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12.3 r-process in the decompression of cold neutron star crusts

The location of the astrophysical site for the rapid neutron capture process (r-process), thought to be responsible for the production of many heavy neutron-rich nuclei with A > 60 in the universe, still remains uncertain (for a recent review, see, for instance, [23Jump To The Next Citation Point]). Many possible sites have been considered, but they all have serious problems. The most studied scenarios are related to neutrino-driven wind during type II supernova explosions or γ-ray bursts. Nevertheless, apart from many uncertainties in the explosion mechanism, the conditions for the r-process to occur are difficult to reach and require a fine tuning of model parameters. Lattimer et al. [253] suggested a long time ago that the r-process could also occur during the decompression of cold crustal matter ejected into the interstellar medium. This possibility has remained largely unexplored until very recently (see [170Jump To The Next Citation Point23Jump To The Next Citation Point] and references therein). This scenario is, however, promising because the presence of neutron-rich nuclei, the large neutron-to-seed ratio and the low electron fraction in the decompressing crustal matter are favorable conditions for the r-process to occur. Various scenarios can be envisioned. Matter could be ejected into the interstellar medium by outflows from newly-born proto-neutron stars or jets such as those recently observed in Circinus X-1 [194]. Neutron stars very rapidly spinning beyond the mass-shedding limit would also expel matter. In the early years of pulsar astronomy, Dyson [131] suggested that neutron stars might have volcanic activity. This idea of cataclysmic events has been more recently revived by the observations of giant flares in magnetars, thought to be the signature of magnetic crustquakes. From observations of the radio afterglow [156] it has been estimated that more than −9 10 M ⊙ was ejected during the December 27, 2004 event in SGR 1806–20. More exotic astrophysical events have been proposed, such as the explosion of a neutron star below the minimum mass [401] or the phase transition into a strange quark star (quark-novae) [218]. However the merging of a neutron star and a black hole or of two neutron stars (see [337] for a recent review on compact binaries) is probably the most likely scenario for the ejection of large amounts of matter. This tidal disruption of two merging neutron stars has recently been studied in detail [17023Jump To The Next Citation Point], motivated by the results of hydrodynamic simulations, which show that up to 10 −2M ⊙ could be ejected in this manner. This study has proven that the solar system abundance pattern can be qualitatively reproduced by considering the decompression of clumps of neutron-star–crust matter with different initial densities, as shown in Figure 73View Image.
View Image

Figure 73: Final composition of clumps of ejected neutron star crust with different initial densities (solid squares). The open circles correspond to the solar system abundance of r-elements. From [23].

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