Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 443
sense that less additional energy is transferred because the stochastic
density field can prevent a complete swap of the neutrino spectra. On
the other hand, for the r-process prevention a complete swap is not
necessary. Therefore, the mixing parameters for which r-process nucleosynthesis
is prevented by oscillations is not significantly changed, even
if the amplitude of the stochastic density component is as large as 1%.
11.5 Neutrino Propulsion of Neutron Stars
Shortly after the discovery of pulsars (neutron stars) it became clear
that they tend to have the largest peculiar velocities of all stellar populations.
Recent determinations of pulsar proper motions by radiointerferometric
methods (Bailes et al. 1990; Fomalont et al. 1992; Harrison,
Lyne, and Anderson 1993) and by interstellar scintillation observations
(Cordes 1986) reveal typical speeds of a few 100 km s −1 . Taking
into account selection effects against high-velocity pulsars, Lyne
and Lorimer (1994) argued that the mean pulsar velocity at birth was
450 ± 90 km s −1 . Associating certain pulsars and SN remnants would
indicate velocities of up to 2000 km s −1 (Frail and Kulkarni 1991; Caraveo
1993; Stewart et al. 1993) while the interaction of PSR 2224+65
with its local environment produces a nebula which reveals a transverse
velocity of at least 800 km s −1 (Cordes, Romani, and Lundgren 1993).
Therefore, the distribution of pulsar peculiar speeds appears to
have a mean of 400−500 km s −1 with the largest measured values of
1000−2000 km s −1 . Recall that the galactic rotation velocity is about
200 km s −1 , the escape velocity about 500 km s −1 . Therefore, the fastest
pulsars will eventually escape from the galaxy. In most cases the migration
is away from the galactic disk in agreement with the picture
that pulsars are born in the disk where massive stars can form from
the interstellar gas. However, there seem to be a few puzzling cases of
pulsars which move toward the disk, apparently having formed in the
galactic halo.
Massive stars probably do not form with much larger velocities than
other stars and so pulsars are likely accelerated in conjunction with the
SN collapse that produced them or during their early evolution. One
suggestion for an acceleration mechanism holds that large neutron-star
“kick velocities” are related to the breakup of close binaries, notably
during the SN explosion of the second binary member (Gott, Gunn,
and Ostriker 1970; Dewey and Cordes 1987; Bailes 1989; see also the
review by Bhattacharya and van den Heuvel 1991).