The decompression of the outer neutron star crust and r-process nucleosynthesis

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050, Brussels, Belgium
e-mail: sgoriely@astro.ulb.ac.be
² Max-Planck-Institut für Astrophysik, Postfach 1317, 85741 Garching, Germany
³ Département de Physique, Université de Montréal, Montréal (Québec), H3C 3J7, Canada

Received: 16 March 2011
Accepted: 6 May 2011

Abstract

Context. The rapid neutron-capture process, or r-process, is known to be fundamental for explaining the origin of approximately half of the A > 60 stable nuclei observed in nature. In recent years nuclear astrophysicists have developed more and more sophisticated r-process models, by adding new astrophysical or nuclear physics ingredients to explain the solar system composition in a satisfactory way. Despite these efforts, the astrophysical site of the r-process remains unidentified.

Aims. The composition of the neutron star outer crust material is investigated after the decompression that follows its possible ejection.

Methods. The composition of the outer crust of a neutron star is estimated before and after decompression. Two different possible initial conditions are considered, namely an idealized crust composed of cold catalyzed matter and a crust initially in nuclear statistical equilibrium at temperatures around 10¹⁰ K.

Results. We show that in this second case before decompression and at temperatures typically corresponding to 8 × 10⁹ K, the Coulomb effect owing to the high densities in the crust leads to an overall composition of the outer crust in neutron-rich nuclei with a mass distribution close to the solar system r-abundance distribution. Such distributions differ, however, from the solar one due to a systematic shift in the second peak to lower values. After decompression, the capture of the few neutrons per seed nucleus available in the hot outer crust leads to a final distribution of stable neutron-rich nuclei with a mass distribution of 80 ≤ A ≤ 140 nuclei in excellent agreement with the solar distribution, provided the outer crust is initially at temperatures around 8 × 10⁹ K and all layers of the outer crust are ejected.

Conclusions. The decompression of the neutron star matter from the outer crust provides suitable conditions for a robust r-processing of the light species, i.e., r-nuclei with A ≤ 140. The final composition should carry the imprint of the temperature at which the nuclear statistical equilibrium is frozen prior to the ejection.