Pulsar bow-shock nebulae -: III.: Inclusion of a neutral component and Hα luminosity

The interaction of the wind from a pulsar (or more generally from a star) with the ambient medium gives rise to the formation of a bow-shock nebula. We present a model 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 as the characteristics of the interstellar medium (ISM), play an essential role in determining the properties of bow shock nebulae. In particular, the Balmer emission is strongly affected by the rates of hydrogen charge-exchange and collisional excitation. So far, only one-component models have been proposed, treating the neutrals as a small perturbation. The distribution of neutral hydrogen in the nebula, derived by our model, is then used to determine how the Halpha luminosity varies along the bow shock. We find that the luminosity trend is different for charge-exchange and collisional excitation, and that processes like diffusion or ionization may reduce the emission in the head of the nebula.