  The interaction of the wind from a pulsar (or more generally from a star)
with t he amb ient medium gives rise to the formation of a bow-shock nebula. We
present a mode l of adiabatic bow-shock nebulae, including the presence of a
neutral component in the interstellar medium. As demonstrated in a previous
paper (Bucciantini & Bandiera 2001}, hereafter Paper I), the velocity and the
luminosity of the pulsar, as well a s the characteristics of the interstellar
medium (ISM), play an essential role in determinin g the properties of bow
shock nebulae. In particular, the Balmer emission is strongly affect ed by the
rates of hydrogen charge-exchange and collisional excitation. So far, only
one-com ponent models have been proposed, treating the neutrals as a small
perturbation. The distrib ution of neutral hydrogen in the nebula, derived by
our model, is then used to determine how the h_{alpha} luminosity varies along
the bow shock. We find that the luminosity trend is d ifferent for
charge-exchange and collisional excitation, and that processes like diffusion o
r ionization may reduce the emission in the head of the nebula.

Astrophysics