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Astron. Astrophys. 334, 159-168 (1998)


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The fate of a neutron star just below the minimum mass:
does it explode?

K. Sumiyoshi 1, 2, S. Yamada 1, 3, H. Suzuki 1, 4 and W. Hillebrandt 1

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzshild-Strasse 1, D-85740 Garching, Germany
2 The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-01, Japan
3 Department of Physics, School of Science, The University of Tokyo, Hongo, Bunkyo, Tokyo 113, Japan
4 High Energy Accelerator Research Organization (KEK), Oho, Tsukuba, Ibaraki 305, Japan

Received 17 July 1997 / Accepted 9 February 1998

Abstract

First results of numerical simulations are presented which compute the dynamical evolution of a neutron star with a mass slightly below the minimum stable mass by means of a new implicit (general relativistic) hydrodynamic code. We show that such a star first undergoes a phase of quasi-static expansion, caused by slow nuclear [FORMULA] -decays, lasting for about 20 seconds, but then explodes violently. The kinetic energy of the explosion is around [FORMULA] erg, the peak luminosity in electron anti-neutrinos is of order [FORMULA] erg/s, and the thermodynamic conditions of the expanding matter are favorable for r-process nucleosynthesis. These results are obtained for the Harrison-Wheeler equation of state and a simple and, possibly, unrealistic treatment of [FORMULA] -decay rates and nuclear fission, which were adopted for comparison with previous works. However, we do not expect that the outcome will change qualitatively if more recent nuclear input physics is used.

Although our study does not rely on a specific scenario of how a neutron star starting from a bigger (and stable) mass can reach the dynamical phase, we implicitly assume that the final mass-loss event happens on a very short time scale, i.e., on a time scale shorter than a sound-crossing time, by removing a certain amount of mass as an initial perturbation. This assumption implies that the star has no time to adjust its nuclear composition to the new mass through a sequence of quasi-equilibria. In the latter case, however, there exists no stable configuration below the minimum mass, because the equation of state of fully catalyzed matter is too soft. Therefore, the dynamics of the explosion will not be too different from what we have obtained if different initial perturbations are assumed.

Key words: stars: neutron – equation of state – hydrodynamics – instabilities – nuclear reactions, nucleosynthesis, abundances

Send offprint requests to: K. Sumiyoshi

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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