RS Ophiuchi: Thermonuclear Explosion or Disc Instability?

📝 Abstract

Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophiuchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal–viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate.

💡 Analysis

Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophiuchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal–viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate.

📄 Content

arXiv:0905.0637v1 [astro-ph.EP] 5 May 2009 Mon. Not. R. Astron. Soc. 000, 1–?? (2005) Printed 2 November 2018 (MN LATEX style file v2.2) RS Ophiuchi: Thermonuclear Explosion or Disc Instability? A.R. King1 & J.E. Pringle1,2 1Theoretical Astrophysics Group, University of Leicester, Leicester LE1 7RH 2 Institute of Astronomy, Madingley Road, Cambridge, CB1 0HA 2 November 2018 ABSTRACT Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophi- uchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal– viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate. Key words: accretion, accretion discs – binaries: close – novae, cataclysmic variables – stars: dwarf novae 1 INTRODUCTION Sokoloski et al (2008) have presented evidence that within days of its 2006 outburst the recurrent nova RS Ophiuchi produced a pair of highly collimated (’half–opening angles of a just few degrees’) and high velocity (v ∼5000 km s−1) jets. Taylor et al. (1989) made similar deductions after the previous (1985) outburst. We argue here that this provides compelling evidence about the cause of the outburst in this object. The source of the energy driving the outbursts in re- current novae has been under discussion for some time (e.g. Livio, Truran & Webbink, 1986). There are two basic possi- bilities: either the outburst is caused by a nuclear explosion on the surface of a white dwarf, akin to a classical nova ex- plosion, but smaller in scale, or the outburst is powered by accretion energy released by a change in the accretion rate on to the central white dwarf, akin to dwarf nova outbursts but larger in scale, and similar in nature to the transient X– ray binaries (which differ in having a central neutron star or black hole as the accretor). It is, however, difficult to see how a nuclear–powered ex- plosion on the surface of an accreting white dwarf could give rise to such well–collimated jets. In general one would ex- pect nuclear burning in the hydrogen–rich surface shell to be quasi–spherically symmetric. The nuclear energy yield from the hydrogen–rich matter considerably exceeds its binding energy at the surface of a white dwarf. In a nuclear–powered nova, the mass of explosively burning material is compara- ble to that accreted since the last outburst. The mass in the central regions of any disc must be much smaller than this, and is even less gravitationally bound than the matter on the white dwarf surface. Thus the mass in such a disc does not have enough inertia to provide strong collimation of the flow. This point is underlined by the lack of any strongly collimated flows to be seen in ejecta from classical novae (Slavin et al., 1995; Gill & O’Brien, 1998). One only sees slight asymmetries in the shells of material expelled by the nova explosion, presumably caused by the presence of a close binary companion, and lumpiness in the shells (presumably caused by instabilities in the ejection process). In contrast, the jets seen in RS Oph are high velocity and strongly collimated. As such they are strongly reminis- cent of jets seen in other astronomical objects such as radio galaxies, active galactic nuclei (AGN), microquasars, some binary X–ray sources and young stellar objects. These jets all have the following in common (Pringle, 1993; Livio, 1999, 2000; Price, Pringle & King, 2003): the source of the jet has accretion occurring through a disc; the jet velocities are com- parable to, or slightly higher than, the escape velocity from the central accreting object; and about 10 per cent of the accreting material is being ejected in the jet. There is also a suggestion that the collimation of such jets require a large 2 King & Pringle ratio of inner to outer disc radii. It is immediately clear that the jets seen in RS Oph fall precisely into this category. We therefore argue here that we should abandon the view that the recurrent outbursts of RS Oph are nuclear– powered. We propose instead that

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