Mass Loss in Evolved Stars

Abstract

This thesis investigates the topic of mass loss in evolved stars using two different ways. First, we study mass loss in a sample of 313 evolved stars observed in low-excitation rotational transitions (J = 2-1 and J = 3-2) of circumstellar 12CO and 13CO lines. The observations has been carried out using the JCMT telescope, USA. The observed lines have been fitted to determine the mass loss rates and other parameters of the outflow wind. We found that the second moment, that can be easily calculated from spectra, could be used as a parameter to investigate the CO lines profiles. A significant difference in average expansion velocities is found between oxygen-rich and carbon-rich stars, where the latter expand at higher velocities. We found that stars with pulsation periods more than 300 days have higher expansion velocities. In additional to the presentation of the results of our millimetre observations, we calculated mass loss rates for 118 objects using two different equations. The results from both equations show a strong linear log-log relation between the momentum rate of the gas outflow and the total momentum of the photons emitted by the star per second. A comparison between the calculated mass loss rates was done using the momentum transfer efficiencies. The isotopic 13/12C line intensity ratios is calculated for about 130 objects in our sample. The second part of the thesis studies mass loss in binary systems where the primary is a mass-losing asymptotic giant branch (AGB) star and part of the AGB star ejected mass is likely to be accreted by the companion. Binary companions to AGB stars are an important aspect of their evolution. Few AGB companions have been detected, and in most cases it is difficult to distinguish between main-sequence and white dwarf companions. Detection of photometric flickering, a tracer of compact accretion disks around white dwarfs, can help identify the nature of these companions. In this work, we searched for flickering in four AGB stars suggested to have likely accreting companions. We found no signs for flickering in two targets: R Aqr and V1016 Cyg. Flickering was detected in the other two stars: Mira and Y Gem. We investigated the true nature of Mira's companion using three different approaches. Our results for Mira strongly suggest that its companion is a white dwarf.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Gary Fuller (Supervisor) & Albert Zijlstra (Supervisor)