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 in the universe, still
remains uncertain (for a recent review, see, for instance, [23]). 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 -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 [170, 23] 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 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 [170, 23], motivated by the results of hydrodynamic
simulations, which show that up to 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 73.
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].