Constraints on Pulsar Evolution: The Joint Period-Spindown Distribution
  of Millisecond Pulsars
  We calculate the joint period-spindown (P-Pdot) distributions of millisecond
radio pulsars (MSRP) for the standard evolutionary model in order to test
whether the observed MSRPs are the unequivocal descendants of millisecond X-ray
pulsars (MSXP). The P-Pdot densities implied by the standard evolutionary model
compared with observations suggest that there is a statistically significant
overabundance of young/high magnetic field MSRPs. Taking biases due to
observational selection effects into account, it is unlikely that MSRPs have
evolved from a single coherent progenitor population that loses energy via
magnetic dipole radiation after the onset of radio emission. By producing the
P-Pdot probability map, we show with more than 95% confidence that the fastest
spinning millisecond pulsars with high magnetic fields, e.g. PSR B1937+21,
cannot be produced by the observed MSXPs within the framework of the standard
model.
