Neutron star population dynamics. II. Three-dimensional space velocities of young pulsars

We use astrometric, distance, and spindown data on pulsars to (1) estimate three-dimensional velocity components, birth distances from the Galactic pl ane, and ages of individual objects; (2) determine the distribution of spac e velocities and the scale height of pulsar progenitors; (3) test spindown laws for pulsars; (4) test for correlations between space velocities and ot her pulsar parameters; and (5) place empirical requirements on mechanisms t han can produce high-velocity neutron stars. Our approach incorporates meas urement errors, uncertainties in distances, deceleration in the Galactic po tential, and differential Galactic rotation. We focus on a sample of proper motion measurements of young (<10 Myr) pulsars whose trajectories may be a ccurately and simply modeled. This sample of 49 pulsars excludes millisecon d pulsars and other objects that may have undergone accretion-driven spinup . We estimate velocity components and birth z distance on a case-by-case ba sis assuming that the actual age equals the conventional spindown age for a braking index n = 3, no torque decay, and birth periods much shorter than present-day periods. Every sample member could have originated within 0.3 k pc of the Galactic plane while still having reasonable present-day peculiar radial velocities. For the 49 object sample, the scale height of the proge nitors is similar to 0.13 kpc, and the three-dimensional velocities are dis tributed in two components with characteristic speeds of 175(-24)(+19) km(- 1) and 700(-132)(+300) km s(-1), representing similar to 86% and similar to 14% of the population, respectively. The sample velocities are inconsisten t with a single-component Gaussian model and are well described by a two-co mponent Gaussian model but do not require models of additional complexity. From the best-fit distribution, we estimate that about 20% of the known pul sars will escape the Galaxy, assuming an escape speed of 500 km s(-1). The best-fit, dual-component model, if augmented by an additional, low-velocity (<50 km s(-1)) component, tolerates, at most, only a small extra contribut ion in number, less than 5%. The best three-component models do not show a preference for filling in the probability distribution at speeds intermedia te to 175 and 700 km s(-1) but are nearly degenerate with the best two-comp onent models. We estimate that the high-velocity tail (>1000 km s(-1))may b e underrepresented tin the observed sample) by a factor similar to 2.3 owin g to selection effects in pulsar surveys. The estimates of scale height and velocity parameters are insensitive to the explicit relation of chronologi cal and spindown ages. A further analysis starting from our inferred veloci ty distribution allows us to test spindown laws and age estimates. There ex ist comparably good descriptions of the data involving different combinatio ns of braking index and torque decay timescale. We find that a braking inde x of 2.5 is favored if torque decay occurs on a timescale of similar to 3 M yr, while braking indices similar to 4.5 +/- 0.5 are preferred if there is no torque decay. For the sample as a whole, the most probable chronological ages are typically smaller than conventional spindown ages by factors as l arge as 2. We have also searched for correlations between three-dimensional speeds of individual pulsars and combinations of spin period and period de rivative. None appears to be significant. We argue that correlations identified previously between velocity and (appa rent) magnetic moment reflect the different evolutionary paths taken by you ng, isolated (nonbinary), high-field pulsars and older, low-field pulsars t hat have undergone accretion-driven spinup. We conclude that any such corre lation measures differences in spin and velocity selection in the evolution of the two populations and is not a measure of processes taking place in t he core collapse that produces neutron stars in the first place. We assess mechanisms for producing high-velocity neutron stars, including disruption of binary systems by symmetric supernovae and neutrino, baryonic, or electr omagnetic rocket effects during or shortly after the supernova. The largest velocities seen (similar to 1600 km s(-1)), along with the paucity of low- velocity pulsars, suggest that disruption of binaries by symmetric explosio ns is insufficient. Rocket effects appear to be a necessary and general phe nomenon. The required kick amplitudes and the absence of a magnetic field-v elocity correlation do not yet rule out any of the rocket models. However, the required amplitudes suggest that the core collapse process in a superno va is highly dynamic and aspherical and that the impulse delivered to the n eutron star is larger than existing simulations of core collapse have achie ved.