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Why do Radiopulsars not Usually Form Pairs with Normal Stars?


In the very first years after the discovery of radiopulsars, it became clear that for some reason they avoid forming binary systems. Three hypotheses were put forward to explain such a strange incompatibility.

  1. Binary systems disintegrate after the first explosion which leads to the formation of a NS. Since it was clear from the very beginning that the lighter component explodes first in view of the mass exchange, the binary system certainly cannot lose more than half its total mass. Hence, the disintegration must be associated with the anisotropy  of the collapse (Shklovskii, 1976)[180].
  2. Schwartzman (1971)[176] interpreted the absence of radiopulsars in binary systems from an entirely different position. NS remain in the binary systems, but the radiopulsar stage is much shorter than the stage for a single NS due to the presence of accretion flows. Hence, the probability of finding a radiopulsar in a binary system is much lower.
  3. Illarionov and Sunyaev (1975)[76] noted the fact that coherent radiowaves with a wavelength longer than 75 cm, which make it possible to observe most of the radiopulsars, are absorbed effectively in the stellar wind of a normal star.

Suppose that a normal star loses matter in a spherically symmetric way. In this case, the optical thickness associated with the free-free absorption in the stellar wind is


where tex2html_wrap_inline11080  K is the temperature of the stellar wind, tex2html_wrap_inline11082  cm is the wavelength of the radiofrequency radiation, and a is the semi-major axis of the orbit expressed in units of solar radii. In massive binary systems with tex2html_wrap_inline11086 , we have tex2html_wrap_inline11088 . Hence the stellar wind is opaque even for very wide systems ( tex2html_wrap_inline11090 ).

Which of the above three mechanisms is responsible for the deficit of radiopulsars in binary systems with normal stars? To answer this question correctly, a more detailed analysis of the evolution of normal and neutron stars in binary system is needed. Recent studies (see Lipunov et al., 1995d and Section 7 for more detail) suggest that the ill-defined conditions of stellar wind transparency turn out to be of a rather small significance since after the first supernova  explosion the distribution of binaries by optical thickness of the wind is fairly flat on a logarithmic scale (which reflects the initial flat binary distribution on semimajor axes; Abt, 1983).

next up previous contents index
Next: Caverns in Binary Systems Up: Ejectors in Massive Binary Previous: The Number of Ejectors

Mike E. Prokhorov
Sat Feb 22 18:38:13 MSK 1997