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Spectroscopic studies of RV TAU and related objects a thesis submitted for the award of the degree of doctor of philosophy in physics, the Mangalore university

Sumangala Rao, S

Spectroscopic studies of RV TAU and related objects a thesis submitted for the award of the degree of doctor of philosophy in physics, the Mangalore university [Ph.D Thesis] S. Sumangala Rao - Bangalore Indian Institute of Astrophysics 2013 - xvi, 245p.

Thesis Supervisor Giridhar, Sunetra

Post-AGB stars represent a crucial phase in the evolution of low and intermediate mass stars (0.8-8M ) where rapid change in the surface composition of the star and its circumstellar envelope takes place. A very small fraction of the known post-AGB objects show hard core post-AGB abundance pattern i.e. the C/O larger than 1 and enhancement of s-process elements. Many of them show chemical peculiarities not caused by the nucleosynthesis. From the spectroscopic studies of a number of RV Tauri and post-AGB objects it is found that many of these show abundance anomalies due to selective depletion of condensable elements. These abundance anomalies are found to be correlated with their condensation temperatures and this process is known as the Dust-Gas Winnowing. A large fraction of these stars are found to be binaries and this depletion is thought to be occurring in a dusty circumbinary disk. A few RV Tau stars show abundance anomalies correlated with the ionization potential of the neutral atoms. This process is known as the First Ionisation Potential (FIP) effect. In our spectroscopic study of RV Tauri and post-AGB candidates, we try to find the signature of various processes that may affect the stellar composition during the course of its evolution like the initial composition of the Interstellar Medium (ISM), effect of First Dredge Up (FDU), effect of thermal pulses and Third Dredge Up (TDU), effects of dustgas winnowing and FIP effect and also try to refine the boundary conditions (temperature, metallicity, binarity) for these processes to operate. We have undertaken the study of unexplored (as well as the ones lacking comprehensive abundance analysis) RV Tau stars and post-AGB objects to understand the chemical changes taking place during the late stages of stellar evolution of low and intermediate mass stars. We have tried to enlarge the RV Tau and post-AGB sample and also present
a critical compilation of all these objects studied till now to understand the importance of the stellar parameters and of the conditions such as binarity on post-AGB evolution. We have divided our compilations into several sub-groups such as those showing s-process enrichment, dust-gas winnowing, FIP effect and those having normal compositions. This compilation has enabled us to explore the evolutionary link between RV Tau and post-AGB stars; the latter appear to be in a more advanced stage of evolution. This finding has been further supported by their photometric behaviour. We also find that RV Tau stars generally do not exhibit s-process enhancement but they outnumber post-AGBs in dust-gas winnowing and exhibit different families of depletion curves. Starting with the depletion curve of R Sct with an almost precipice like fall at high condensation temper atures, the length and height of low and intermediate condensation temperature plateau show considerable variations. There are another set of RV Tau stars with depletion curve asymptoting at higher condensation temperatures at different [X/H] levels. An attempt has been made to explain the observed depletion curves taking into consideration conditions such as the temperature and composition of the circumstellar environment as well as the efficiency of mixing between the accreted clean gas with the atmosphere/envelope
of the star. In our exploration of post-AGB candidates we also discovered two mildly hydrogendeficient
stars: SAO 40039 and HD 187885 (with strong He I lines). The detections of these stars are very important as they provide an important evolutionary link between hydrogen-normal and hydrogen-deficient stars.

RV Tauri stars

043:524.358 / SUM

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