Galactic Electron Density:
Most pulsar distances are estimated from their dispersion measures
(DM), using a model for the Galactic electron density such as
Taylor and Cordes (1993, hereafter TC93). In turn, because of its explicit
incorporation of Galactic spiral structure, the TC93 model has been
used as a surrogate for modeling the warm ionized medium, both in our
Galaxy and others (e.g. Duncan et al. 1999, Wood and Reynolds 1999,
Roshi and Anantharamaiah 2000). Model-independent
distances obtained from parallaxes provide essential calibration
points for the electron density model. In particular, more parallaxes
will allow much better modeling of the local interstellar medium (a
weakness in the current model), thus providing better DM distances for
nearby pulsars which are too weak to obtain parallaxes.
Hybrid VLBI and Interstellar Scintillation Analysis:
The characteristics of pulsar scintillation, i.e., scintillation time
and bandwidth, depend in part upon the distribution of scattering
material along the line of sight. Combining proper motion and
parallax measurements with interstellar scintillation (ISS)
observations of pulsars allows modeling of the distribution of
scattering material, and thus contributes to modeling of the free
electron density in the Galaxy. Identification of regions of enhanced
scattering is particularly important, because they evidently signify
locales where the interstellar medium has been stirred significantly
by shocks. Chatterjee et al. (2001) demonstrate this analysis for
B0919+06, from which they find evidence for ``clumps'' with a probable
scale size of 10 pc at the edges of the Local Bubble.
NS Population Velocities: Parallaxes and proper motions
lead to accurate model-independent transverse velocities for pulsars,
which constrain the shape of the population velocity distribution.
Additionally, improved ne models provide improved distances, and
hence, improved velocity estimates for the entire pulsar
population. Pulsar velocities also represent fossil information about
the evolution of close binary systems and core collapse processes in
supernovae.
For high magnetic field objects, the entire range of velocities is of
interest: supernova core collapse processes, interstellar accretion
onto once-active but now dead pulsars, and the apparent bimodality of
the NS velocity distribution (Arzoumanian, Chernoff and Cordes 2001,
Cordes and Chernoff 1998) are all constrained by model-independent
velocity measurements.
For low-field binary millisecond pulsars (MSPs), measured transverse
velocities provide information on the degree of mass loss during the
formation of the neutron star that also allowed a low-mass binary to
remain bound. For solitary MSPs, velocity measurements may provide
clues about any peculiarities in their formation process.
Measurements also characterize the role of diffusion of stellar orbits
in the Galaxy's gravitational potential, which is important for old
stars.
Reference Frame Ties: Astrometry on MSPs allows the
verification of solar system--extragalactic reference frame ties and
the accuracy of timing parallaxes. The measurements also constrain
model fitting for orbital and relativistic parameters of MSPs in
binary systems.
Pulsar--Supernova Remnant Associations and NS Birth Sites:
Knowing the proper motion and distance to pulsars can help identify
their birth sites, and clarify putative pulsar--SNR associations.
True ages of both pulsars and their associated SNRs may also be
estimated (or constrained) from the angular separation of the SNR and
pulsar, and the pulsar proper motion. Similarly, accurate astrometry
allows some pulsars to be traced back to their birth sites in stellar
clusters (e.g. Hoogerwerf, de Bruijne and de Zeeuw 2001).
Nuclear Astrophysics: Accurate distance measurements, in
combination with observed thermal radiation from the neutron star
surface, can be used to constrain the `size' of the neutron star
photosphere, with important implications for the NS Equation of
State. For example, ongoing VLBA observations of PSR B0656+14 (Brisken
et al.) will complement extensive X-ray/EUV/optical datasets with this
aim.
Other flavors of NS: Radio pulsars represent just one
of several manifestations of neutron stars. Others include Anomalous
X-ray Pulsars (6 known objects), Soft Gamma-ray Repeaters (4 known
objects) and 10 radio-quiet gamma-ray pulsars (Gaensler, Gotthelf and
Vasisht 1999, and references therein). Although proper motions
have not yet been measured for any of the objects, it is possible that
they will display different kinematics. Therefore, it is of interest
to quantify and understand the kinematics of radio pulsars as well as
possible, in order to compare and contrast them with other NS
sub-classes.
