Recurrent Novae: Are They Related To Type Ia Supernovae?

One of the most dramatic events of a cataclysmic star's lifetime is the nova eruption (Hellier 2001). The believed cause of such outbursts is a nuclear chain-reaction on the surface of a white dwarf star. Within the nova class of outbursts exists the sub-class of recurrent novae. As the name suggests, such outbursts occur when a system undergoes multiple novae eruptions (the term classical nova is applied to novae which have not been seen to repeat). Are recurrent novae and classical novae just two of the same thing, differing only in recurrence timescale? And are such events linked to Type Ia supernovae outbursts?

To answer the first question: some of them probably are. For example, the system T Pyx has a recurrence time which drops to < 100 years. The T Pyx system has a typical mass of around 1.4M* and and typical accretion rate of ≤10^-8M* yr^-1 (O'Brien 1993; Prialnik & Kovetz 1995; Hellier 2001) Hence, rates of accretion higher than this are suspected to recurrent novae.

However, most other recurrent novae are not typical cataclysmic variables but have very long orbital periods of days to hundreds of days with a companion star which is evolving to or is a giant star (Hellier 2001). An example of such a system is RS Oph, a 230 day binary with a red-giant secondary (Webbink et al. 1987; Bode et al. 2006). The nature of the outburst in such systems remains unclear. Although many of such recorded recurrent events could be thermonuclear runaway on the white dwarf surface, many could be partly due to disc instabilities or other accretion events (e.g. Roche-lobe overflow).

However, as well as classical and recurrent nova, there is one other causality linked eruption event: Type Ia supernovae. Type Ia supernovae (SNe Ia) are one of the most luminous explosive events of stars. The last event of this type recorded in our Galaxy was 400 years ago. Such events are the total disintegration of entire white dwarfs stars, pushed over the Chandrasekhar limit. Does the accretion of material onto the white dwarf star in a cataclysmic variable system ultimately lead to the fabled Type Ia supernovae explosion?

The answer lies in analysing four cataclysmic variable systems, or rather, four recurrent novae which have a red giant companion: T Coronae Borealis (TCrB), RS Ophiuchi (RS Oph), V745 Scorpius and V3890 Sagittarius. Research conducted by Hachisu & Kato (2001) has concluded that their analysis of the four recurrent novae mentioned indicate that the white dwarfs in these particular recurrent novae have now grown up to near the Chandrasekhar mass limit and will soon explode as a Type Ia supernova if the white dwarf consists of carbon and oxygen. Hence, certain sub-classes of recurrent novae are now being strongly considered to be the progenitor systems of Type Ia supernova explosions.