Understanding The Recurrent Nova RS Ophiuchi
Recurrent novae are a rare sub-class of cataclysmic variable stars; white dwarfs accreting material from a binary companion in which more than one classical nova-type outburst has been observed (see Hellier 2001 for a comprehensive review of CVs). Nova outbursts are suspected to be due to a thermonuclear runaway on the surface of the white dwarf, which releases huge amounts of thermal energy once a critical pressure is reached at the base of the shell of accreted material.
Classical novae are expected to recur on timescales from 100,000 years to just a few decades. The most important physical parameters controlling this recurrence timescale are the white dwarf mass, and the mass accretion rate from the secondary (see e.g. Yaron et al. 2005). Once classical nova is recorded more than once, it can be designated as “recurrent”. Such a recurrent-nova is RS Ophiuchi (RS Oph).
RS Oph is an amazingly prolific recurrent nova, with recorded outbursts in 1898, 1907, 1933, 1945, 1958, 1967, 1985 and 2006 (Schaefer 2010). The short time between outbursts (∼20 yrs) suggests that RS Oph hosts a massive white dwarf accreting material at a significantly high rate, providing a wealth of interest from astronomers worldwide on this process.
During an outburst in 2006, Nelson et al. (2008) presented high resolution X-ray spectra obtained during the supersoft phase (the phase where the still burning hydrogen shell is directly observable), and found a very high effective temperature for the source (~800,000K), consistent with shell burning on a white dwarf of at least 1.2 (see e.g. Paczynski 1971; Nomoto et al. 2007; Osborne et al. 2011). Furthermore, Sokoloski et al. (2006) estimated the mass of the ejected envelope based on the hard X-ray light curve of the outburst, and found that no more than a few 10-7 could have been ejected, a result consistent with previous work on accretion rates in novae by Yaron et al. (2005).
Nevertheless, there is less evidence of the high accretion rate required to power an outburst every ∼20 yrs. Cataclysmic variables are well known X-ray sources, and are understood to be powered by accretion. For a slowly rotating white dwarf, approximately half of the available accretion luminosity is emitted in the accretion disk boundary layer at X-ray wavelengths (see e.g. Shakura & Sunyaev 1973; Lynden-Bell & Pringle 1974). However, although the presence of a disk in RS Oph seems highly probable, the mechanism by which material is fed into it still remains unknown.
Current understanding suggests that most giant symbiotic stars do not appear to fill their Roche Lobes, although a number of systems show near-infrared ellipsoidal variations, suggesting that they are at least partially tidally distorted (Mikolajewska 2007). If Roche lobe overflow (RLO) is not occurring, it is normally assumed that material is fed to the white dwarf through Bondi-Hoyle accretion of the red giant wind (Edgar 2001). Since this mode of accretion is much less efficient than RLO, it is unclear that it can provide enough mass to produce an outburst in RS Oph every 20 years.
However, no ellipsoidal variations have been observed in RS Oph, which is generally assumed to indicate that it does not fill its Roche lobe. If this is the case, then the accretion disk must be fed by some mechanism other than RLO. Work by Podsiadlowski & Mohamed (2007) explores one such alternative. They presented a scenario dubbed “wind Roche lobe overflow, where the stellar wind of the giant fills the Roche lobe, where the wind is gravitationally focused towards the L1 (primary Lagrange point) point, where it is then captured by the potential well of the white dwarf. At this point, the material can form an accretion disk, as in RLO systems. This mode of mass transfer can be up to 100 times more efficient than Bondi-Hoyle accretion.
Perhaps if such mass transfer is occurring in RS Oph, it can feed a disk, and the white dwarf, at a sufficiently high rate to power the observed outburst frequency without the red giant filling its Roche lobe?
- Mikolajewska, J. (2006) The Place Of Recurrent Novae Amont Symbiotic Stars. ASP Conference Series, 401 pp.42.
- Voloshina, I. et al. (2008) Study of the Recurrent Nova RS Ophiuchi. Romanian Astronomical Journal, Vol. 18, Supplement, p. 101-111.
- King, A. R.; Pringle, J. E. (2009) RS Ophiuchi: Thermonuclear Explosion Or Disc Instability? Monthly Notices Royal Astronomical Society: Letters, 397 (1) pp. L51-L54.
- Schaefer, B.E. (2009) Orbital Periods Of Three Recurrent Novae. The Astrophysical Journal, 697 (1) pp. 721-729.
Suggested Further Reading:
- Hellier, C. (2001) Cataclysmic Variable Stars: How & Why They Vary. Springer-Praxis Publishing, London.