Pulsar bow-shock nebulae. III. Inclusion of a neutral component, and H_{a lpha} luminosity

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.