Tracing Historic Mass Loss from Evolved Stars with Thermal Emission from Cold Dust
Creators: name: Dharmawardena, Thavisha Erandinie
Publisher: name: Zenodo
Year: 2019
Subjects: Evolved stars
Thermal dust emission
Mass loss
sub-mm
far-IR

description: Evolved stars play a key role in the life cycle of dust in the universe. Through strong winds and supernovae they inject the material reprocessed in their cores to the interstellar medium, replenishing the interstellar medium with heavy elements. As evolved stars are so numerous they are extremely effective in this role. In this thesis we study this mass loss delving deep into their past, exploring dust mass-loss events which occurred throughout their lifetime as evolved stars. The typical treatment of evolved stars has been to only account for present day mass loss or assume they are quasi-stable undergoing constant mass loss throughout their lifetime. Historic mass loss can be effectively traced using thermal emission from the dust which cools down as it moves away from the central star. We exploit Herschel/PACS far-infrared (\(70~\mu m, 100~\mu m \ and \ 160~\mu m\)) and JCMT/SCUBA-2 sub-millimetre (\(450~\mu m \ and \ 850~\mu m\)) observations to study this historic mass loss. These two sets of observations are especially powerful, tracing the radial variation in the circumstellar envelopes of evolved stars. We establish the presence of variations in mass loss for a sample of Milky Way evolved stars by studying features observed in surface-brightness, temperature, dust mass-column density and spectral index of dust emissivity radial profiles. By comparing these results to predictions for a constant-outflow model we find significant deviations between the total dust masses determined for the two scenarios, demonstrating that the effect of mass-loss variations cannot be ignored. We determine the complexities of the detached shell of U Ant with the aid of radiative transfer modelling, prompting the need for further study at higher angular resolution. The analysis of the sub-mm periodicity of IRC+10216 and o Ceti, two well-known evolved stars, shows that it may be possible to use sub-mm light curves of evolved stars to study the nature of dust formation/destruction in their inner envelope effectively. The optical and sub-mm light curves of IRC+10216 shows an offset in phase of ~ 3/4. Using radiative transfer modelling as well as findings in literature we attribute the sub-mm variability and phase lag to both the relationship between stellar pulse and dust formation/destruction cycle as well as a second unknown mechanism which needs to be narrowed down in the future. Moving on from the Milky Way to the Magellanic clouds we combine cutouts of evolved stars identified in the mid-infrared but not detected in the far-IR. The cutouts are generated from the Herschel HERITAGE survey at \(100 \ \mu m\) allowing a statistical study of the existence or absence of a far-IR component as a result of cold dust in these evolved stars. We successfully produce detections for three stacks comprised of the highest mass-loss rate sources, showcasing the merits of the stacking method. With this thesis we emphasise the variations in structure and hence historic mass loss of evolved stars. By doing so we highlight the importance of including these variations in studies which include dust production by evolved stars in the future.