Transcript Magnetars origin and progenitors with enhanced rotation'

Magnetars origin and
progenitors with
enhanced rotation
S.B. Popov, M.E. Prokhorov
(Sternberg Astronomical Institute)
(astro-ph/0505406)
Abstract
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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.
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The fraction of such neutron stars in our calculations is about 13-16 %.
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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.
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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.
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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.
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A question:
Why do all magnetars are isolated?
• 10 % of NSs are
expected to be binary.
• All known magnetars
(or candidates) are
single objects.
• At the moment from
the statistical point of
view it is not a miracle,
however, it’s time to
ask this question.
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Two possible explanations
• Large kick velocities
• Particular evolutionary path
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
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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.
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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.
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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).
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Results of calculations
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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
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Conclusions
• We made population synthesis of binary systems to
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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.
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