Magnetars origin and progenitors with enhanced rotation' by rt3463df

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									Magnetars origin and
progenitors with
enhanced rotation
S.B. Popov, M.E. Prokhorov
(Sternberg Astronomical Institute)



                       (astro-ph/0505406)
            Abstract
•   We present population synthesis calculations of binary systems.
•   Our goal is to estimate the number of neutron stars originated from
    progenitors with enhanced rotation, as such compact objects can be
    expected to have large magnetic fields, i.e. they can be magnetars.

•   The fraction of such neutron stars in our calculations is about 13-16 %.

•   Most of these objects are isolated due to coalescences of components
    prior to a neutron star formation, or due to a system disruption after a
    supernova explosion.

•   The fraction of such neutron stars in survived binaries is about 1% or
    lower, i.e. magnetars are expected to be isolated objects.
    Their most numerous companions are black holes.
            2                  Popov, Prokhorov astro-ph/0505406
Magnetars in the Galaxy

• 4 SGRs, 8 AXPs, plus candidates, plus
  radio pulsars with high magnetic fields …
• Young objects (about 104 yrs).
• Probably about 10% of all NSs.




3            Popov, Prokhorov astro-ph/0505406
       A question:
       Why do all magnetars are isolated?
• 10 % of NSs are                        Two possible explanations
  expected to be binary.
• All known magnetars                  • Large kick velocities
  (or candidates) are
  single objects.                      • Particular evolutionary path
• At the moment from
  the statistical point of
  view it is not a miracle,
  however, it‟s time to
  ask this question.
       4               Popov, Prokhorov astro-ph/0505406
    Magnetars origin

• Probably, magnetars are
  isolated due to their origin
• Fast rotation is necessary
  (Thompson, Duncan)
• Two possibilities to spin-up
  during evolution in a binary
  1) Spin-up of a progenitor star
  in a binary via accretion or
  synchronization
  2) Coalescence

    5              Popov, Prokhorov astro-ph/0505406
  The code

We use the “Scenario Machine” code.
Developed in SAI (Moscow) since 1983
by Lipunov, Postnov, Prokhorov et al.
(http://xray.sai.msu.ru/~mystery/articles/review/ )


We run the population synthesis of
binaries to estimate the fraction of NS
progenitors with enhanced rotation.


  6                 Popov, Prokhorov astro-ph/0505406
  The model
  Among all possible evolutionary paths that result in
  formation of NSs we select those that lead to
  angular momentum increase of progenitors.
• Coalescence prior to a NS formation.
• Roche lobe overflow by a primary.
• Roche lobe overflow by a primary with a common
  envelope.
• Roche lobe overflow by a secondary without a
  common envelope.
• Roche lobe overflow by a secondary with a
  common envelope.
  7             Popov, Prokhorov astro-ph/0505406
 Parameters
We run the code for two values of the parameter
αq which characterizes the mass ratio distribution
of components, f(q), where q is the mass ratio.
At first, the mass of a primary is taken from the
Salpeter distribution, and then the q distribution is
applied.
            f(q)~q αq , q=M1/M2<1
We use αq=0 (flat distribution, i.e. all variants of
mass ratio are equally probable) and αq=2 (close
masses are more probable, so numbers of NS
and BH progenitors are increased in comparison
with αq=0).
 8              Popov, Prokhorov astro-ph/0505406
Results of calculations




9        Popov, Prokhorov astro-ph/0505406
Coalescence of helium stars

Fryer and Heger (2005) suggested a scenario
  in which a GRB progenitor is formed after
    a coalescence of two helium stars.

We estimate the rate of BH formation after
 a coalescence of two helium stars as
              10-6 yr-1 for αq =0
            5 10-6 yr-1 for αq =2
 It is too low to explain the rate of GRB
10            Popov, Prokhorov astro-ph/0505406
          Conclusions
• We made population synthesis of binary systems to
    derive the relative number of NSs originated from
    progenitors with enhanced rotation -``magnetars''.
•    With an inclusion of single stars (with the total
    number equal to the total number of binaries) the
    fraction of ``magnetars'„ is ~13-16%.
•    Most of these NSs are isolated due to coalescences of
    components prior to NS formation, or due to a system
    disruption after a SN explosion.
•    The fraction of ``magnetars'' in survived binaries is
    about 1% or lower.
•    The most numerous companions of ``magnetars'' are
    BHs.
          11            Popov, Prokhorov astro-ph/0505406

								
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