SCENARIOS FOR THE FORMATION OF BINARY AND MILLISECOND PULSARS - A CRITICAL-ASSESSMENT

Citation
Epj. Vandenheuvel, SCENARIOS FOR THE FORMATION OF BINARY AND MILLISECOND PULSARS - A CRITICAL-ASSESSMENT, Journal of astrophysics and astronomy, 16(2), 1995, pp. 255-288
Citations number
63
Categorie Soggetti
Astronomy & Astrophysics
ISSN journal
02506335
Volume
16
Issue
2
Year of publication
1995
Pages
255 - 288
Database
ISI
SICI code
0250-6335(1995)16:2<255:SFTFOB>2.0.ZU;2-Q
Abstract
The evolution of high- and low-mass X-ray binaries (HMXB and LMXB) int o different types of binary radio pulsars, the 'high-mass binary pulsa rs' (RMBP) and 'low-mass binary pulsars' (LMBP) is discussed. The HMXB evolve either into Thorne-Zytkow objects or into short- period binari es consisting of a helium star plus a neutron star (or a black hole), resembling Cygnus X-3. The latter systems evolve (with or without a se cond common-envelope phase) into close binary pulsars, in which the co mpanion of the pulsar may be a massive white dwarf, a neutron star or a black hole ( some final systems may also consist of two black holes) . A considerable fraction of the systems may also be disrupted in the second supernova explosion. We discuss the possible reasons why the ob served numbers of double neutron stars and of systems like Cyg X-3 are several orders of magnitude lower than theoretically predicted. It is argued that the observed systems form the tip of an iceberg of much l arger populations of unobserved systems, some of which may become obse rvable in the future. As to the LMBP, we consider in some detail the o rigins of systems with orbital periods in the range 1 - 20 days. We sh ow that to explain their existence, losses of orbital angular momentum (e.g., by magnetic braking) and in a number of cases: also of mass, h ave to be taken into account. The masses of the low-mass white dwarf c ompanions in these systems can be predicted accurately. We notice a cl ear correlation between spin period and orbital period for these syste ms, as well as a clear correlation between pulsar magnetic field stren gth and orbital period. These relations strongly suggest that increase d amounts of mass accreted by the neutron stars lead to increased deca y of their magnetic fields: we suggest a simple way to understand the observed value of the 'bottom' field strengths of a few times 10(8) G. Furthermore, we find that the LMBP-systems in which the pulsar has a strong magnetic field (> 10(11) G) have an about two orders of magnitu de larger birth rate (i.e., about 4 x 10(-4) yr(-1) in the Galaxy) tha n the systems with millisecond pulsars (which have B < 10(9) G). Using the observational fact that neutron stars receive a velocity kick of similar to 450 km/s at birth, we find that some 90% of the potential p rogenitor systems of the strong-field LMBP must have been disrupted in the supernovae in which their neutron stars were formed. Hence, the f ormation rate of the progenitors of the strong-field LMBP is of the sa me order as the galactic supernova rate (4 x 10(-3) yr(-1)). This impl ies that a large fraction of all supernovae take place in binaries wit h a close low-mass (< 2.3 M(.)) companion.