Radio observations as a tool to study shock interactions and mass ejections in novae

This thesis presents radio frequency studies of three novae, namely V445 Puppis, V3890 Sagitarii and V339 Delphini. The new data, in this thesis, represent some of the most detailed and comprehensive radio light curves to date. In these systems a thermonuclear eruption occurs on the surface a white dwarf following extensive periods of accretion of material from a companion star. The result of the thermonuclear eruption is the explosive ejection of the outer layers of the accumulated material. Since the ejected material emits at radio wavelengths at some point in its evolution, radio data obtained for the three objects are utilised to test the hypothesis of radio emission models. First, radio data of V445 Pup are presented. V445 Pup is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km s°1, and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multi-frequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5°43.3 GHz, to 2001 January and 2008 March (ª 89 ° 2700 days after eruption). The radio light curve is dominated by synchrotron emission over these seven years, and shows four distinct radio flares. Resolved radio images obtained in the VLA's A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons to near-IR images of the nova clearly demonstrate that it is the densest ejecta—not the fastest ejecta—that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae, but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae. Second, the radio observations following the 2019 August eruption of V3890 Sgr obtained with the MeerKAT radio telescope at 1.28 GHz are presented. The radio light curve spans from day 1 to 300 after discovery and is dominated by synchrotron emission produced as the expanding ejected nova envelope interacts with the dense wind from an evolved companion in the binary system. The radio emission is detected early on day 6 and increases rapidly with the radio flux peaking after day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock losses energy and decelerates. Modelling of the light curve provides an estimated mass-loss rate of M ˙ wind º 10°8 MØ yr°1 for the red giant wind. V3890 Sgr hosts a massive white dwarf, shows presence of structured circumbinary material consistent with the surrounding environment present in some supernovae type Ia (SNe Ia) and therefore it is possible to be a SNe Ia progenitor. Finally, radio observations of V339 Del obtained using the Karl G. Jansky Very Large Array and the Arcminute Microkelvin Imager Large Array (AMI) following the 2013 eruption are presented. The AMI data were obtained every two days resulting in the most detailed radio light curve of a classical nova known to date. The radio data are used to test the hypothesis that the observations were due to a bipolar shaped ejecta as suggested from emission line modelling at optical wavelength. Here, the morphology is utilised in predicting the ejected mass of V339 Del during the eruption. The radio light curve is modelled in the morpho-kinematical program SHAPE using a linearly expanding ionized ejecta, assuming that the nova emission is dominated by thermal free-free emission. Considering a bipolar geometry for the nova ejecta, the radio light curve fluxes can be replicated using an ejected mass of Mej º 0.1°8£10°5 MØ. The mass estimate of the ejecta compares well with estimates of emission line modelling in optical spectroscopy and also ∞-ray emission modelling. High resolution radio images of V339 Del obtained with the VLA's A configuration initially show a spherical structure when the ejecta is optically thick and later a ring structure when the ejecta is optically thin. The change in optical depth and apparent morphology is a further indication of a non-spherical geometry. V339 Del shows evidence of shock powered emission in the first 100 days after the nova eruption. This thesis emphasizes the role of radio data towards a better understanding of the effects of thermonuclear eruptions in novae. This work highlights the importance of obtaining well sampled radio light curves of novae which give more insight into the physical processes that follow nova explosions, which may have been overlooked before due to sparsely sampled radio light curves. Resolved radio images of novae are also crucial in determining the morphology of the ejecta hence provide clues on the approach to modelling radio emission from novae. This is particularly relevant given the new generation of sensitive radio telescope arrays such as MeerKAT, the next generation Very Large Array, and ultimately the Square Kilometre Array