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Ejector-propeller hysteresis

     

As mentioned earlier, the transition of the rotator from the ejector state to the propeller state is not symmetrical. Here we consider this effect in more detail. In terms of our approach, we must study the dependence of tex2html_wrap_inline8859 on tex2html_wrap_inline9294 . To find tex2html_wrap_inline8859 , we must match the ram pressure of the accreting plasma with that caused by the relativistic wind or by the magnetosphere  of the rotator (see Section 4.3). This dependence tex2html_wrap_inline10006 will be substantially different for rapidly ( tex2html_wrap_inline8869 ) and slowly ( tex2html_wrap_inline8867 ) rotating stars (see Figure 12). One can see that in the case of a fast rotator, an interval of tex2html_wrap_inline9294 appears where three different values of tex2html_wrap_inline8859 are possible, the upper value tex2html_wrap_inline10016 corresponding to the ejector state and the bottom value tex2html_wrap_inline10018 to the propeller state; the intermediate value tex2html_wrap_inline10020 is unstable. This means that the rotator's state is not determined solely by the value of tex2html_wrap_inline9294 , but also depends on previous behavior of tex2html_wrap_inline9294 .

  figure1501
Figure 12: Dependence of the stopping radius tex2html_wrap_inline8859 on the modified gravimagnetic radius tex2html_wrap_inline8861 for two possible relations between the light cylinder radius tex2html_wrap_inline8863 and the gravitation capture radius tex2html_wrap_inline8865 : tex2html_wrap_inline8867 (left-hand panel) and tex2html_wrap_inline8869 (right-hand panel; here the ejector-propeller hysteresis becomes possible). 

Now consider a periodic changing of tex2html_wrap_inline9294 caused, for example, by the rotator's motion along an eccentric orbit, and large enough for the rotator to transit from the ejector  state to the propeller state and vice versa. Initially, the rotator is in the ejector state. By approaching the normal star, the accretion rate tex2html_wrap_inline9294 increases and reaches a critical value tex2html_wrap_inline10042 , where the equilibrium points tex2html_wrap_inline10016 (stable point corresponding to the ejector state) and tex2html_wrap_inline10020 (unstable) approach tex2html_wrap_inline8865 (upper kink), where they merge (see Figure 12). After that only one equilibrium point remains in the system, the stopping radius tex2html_wrap_inline8859 jumps from tex2html_wrap_inline9044 tex2html_wrap_inline8865 down to tex2html_wrap_inline10056 , and the rotator changes to the propeller state.

As tex2html_wrap_inline9294 decreases further along the orbit and reaches the critical value tex2html_wrap_inline10042 once again, the reverse transition from propeller to ejector does not occur. The transition only occurs when tex2html_wrap_inline9294 reaches another critical value, tex2html_wrap_inline10064 where the unstable point tex2html_wrap_inline10020 meets the stable propeller point tex2html_wrap_inline10018 , and the stopping radius tex2html_wrap_inline8859 jumps from tex2html_wrap_inline8945 tex2html_wrap_inline8863 up to tex2html_wrap_inline10076 .

It should be noted that for fast enough rotators, a situation is possible when the step down from the ejector state occurs in such a manner that the stopping radius tex2html_wrap_inline10078 and the rotator passes directly to the accretion state.  The reverse transition always passes through the propeller stage: tex2html_wrap_inline10080 . In principle, transitions from the ejector state to supercritical states SP or SA are also possible (Prokhorov, 1987a; Osminkin and Prokhorov, 1995)[166, 147]. In the case of slow rotators ( tex2html_wrap_inline8867 ), the ``E-P'' hysteresis is not possible, and transitions between these states are symmetrical.


next up previous contents index
Next: E-P transitions for different Up: Evolution of Magnetic Rotators Previous: Mixing types of E-P-A

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