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THE EVOLUTIONARY STATUS OF RCBS

Nuclear processes within stars first convert hydrogen to helium, then helium to carbon, and eventually to heavier elements. RCB surfaces consist primarily of a mixture of helium and carbon-rich layers, with a trace of hydrogen. Single-star evolution does not normally succeed in mixing such different layers of a star, so special models have been proposed to explain their origin.

In one model, a low-mass star which has finished its evolution as a red giant contracts to become a white dwarf, passing through a phase when it illuminates a planetary nebula. It may happen that sufficient unprocessed helium remains on the surface of the white dwarf that nuclear reactions can be reignited. The star then expands suddenly to become a helium-burning red giant for a second time. Convection will thoroughly mix the outer layers of this star to give the mixture of helium and carbon seen in RCBs. This model is sometimes known as the Final Flash (FF) or last thermal pulse (LTP) model ([Iben et al. 1983]).

In another model, it is supposed that two white dwarfs are in orbit around one another. Over a long timescale ( $\sim10^{10}$ years), either gravitational radiation or magnetic braking will make the orbit decay and the stars will spiral in towards one another. If one star is a helium white dwarf (HeWD), and the other a carbon/oxygen white dwarf (COWD), the HeWD will be cannibalized by the more massive COWD. This helium will be capable of initiating new nuclear reactions and, like the previous model, the star will expand to become a helium-burning giant, with a helium- and carbon-rich surface. This model is sometimes known as the Double Degenerate (DD) or Merged Binary White Dwarf (MBWD) model ([Webbink 1984], [Iben & Tutukov 1984]).

Figure 3: Internal structure of an RCB star

It is difficult to resolve which model is correct, if either, because RCBs show a range of surface compositions, and the crucial carbon abundance is not well known. However it is agreed that RCBs probably have a degenerate carbon-oxygen core containing upwards of 90% of the mass of the star. Their energy comes from a nuclear-burning shell at the bottom of a tenuous helium-carbon envelope (Fig. 3). Whilst the core has a radius of approximately $0.01 {\rm R_{\odot}}$, the star has an overall radius of some $50 {\rm R_{\odot}}$.