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Title: Extragalactic novae and their progenitors
Author: Williams, S. C.
ISNI:       0000 0004 5992 9359
Awarding Body: Liverpool John Moores University
Current Institution: Liverpool John Moores University
Date of Award: 2014
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Novae are binary systems containing a white dwarf (WD) and a less-evolved companion star, either a main-sequence, sub-giant or red giant star. The WD accretes matter from the companion through Roche lobe overflow or via a stellar wind. As material is accreted, the pressure and temperature at the base of the accreted envelope increase until a thermonuclear runaway occurs. This causes a sudden increase in brightness (the outburst), which ranks among the most luminous stellar astrophysical phenomena. Following the outburst, some novae form detectable dust in the ejecta. Observationally, there is a correlation between the dust-formation timescale and the time it takes the nova to fade optically by two magnitudes, which was emphasised in a study of infrared emission from novae in the Andromeda Galaxy (M31). In the first part of this thesis, a simple theoretical model is presented, which considers the higher-energy photons produced by the nova being absorbed by neutral hydrogen in the ejecta, before they can reach the potential dust-formation sites. This new model successfully replicates the observed trend between these two parameters and agrees well with the observational data. The majority of novae are thought to consist of a WD and a main-sequence star, although some systems harbour a sub-giant (SG-novae) or red giant (RG-novae) companion instead. In the Milky Way galaxy, relatively few RG-novae have been confirmed, although in many systems, the evolutionary state of the secondary is simply not known. There is evidence that the progenitors of some Type Ia supernovae (SNe Ia) may be RG-nova systems (e.g. SN PTF11kx), therefore it is important to understand the population of such systems. In this thesis, archival Hubble Space Telescope (HST) data are used to search for RG-novae in M31. Many more novae are discovered in M31 each year (~30) than in the Milky Way (~10). Distance determination is a major complication when studying Galactic novae. However, at the distance of M31 all the novae may be considered to be at the same distance, making M31 an excellent environment for studying nova populations. We conducted a survey of 38 spectroscopically confirmed M31 novae in quiescence. We determined that 11 of these systems had a coincident progenitor candidate whose probability of being a chance alignment with a resolved source in the HST data was ≤5%. As the main sequence and the majority of the sub-giant branch are not resolvable in the HST data, this implies that a significant proportion of these systems contain red giant secondaries. The light curves of several M31 novae are also presented here, some of which use HST data to extend the light curves far deeper than is typically possible for extragalactic systems. A statistical study was then carried out to test the results of the survey and derive an estimate of the proportion of M31 novae associated with a resolved source in the HST data. This includes, for example, models of the spatial distribution, speed class and peak magnitude of the M31 nova population, as well as considering biases introduced by the HST coverage of M31. The initial results suggest about 0.38 of M31 novae are associated with a source in the HST data, a class of objects expected to be dominated by RG-novae. This is a much greater proportion than that observed so far in our Galaxy, and will be important when considering such systems as potential SN Ia candidates. The spatial distribution of novae that have resolved progenitor candidates is consistent with these systems being associated with the M31 disk, rather than the bulge. The method used to locate the progenitors of M31 novae was also used to study three additional systems. The M31 nova, M31N 2008-12a, which appears to be a recurrent nova (RN) with a very short inter-outburst period, produced an outburst in November 2013. This outburst was studied and a candidate progenitor system was found in HST data when it was apparently in quiescence, supporting its classification as a RN with a high accretion rate. The method was also used to explore upper limits on the brightness of the progenitor of SN 2014J, a SN Ia in M82, although no progenitor was found, a RG-nova (or in-fact any type of system) could not be ruled out due to the limitations of the data. For the M31 transient TCP J00403295+4034387, which showed an unusual spectrum, archival HST data were used to show the object was probably a blend of two objects with a very small apparent separation. Finally, the thesis is summarised, and future work on both dust formation and the progenitor search are discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy