Spatial Offsets in Bow Shocks
Spatial offsets of high radial velocity material in paraboloid
bow shocks. Top: A paraboloid viewed from the side is shown, where the
thin arrows indicate the velocity of the material immediately behind the bow shock.
For a viewing angle phi, material along the projected apex of the bow
moves tangentially to the line of sight, so the maximum negative radial
velocity appears at some point interior to the bow.
Bottom: The paraboloid appears as a fatter parabola when projected
onto the plane of the sky. The intersection of the line x' = z'
with the parabola defines a characteristic size h of the bow. For
arbitrary viewing angles and arbitrary paraboloids, as long as the
cooling distance behind the shock is small compared with h, the location of largest
radial velocity V_max (the maximum redshift for a redshifted bow or the maximum blueshift
for a blueshifted bow) is always offset from the apex of the projection of the
paraboloid on the plane of the sky by an amount d = 0.25 h. This result is
very useful for testing the applicability of a bow shock model when
velocity-resolved images are available.