• Accretion
• Astrophysics
• binaries: close
• Cataclysmic Variables
• Hibernation
• Novae
• Nucleosynthesis
• Stars
• Stellar Physics
• T Pyx

A nova is a stellar system whose luminosity suddenly increases and it is thought to consist of a white dwarf which is accreting mass from a close companion. The eruption of the system is due to thermonuclear reactions at the white dwarf surface, from which an envelope of material is ejected in a ballistic way during the process.

With six recorded eruptions, the latest of which occurred in 2011, T Pyxidis (abbrev. T Pyx) is the most studied Galactic recurrent nova, i.e. a nova with more than one documented outburst. Its visual lightcurve is characterized by a decline timescale of about two months, which is the second longest duration among the ten known Galactic recurrent novae.

The visual light emitted from a nova at the beginning of the eruption consists of the radiation from the ejected envelope. As the envelope expands, its density drops, in particular in the outer layers, and recombination processes become less efficient. The increasing ionization of the outer layers implies that the photosphere moves closer to the white dwarf and therefore the radiation temperature rises, so that the maximum in the emission gradually moves away from the visual part of the spectrum. This process is faster in novae with a low-mass envelope, which is more easily ionized by the incoming radiation and recombines less efficiently. The timescale for the decline of the visual emission is therefore related to the mass of the envelope.

Several independent indications suggest that the mass of the envelope of T Pyx is low. They include a prolonged supersoft X-rays emission, which implies a long duration of the nuclear burning phase and therefore that a significant fraction of the accreted material wasn't ejected; an estimate of the mass of the material accreted between two consecutive outbursts; a direct spectroscopic determination based on ratios of emission line fluxes sensitive to the electron temperature and density in the ejecta. The estimated mass of the ejecta suggests that the decline timescale of T Pyx should be of the same order of most other recurrent novae, i.e. an order magnitude lower than the actually recorded one.

In this thesis, an original explanation for this inconsistency is proposed with the inclusion of a new effect: I suggest that a fraction of the mass transferred from the low mass companion of the white dwarf might surround spherically the white dwarf itself and absorb the radiation before it reaches the ejecta, preventing them to be directly ionized by the photons from the white dwarf. The efficacy of this shielding effect depends steeply on the density of the material surrounding the white dwarf, which is shown to be higher for systems with a high mass transfer rate from the companion to the white dwarf and a short orbital period. T Pyx presents both of these attributes.

Like T Pyx, the recurrent systems IM Nor and CI Aql have short orbital periods and decline timescales significantly longer than all the other 7 recurrent novae, and might experience this effect. The results of the model suggest that if a significant fraction of the material transferred from the companion doesn't settle in the orbital plane, the radiation from the white dwarf is reprocessed before reaching the ejecta in T Pyx, IM Nor and CI Aql, while it isn't in the other recurrent novae.

The mass transfer rate in the first few days of the outburst is a parameter of the aforementioned model. During the eruption, the companion of the white dwarf is irradiated by an amount of energy per unit time which is at least two orders of magnitude higher than the luminosity of the companion itself; it has been suggested in the literature that the mass transfer rate from the companion may be significantly enhanced because of this irradiation. Since the mass transfer rate cannot be directly measured during the first days of the eruption, it can only be assessed by theoretical means, and the currently accepted model does support a mass transfer enhancement of up to a factor of 10^2.

The discussion of the mass transfer enhancement in the literature was always focussed at evolutionary timescales, which are much longer than those needed for the decline of the lightcurve and the ionization of the ejecta. Moreover, the currently accepted model doesn't include the gravitational force of the white dwarf acting onto the companion nor a correct assessment of the energy dissipation from the outer layers of the companion itself. I discuss the applicability of the currently accepted mass transfer enhancement scenario to the case of T Pyx and discuss an original, exploratory model which takes into account these effects and suggests that the irradiation from the white dwarf doesn't enhance the mass transfer rate in a significant way. The mass transfer rate at quiescence is therefore used to study the shielding effect.