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Astron. Astrophys. 342, 192-200 (1999)

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Direct Urca process in strong magnetic fields and neutron star cooling

D.A. Baiko and D.G. Yakovlev

A.F. Ioffe Physical Technical Institute, Politekhnicheskaya 26, 194021 St.Petersburg, Russia

Received 14 August 1998 / Accepted 3 November 1998


The effect of the magnetic field on the energy loss rate in the direct Urca reactions is studied. The general expression for the neutrino emissivity at arbitrary magnetic field B is derived. The main emphasis is laid on a case, in which the field is not superstrong, and charged reacting particles (e and p) populate many Landau levels. The magnetic field keeps the process operative if [FORMULA] ([FORMULA] is the number of the Landau levels populated by protons and [FORMULA]), that is beyond the well-known switch-on limit in the absence of the field, [FORMULA]. Cooling of magnetized neutron stars with strong neutron superfluid in the outer cores and nonsuperfluid inner cores is simulated. The magnetic field near the stellar center speeds up the cooling if the stellar mass M is slightly less than the minimum mass [FORMULA], at which the direct Urca reaction becomes allowed for [FORMULA]. If [FORMULA] G, the affected mass range is [FORMULA], while for [FORMULA] G the range is [FORMULA]. This may influence a theoretical interpretation of the observed thermal radiation as illustrated for the Geminga pulsar. The case of superstrong magnetic fields ([FORMULA] G), such that e and p populate only the lowest Landau levels is briefly outlined.

Key words: nuclear reactions, nucleosynthesis, abundances – stars: neutron – pulsars: individual: Geminga = 1E 0630+1

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998