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Astron. Astrophys. 338, 69-74 (1998)


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Evolution of black hole low-mass binaries

E. Ergma 1, 2 and A. Fedorova 3

1 Physics Department, Tartu University, Ülikooli 18, EE-2400 Tartu, Estonia (ene@physic.ut.ee)
2 Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 Institute of Astronomy RAN, Pyatnitskaya 48, 109017 Moscow, Russia

Received 18 July 1997 / Accepted 29 May 1998

Abstract

Several evolutionary sequences with low mass secondaries as donors and black holes as accretors are calculated. Adopting a simple estimate for the efficiency of magnetic braking we have determined the bifurcation period [FORMULA], separating converging from diverging binary systems, to be [FORMULA] 1 day. It is shown that in converging binary systems, similar to neutron star binaries with low-mass secondaries, evolution proceeds towards very short orbital periods with continous accretion or it ends by formation of system with a low mass helium white dwarf and a black hole. If the final orbital period of the latter system is less than 10 hours, it is possible that the system will reappear as a persistent X-ray source with ultra-short orbital period. For systems which evolve towards very short orbital periods there always is a phase in which the mass accretion rate exceeds a certain [FORMULA] above which the system is a persistent X-ray source. By comparing evolutionary sequences for different black hole masses it is shown that for a more massive black hole as accretor the range in which mass transfer is unstable is larger than for a less massive black hole.

It is shown that after the common envelope (CE) phase, during the extensive high-velocity wind stage (before the collapse to a black hole) only helium stars of rather low mass end up with orbital periods lower than [FORMULA] which enables them to evolve towards the short orbital period range where the majority of the low-mass X-ray binaries (LMXB) with black holes as accretors are observed. This implies that the pre-CE progenitors of black holes in these systems had masses [FORMULA] 25 [FORMULA]

Key words: stars: evolution – black hole physics – X-rays: stars

Send offprint requests to: E. Ergma

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998

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