EDP Sciences
Free access
Volume 416, Number 3, March IV 2004
Page(s) 1013 - 1022
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20034387

A&A 416, 1013-1022 (2004)
DOI: 10.1051/0004-6361:20034387

Hyperon softening of the EOS of dense matter and the spin evolution of isolated neutron stars

J. L. Zdunik1, P. Haensel1, 2, E. Gourgoulhon2 and M. Bejger1

1  N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
    e-mail: bejger@camk.edu.pl
2  LUTH, UMR 8102 du CNRS, Observatoire de Paris, 92195 Meudon Cedex, France
    e-mail: haensel@camk.edu.pl;Eric.Gourgoulhon@obspm.fr

(Received 24 September 2003 / Accepted 12 November 2003)

The effect of the hyperon softening of the equation of state (EOS) of dense matter on the spin evolution of isolated neutron stars is studied for a broad set of hyperonic EOSs. We use a multidomain 2-D code based on a spectral method, and show how important the precision of solving the equations of stationary motion is for the stability analysis. For some EOSs, the hyperon softening leads to spin-up by angular momentum loss, in the form of the back-bending phenomenon, for a rather broad range of stellar baryon mass. We show that large segments of the evolutionary tracks exhibiting the back-bending behaviour in the moment-of-inertia - rotation-frequency plane are unstable and therefore not astrophysically relevant. We show also that during the spin-up - angular-momentum-loss epoch, an isolated neutron star (e.g. a radio pulsar) can lose a sizable part of its initial angular momentum without significantly changing its rotation period. We propose also simple arguments and criteria allowing one to connect the presence of a back-bending epoch with the mass-radius relations and the stiffness and/or softness of the nucleon and hyperon EOSs of the neutron star core.

Key words: dense matter -- equation of state -- stars: neutron -- stars: rotation

Offprint request: J. L. Zdunik, jlz@camk.edu.pl

© ESO 2004