Stock, D.J.; (2011) The cosmic origins of carbon and the evolution of dust, gas and the CNO elements in galaxies. Doctoral thesis , UCL (University College London).

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Abstract

Carbon, along with nitrogen and oxygen, is produced by stars of differing mass and metallicity throughout the evolutionary history of galaxies. The production of oxygen and nitrogen is believed to be dominated by stars of high and low mass respectively, while the origin of carbon is less settled, as it can be produced by both low and high mass stars. An observational approach to determining whether low or high mass stars dominate carbon production is desirable, via studies of the nebulae that such stars produce during their advanced evolutionary stages. However, ionized carbon does not have forbidden emission lines in the optical range, making optical carbon abundance measurements reliant on the use of carbon recombination lines or neutral carbon forbidden lines. Carbonaceous dust is inferred to exist in many nebulae, though the amount of carbon in such dust can be difficult to determine. This thesis presents observations and numerical modelling results aimed at tracing the origins of carbon in galaxies. The contribution of individual stars is probed, focusing first on nebulae around massive Wolf-Rayet (WR) stars, particularly those with C-rich WC stars. The properties of the population of Galactic and LMC circumstellar nebulae around WR stars are examined, followed by a spectroscopic investigation of abundances in nebulae around both WN and WC stars. Carbon production rates by low and intermediate mass stars are inferred from published carbon abundance measurements for planetary nebulae. The second approach used to trace the origin and evolution of carbon is through numerical modelling of the chemical histories of galaxies. Using various formulations for the inputs of C, N and O by low and high mass stars, models are constructed which trace the overall abundances of these elements over the history of a galaxy, from their birth to the present day. By tuning the input data for stellar elemental yields to best match observed abundance patterns, the mass and metallicity ranges which are responsible for creating carbon can potentially be diagnosed. Finally, these models are adapted to investigate the evolution of the dust content of galaxies, including galaxies at high redshifts.

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