3.3 Electron-capture supernovae and core-collapse supernovae below 10 – 12 solar masses: Low mass stars

Stars below about 10 –12M ⊙ ([286Jump To The Next Citation Point, 287Jump To The Next Citation Point, 241Jump To The Next Citation Point], Poelarends et al., in preparation) have very diffuse envelopes surrounding their cores when they collapse. The lowest mass stars in this class lie on the border between white dwarf (and planetary nebula) formation and the formation of an iron core (see Herwig [141] for a review). These stars tend to form ONe white dwarfs, ejecting most of their envelope. They are supported by electron degeneracy pressure and electron capture causes them to collapse down to neutron stars. Their resultant explosions are termed Electron Capture Supernovae [152Jump To The Next Citation Point]. The more massive stars in this class do form iron cores, but the envelopes surrounding these core have low densities (when compared to normal supernovae progenitors) at the time of collapse.

3.3.1 Rate of supernovae from low-mass stars

The rate of core collapse from low mass stars has the same uncertainties of star formation rate and initial mass function of stars that made it difficult to estimate the normal supernova rate. But it also has an additional uncertainty: understanding the lower limit at which these collapses occur [197, 286, 287, 241Jump To The Next Citation Point]. This lower limit marks the dividing line between white dwarf and neutron star formation and the explosive ejection of the star’s envelope (part of the white dwarf formation process) has proven very difficult for traditional stellar evolution codes to model. The actual position of this lower limit has been a matter of debate for over three decades (see Iben & Renzini [152] for a review). The metallicity dependence on this limit is even more controversial. Heger et al. [136Jump To The Next Citation Point] argued that the metallicity dependence was negligible and that the lower limit was roughly at 9M ⊙ at all metallicities. Poelarends et al. [241] have studied this matter in more detail and have a range of solutions for the metallicity dependence. For neutron stars forming through electron capture supernovae, their “preferred” model predicts that the limit drops from 9M ⊙ at solar metallicity down to 6.3M ⊙ at zmetal = 10− 3z⊙. This means that for most initial mass functions, these low mass core-collapse progenitors dominate core-collapse at low metallicities.

Unfortunately, these low-mass supernovae do not produce very much radioactive Ni, the power source for supernova light-curves. Although their envelopes are likely to be denser than the material surrounding AICs, the tenuous envelopes certainly affect the explosion and may limit the shock-powered light-curve. But surveys are now catching these supernovae [288]. These surveys will ultimately place reasonable estimates on the rate of these supernovae. Based on current estimates of the mass limits and the initial mass function in this mass range, we expect the rate of these supernovae to be roughly equal to that of the core-collapse supernova rate.

3.3.2 Low-mass star collapse: Evolution

The structure of the core of low-mass stars is very similar to the structure of AIC cores, and a large set of simulations of both have been conducted in the past few decades [142Jump To The Next Citation Point, 11, 334, 102Jump To The Next Citation Point, 63Jump To The Next Citation Point, 61Jump To The Next Citation Point]. The core collapses and bounces, but the envelope is not massive enough to prevent a quick revival. A convective region is unlikely to develop above the proto neutron star. But these stars are probably not rotating as fast as most AICs, and their subsequent GW emission is likely to be weaker (e.g., smaller chance of bar-mode instabilities, etc.).

3.3.3 Low-mass star collapse: Gravitational wave emission mechanisms

The GW emission mechanisms at bounce and in the neutron star are the same as AICs but the likelihood of the development of instabilities (and the strength of the GW emission from these instabilities) will be affected by slower rotation.

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