Extreme Explosions by David S. Stevenson

Author:David S. Stevenson
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Limiting Factors

Before we investigate the mysterious process of electron-capture it’s worth examining what evolutionary force limits the growth of the neon core, preventing the headlong rush to iron-core collapse.

As previously mentioned, the precise fate of stars born with 9–9.25 solar masses has interested astronomers for decades. Modeling the lives of these stars is complex primarily because they really do teeter on a knife-edge. Lose too much mass and they become oxygen-neon-magnesium (or ONeMg) white dwarfs, but lose too little and they pursue the well-trodden path of more massive stars, ultimately exploding as Type II supernovae.

The problem is twofold. On the one hand, the force that whittles away at the envelope from the outside – mass loss – is as poorly understood as the process that whittles away at the envelope from the inside – nuclear reactions. Unless you know exactly which rates are which you can’t balance mass loss accurately against core growth. Clearly, without this, and to put it bluntly, you’re stuck.

The initial key to understanding this fate was modeling work done by Icko Iben Junior and colleagues (Enrique Garciá-Berro, Claudio Ritossa) in the late 1990s. Iben built on earlier work on these transitional mass stars carried out by Ken’ichi Nomoto. In 1984 Ken’ichi showed that stars in the mass range of 9–12 solar masses would develop a degenerate oxygen-neon-magnesium core following carbon burning and end their lives as oxygen-neon-magnesium-rich white dwarfs. Iben and colleagues then extended the analysis, examining a succession of models of stars with 8–11.5 solar masses. At the lower end of the mass range, carbon burning produced the oxygen-neon core, but it could never grow big enough to ignite neon and move forward to iron-core collapse. However, in Iben and colleagues’ models, stars at the top end of the range (11–11.5 solar masses) were able to grow the mass of the helium-depleted core to the point at which it could collapse through the exotic electron-capture mechanism.

Subsequent work by Arend Poelarends and Alex Heger has refined the calculations used in the earlier work and shown that the critical mass range for SAGB evolution is likely to extend from 7–9.25 solar masses, somewhat lower than Iben and colleagues initially proposed. The obvious difference in the mass range (and the one that is used here) reflects better understanding of the rates of nuclear reactions, mass loss and the inclusion of stellar rotation in the models analyzed.

In the 1990s Iben used non-rotating stars to cut down on the amount of computations needed to run the models. However, clearly, real stars rotate, and when rotation is taken into account the core mass can grow larger because of mixing of hydrogen from the envelope into the stellar core while the star is on the main sequence. Furthermore, improvements in the understanding of processes such as convection, mass loss and what are called dredge-outs (described in detail below) mean that more realistic models have now been derived.

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