Pulsar bow-shock nebulae - I. Physical regimes and detectability conditions

Pulsar bow-shock nebulae originate from the interaction of the ambient medi um with the wind of a moving pulsar. The properties of these nebulae depend on both the physical conditions in the ambient medium and the characterist ics of the pulsar wind, and may thus represent a powerful diagnostic tool. The main limits to this study are, on the observational side, the very limi ted sample of pulsar bow-shock nebulae known so far and, on the theoretical one, a still limited understanding of the physical conditions in these neb ulae: in particular, classical bow-shock models are not appropriate to desc ribe these objects. In this paper we outline the most likely physical regim es for nebulae associated with typical pulsars, and discuss why classical m odels cannot be applied to these objects. We also discuss how microphysical processes may affect both the dynamics of the ow and the properties of the emission (mostly in Balmer lines). We finally put forward a criterion to s elect among the known pulsars those with the highest probability of having a detectable bow-shock nebula.