title: Novae, the Super-Remnant Phenomenon, and the Link to Type Ia Supernovae
Creators: name: Healy, M
Publisher: name: Liverpool John Moores University
Year: 2021

description: A classical nova describes the sudden brightening of a star in the night sky, before it fades away to quiescence. As systems exhibiting nova outbursts are binary, this steep increase in luminosity is attributed to a thermonuclear runaway on the surface of a white dwarf (WD), resulting in the violent ejection of material previously accreted from a non-degenerate companion. After the system has experienced a nova outburst, both the WD and companion remain intact, allowing for further accretion and subsequent eruptions. Though all nova systems can theoretically experience repeated outbursts, only those systems that have been observed to erupt more than once are known as recurrent novae (RNe). Recent theoretical studies have demonstrated that the WD in a RN system can grow in mass, due to retaining a proportion of the ejected material, up to the Chandrasekhar limit. Consequently, some novae are considered to be single-degenerate progenitors of type Ia supernovae (SNIa). With novae being progenitor candidates of SNIa, it is vital to understand how the underlying system is affected by its environment. Galactic, Magellanic Cloud and M31 novae are routinely studied, yet those in other Local Group galaxies are not. We address this by conducting a multiwavelength observational campaign of a classical nova observed in the Local Group irregular dwarf galaxy, NGC 6822. This consisted of determining the photometric evolution of the system; identifying lines in the spectra at various epochs, including the nebular phase, and their associated velocities; attempting to detect X-ray emission and searching for a progenitor system within archival Hubble Space Telescope images. Even though displaying many characteristics seen in regular classical novae, AT 2017fvz did reveal tentative evidence for belonging to the 'faint and fast' group of novae, and possibly as being a recurrent nova. Currently, the system with the shortest recurrence period known is M31N 2008-12a. A WD close to the Chandrasekhar limit and a very high accretion rate combine to ensure an outburst once every year. As well as being the most rapidly recurring nova we know of, 12a is surrounded by a vast unique nova super-remnant (NSR), at least two orders of magnitude larger than any other nova shell. We show, through Strömgren sphere analysis, that this enormous structure could not be formed solely through photoionisation from the RN system, giving further credence to the dynamical formation of the structure through many previous outbursts. Similar to M31N 2008-12a, any recurrent nova system with a massive WD and high accretion rate should also be surrounded by a vast dynamic remnant. Even so, the characteristics of the underlying nova system will influence its scale and structure. To test this, we employ a large suite of hydrodynamical simulations to model the growth of many NSRs. Each simulation incorporates a growing WD, and has various combinations of mass accretion rate, local interstellar medium (ISM) density, initial WD mass and WD temperature in order to capture parameter dependencies. These models reveal that the higher the ISM density and accretion rate, the smaller the resulting NSR, whereas the temperature and initial mass of the WD have little impact on the final structure. If the NSR around M31N 2008-12a is actually commonplace, then finding more of these phenomena would present a vital new method for not only locating previously unknown recurrent nova systems, but also for finding once active systems (before depleting their donor) as well as the sites for upcoming SNIa. With this motive in mind, we use the suite of simulations employed to model NSRs in order to predict the radial sizes and emission characteristics of other remnants, utilised in the first ever targeted search for these vast structures around a number of Galactic RNe. Finally, we look at two post-nova evolutionary scenarios, one involving the complete depletion of the donor and the other incorporating a SNIa, to determine observational signatures of these events. We find that the NSR would change very little from the fully formed version after the companion ceases donating material, whereas the SNIa ejecta colliding with the already formed remnant would trigger extreme X-ray emission a few thousand years after the explosion takes place; likely detectable with current X-ray missions.