Formation and coalescence of double neutron star binaries
Tomek Bulik (University of Warsaw)Dorota Rosinska (CAMK, VESF Fellow)Stefan Osłowski (Swinburne University)
Krzysztof Belczynski (Los Alamos)
Double neutron stars
● Very small population, radio observations● Perhaps a subpopulation of short GRBs● GW sources● How do DNS form?● What mew can we learn from observations of
coalescences in GW?● What are the populations seen in GW?
The radio sample
The StarTrack code
● Single stellar evolution: kicks, masses, radii, structure
● Evolution of binaries: mass transfers, supernovae, orbits
● Used to investigate compact object binaries: gammaray bursts, gravitational wave sources, Xray binaries, inidividual binaries, Xray transients, supernovae Ia, ...
● Tested on observed binaries● Developed and maintained by K.Belczynski
Formation of NSNS binaries
Classical path
Possible role of helium star common envelope phases.
Pulsar evolution: initial pulsar properties
● Inital spin: adopt a value of Pini
=10ms
● Starting magnetic field: drawn from the interval B=1011 to 1012 Gauss
● Initial orbits implied by the binary evolution
Single pulsar evolution
● Assume the braking index n=3, and use standard dipole formula
● Magnetic field decay with characteristic timescale of 20 Myrs – treated as parameter
● No decay below 108 Gauss● Radio luminosity and death lines
Pulsar evolution: mass transfers in binaries
● Exponential magnetic field decay due to the accreted mass – mass scale 0.025 or 0.05 M
sun
● Spin up in Roche Lobe overflow ● Final spin determined by orbital frequency at
the Alfven surface● Yes/No spin up in common envelope ● Magnetic field decay in common envelopes● No magnetic field decay after mass transfer
Pulsar evolution: orbits
● Orbit changes during mass transfer episodes (pre NSNS stage)
● Strong tightening in common envelope● Orbit decay due to gravitational wave emission
The P – Pdot diagram
● Death lines● Spin up line● Hubble line● Single pulsar track
Pulsars below Hubble line – magnetic field decrease in MT
Population model
● Constant star formation rate● Population as seen today● Observability proportional to the time spent in a
given phase● Evolution in the Galactic potential: velocities
from binary evolution, positions
Comparison with
observations
Stadard model with Md=0.05
Contours of highest probablity
Likelihood function
Remaining models:SP initial mass spectrum continuous, small Md
HP Spin up in common envelopes
Pulsar masses
Pulsar masses – model SP
Pulsar masses model HP
Conclusions
● Difficult to model both P – Pdot and the mass distribution
● Evidence for non equal mass binaries● Simulations must take into account 0.8<q<1● GW observations may reveal a different
population of DNS ● Masses of merging DNS carry information
about formation and evolution of these systems
Radio vs GW
● Radio observability:– Radio Luminosity– Radio Loud Lifetime – Galactic Distribution
● GW observability– Chirp mass– Coalescence times
Selection effects are widely different.
Can the observed populations be different?
The masses of the radio sample1
0.9
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0.7
0.9
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0.7