نبذة مختصرة : 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 ...
Processing Request
No Comments.