Observational studies of key stages in stellar evolution

Abstract

This thesis presents observational investigations into three poorly-understood, but key stages in stellar evolution. Stars undergoing the post T Tauri phase - which lies between the well-studied T Tauri stage of disc accretion and the arrival onto the Main Sequence - are difficult to identify and thus little-investigated. Knowledge of the processes occurring in this stage is clearly essential to understanding the events that lead a star to the Main Sequence. Optical photometric observations of flaring on post T Tauri stars, obtained with the Southern African Large Telescope and smaller SAAO telescopes, are presented here. The energies and magnitudes of flares on a sample of targets have been derived and compared with more famous flare stars, and a new flare star has been discovered. A preliminary search for oscillations in flare loops has been undertaken.
At the other end of a star's lifetime, the very late thermal pulse (VLTP) is an extremely brief stage that occurs in ' S-' 15% of intermediate-mass stars. Observable examples are thus very rare. Sakurai's Object underwent a VLTP in 1995, evolving
from a white dwarf back to a red giant in only two years. This is described as "born-again" evolution. Sakurai's Object is the first star since 1917 in which the born-again stage has been observed, and therefore the first example to be studied using modern instrumental techniques and at the wide range of wavelengths now accessible. Observational data are essential to constraining models of the VLTP phase, which as yet cannot explain the extremely rapid evolution of Sakurai's Object. Near-infrared spectra of Sakurai's Object are analysed in this work, finding that 2.5 x 10-6 M® of CO, with an isotopic ratio of 12C/ 13C 3, was ejected in a fast wind (500 + 80 km r') during the VLTP. The cooling CO is modelled as a shell, mixed with graphitic dust, at a distance of 1011 km from the central star. The evolution of stars in a symbiotic binary system is profoundly effected by mass-transfer from the red giant component. itS Ophiuchi is such a system, containing a white dwarf that accretes matter from its red giant companion. The accumulation of accreted matter on the white dwarf has resulted in observed eruptions every 10-20 years in the past century, making RS Oph the most active of the recurrent novae. The mechanisms of mass-transfer (Roche lobe overflow or direct wind accretion) and eruption (thermonuclear runaway or accretion models) are the subjects of considerable controversy. Observations of optical flickering are presented here, plaing limits on the time at which accretion resumed following the most recent outburst of RS Oph in 2006. This is the first time the re-establishment of accretion has been observed in a recurrent nova. It is demonstrated that the accretion rate at this time spans the range for Roche lobe overflow and direct accretion mechanisins, and is insufficient to result in eruptions by thermonuclear runaway with such a short outburst interval as is observed. A variable accretion rate is proposed, perhaps
increasing with time as accretion is re-established following the disruption of an outburst.