• stars-evolution of
• physical processes
• novae
• stars-individual ( t pyxidis ; v959 mon )
• stars-nucleosynthesis
• binary systems

Novae are violent explosions occurring on the surface of a white dwarf accreting material from a close companion, in binary stellar systems called cataclysmic variables.

Hydrogen-rich gas from the donor star accretes onto and mixes within the envelope of an electron degenerate star, a white dwarf (WD). With increasing pressure and temperature, a threshold can be reached at which proton capture on CNO nuclei ignites a thermonuclear reaction. This release of energy increases the temperature but, because of the degenerate environment, the pressure is not altered by expansion fro the overpressure. Eventually a thermonuclear runaway occurs, reaction rates go out of equilibrium, the envelope expands and the explosion throttles and stalls. Due to high temperature gradients, convective macro-scale motions mix the expanding envelope with the superficial layers of the WD. Expansion is finally driven by beta-unstable nuclides, mainly CNO isotopes.

The photometric and spectroscopic evolution of the outburst is determined by the physical conditions and dynamics of the ejected envelope. Initially completely ionized and opaque, the freely expanding gas is continuously irradiated by the central hot WD: the luminosity finally reaches the Eddington limit. Once recombination becomes efficient, line absorption from metals becomes the main opacity source. The envelope steadily rarefies, eventually turning transparent, showing broad (~10^3 km s^(-1)) emission lines profiles.

Although these explosions release several orders of magnitude less energy (mass) than supernovae, the physical processes taking place deserve full attention. The complete modeling and comprehension of the outburst is a difficult task given the relevant number of parameters involved (the detailed properties of the WD and its companion, the orbital parameters of the system etc), and computational limits on three-dimensional models. However, with panchromatic high-cadence observations it is possible to build a solid evolutionary model for the outburst.

Among the hundreds (and counting) known novae in the Milky Way, only 10 are known to be recurrent: these novae have displayed two or more outbursts in the last ~150 years. One of these systems, T Pyxidis, which sixth historical outburst occurred in April 2011, has been studied for this thesis. The analysis follows the evolutionary path of the nova from the early stage of the outburst to the late nebular phase (December 2012). The spectroscopic analysis suggests a bipolar geometry for the ejecta, a feature which seems omnipresent for classical and recurrent novae. The study of the interstellar medium along the line of sight has also been conducted.

The WD composition is determined by the evolutionary history of the star: naively, more massive stars develop oxygen-neon rich cores, instead of carbon-nitrogen-oxygen, hence defining two subclasses, ONe and CO novae. Nova Monocerotis 2012 (V959 Mon), a recent ONe nova, is discussed here. It is the first classical nova discovered as high-energy gamma-ray source before optical discovery. The comparison with past ONe novae strongly supports the hypothesis of a specific evolutionary sequence for these objects, perhaps also a new candidate class of cosmologically significant standard candles.

The analysis for these novae has been conducted with multi-wavelength observations (from gamma-rays to infrared) and archival data for comparisons with historical analogs.

The discussion concerns the hydrodynamic and evolutionary processes that are peculiar to this class of cataclysmic variables, their possible relation to supernovae Type Ia, and their importance as laboratories for processes encountered in other areas of astrophysics. The results of the analysis are discussed in the light of several aspects of the phenomenon currently under investigation, particularly radiative/dynamical processes taking place during the optically thick stage, and the maximum magnitude vs distance relation.