Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...456..738i&link_type=abstract
Astrophysical Journal v.456, p.738
Astronomy and Astrophysics
Astronomy
66
Stars: Pulsars: General, Stars: Binaries: Close, Stars: Neutron, Stars: Evolution, Stars: Kinematics, Stars: Supernovae: General
Scientific paper
Most observed radio pulsars have high peculiar space velocities covering the range 20-1000 km s 1, although their unevolved OB star progenitors have peculiar space velocities which are among the lowest in the Galaxy. An evolutionary scenario model of the Galactic population of neutron stars shows that the evolution of massive close binaries which takes into account the recoil velocity achieved in response to mass loss in supernova explosions, but ignores a possible additional "natal kick," can produce single neutron stars with space velocities in the range of 10-1000 km s-1, the highest velocities being achieved in the second supernova explosion which unbinds (most) massive binaries. The theoretical distribution of space velocities is consistent, within the uncertainties, with the transverse velocity distribution for radio pulsars in the 500 pc neighborhood of the Sun. The same model explains observed space velocities of OB runaway stars, massive X-ray binaries, and close binary neutron stars.
Progenitors of neutron stars which become radio pulsars are hydrogen-free and so experience supernova explosions which are presumably of Types Ib and Ic. The predicted birthrate of such supernovae, and therefore also of radio pulsars, is ˜0.007 yr-1, compared with a predicted Type II supernova birthrate of ˜0.021 yr-1. Within the uncertainties, these estimates are consistent, respectively, with existing semiempirical estimates of the pulsar formation rate and estimates of the frequency of supernova explosions. The fact that the predicted birthrate of radio pulsars is 4 times smaller than the predicted combined birthrate of supernovae of Types Ib, Ic, and II, coupled with the fact that pulsar radiation is beamed, suggests that fewer than ˜10% of all radio pulsars are associated with diffuse supernova remnants, regardless of age.
In the scenario model with no ad hoc natal kicks, the absence of observed radio pulsars in wide binaries is interpreted as the result of the slow rotation of young neutron stars formed in Type II supernova explosions of single stars or of components of wide, detached binaries. The absence of a close companion which can supply matter and angular momentum to either the presupernova or to the neutron star leaves the neutron star with insufficient angular momentum to be a radio pulsar.
A final conclusion about the role of single stars and components of wide binaries in the formation of radio pulsars requires that the pulsar space velocity distribution be known for velocities less than ˜20 km 5-1 The scenario model predicts as many neutron stars with velocities less than 20 km s-1 as there are in the 20-1000 km s-1 range (whether or not these neutron stars are pulsars). If further observations show the birthrate of low-velocity neutron stars to be comparable to the birthrate of high-velocity ones, this would be a direct demonstration that nature does not require an ad hoc asymmetric kick. If spin periods of the low-velocity group are typical of the spin periods already known (≤ 3 s), rather than of the order of 100-300 s, as is expected if the angular velocity of presupernova cores is comparable to the angular velocity at the presupernova surface, one could infer that the core of the precursor of a Type II supernova spins much faster than do surface layers, and that, therefore, accretion on a neutron star is not the primary reason for observed radio pulsar spin periods. On the other hand, if further observations reveal a real (rather than a selection-induced) paucity of low-velocity pulsars, this would provide strong support for the current prediction that massive single stars and components of wide binaries do not produce radio pulsars and that most radio pulsars have their origin in close binaries.
Iben Icko Jr.
Tutukov Alexander V.
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