2 carbon cepheids (RU Cam and V553 Cen) , 4 carbon RV Tauri stars (AR Pup, RU Cen, SX Cen and AC Her) and 12 red carbon stars (VX And, UU Aur, T Cae, Y CVn, U Hya, Y Hya, R Lep, RY Mon, W Ori, Y Per, RT Pup and X Vel). Out of these objects, polarimetric observations have been reported in the literature only for RU Cam, AC Her, R Lep, Y CVn, Y Per, W Ori and Y Hya, and except for R Lep, multiwavelength observations have been reported for none of them. A considerable part of the available 102-cm telescope time was devoted to observe AR Pup since it was found to show exceptionally large and highly time dependent polarization. The amplitude of polarization in U band was found to be -14.6%, the highest ever observed in any late-type star.
The IRAS Point Source Catalogue, which gives the fluxes at far Infrared wavelengths - 12, 25, 60 and 100~m - provides a good database for a systematic study of the composition and formation of circumstellar dust; an interpretation of the IRAS data on RV Tauri stars is presented in the fifth chapter. In the same chapter a discussion of the possible polarization mechanisms operating in the objects considered, and a few computational polarization models of circumstellar envelope using Mie theory under single scattering approximation are also presented.
Finally, in the last chapter the conclusions are summarized, and the further investigations needed to answer some of the related problems are discussed.
The equations, tables and figures are numbered sequentially in each chapter, and the corresponding chapter number is prefixed whenever a reference is made to them in any other chapter. The related publications and literature cited are listed at the end.