Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://xray.sai.msu.ru/~mystery/articles/review/node64.html
Дата изменения: Wed Feb 26 20:56:27 1997 Дата индексирования: Tue Oct 2 14:50:25 2012 Кодировка: Поисковые слова: hst |
Next: Binary
Radiopulsars with Black Up: Black
Holes in Binaries Previous: Black
Holes in Binaries
Two different types of stellar BH formation exist: (1) direct core collapse of a massive star and (2) AIC of a NS.
A BH of the first type presumably will have a mass comparable to that
of the collapsing star and can be born both from a single star and in a
binary system. The AIC BH can be formed only in binary systems. Their masses
must be much lower, about the Oppenheimer-Volkoff limit,
, and the number of such AIC BH will be sensitive to this limit. AIC BH
can be formed at the stage of supercritical accretion
in a binary system at high accretion rates
yr
.
An effective means of formation of such BH can be spiral-in
of a NS in the extended atmosphere of the giant companion at the common
envelope (CE) stage (Chevalier, 1993[31]),
when neutrino losses can effectively remove the gravitational
energy released, and a Bondi-Hoyle accretion may be
established. The characteristic time of mass growth of the NS at this stage
is only five to seven times less than the time-scale of angular momentum
drag due to spiral-in (Chevalier, 1993)[31],
so AIC BH with masses close to the OV-limit in circular orbits around the
remaining star core (a Wolf-Rayet star (WR) or a white dwarf (WD))
will form, unless the spiral-in ends by coalescence
of the compact star with the stellar core and formation of a Thorne-Zytkow
object. Some BH binaries thus formed can be disrupted
during a second supernova explosion. The remaining
WR+BH binaries will yield PSR+BH systems. Firm theoretical estimation of
the number of AIC BH is very difficult, however one can evaluate their
possible number from the observational data.
At present about a dozen single-line WR stars with suspected binary companions are known (Cherepashchuk, 1991[30]), some of which can be considered as progenitors of these binary pulsars (namely, those which produce notable X-rays). We can make a simple estimation:
Here
yr and
-3
yr are typical pulsar and WR lifetimes, respectively. N(WR+compact)
relates mainly to WR+NS that produce no X-rays due to fast rotation of
the magnetized NS (see Lipunov, 1992[107],
p. 293). In reality, we know only one WR-star with a compact X-ray companion,
Cyg X-3 (van Kerkwijk, 1993[206]),
so we must reduce the upper limit by 10 times . Taking the total galactic
number of pulsars
3
, we obtain the expected fraction of binary PSR+AIC BH (
0.1 per 1000 isolated pulsars). Although we believe that the real number
of PSR+AIC BH may be much smaller, we note nevertheless that the distribution
of such binary pulsars on orbital periods will significantly
differ from that of PSR+BH formed from massive stars, in having shorter
orbital periods of several hours to several days.