The emergence of a neutral Herbig-Haro jet into a photoionized nebula

Recent observations show the existence of an increasing number of collimate d outflows ejected by young, low-mass stars which are embedded in H II regi ons. At distances of a few tens of au from the star, at least one lobe of t hese outflows will be shielded from the ambient ionizing radiation by the c ompact, high-extinction circumstellar disc. Within these shielded regions, the jets are probably mostly neutral, similar to the jets in 'normal' Herbi g-Haro (HH) objects. At larger distances, these jets emerge into the photoi onized nebula, and start to be photoionized by the radiation from the ioniz ing photon source of the nebula. In this paper, we model the photoionization of an initially neutral HH jet. This process begins as an ionization front at the side of the jet, which i s directed towards the ionizing star of the nebula, and progresses into the beam of the jet. There are two possible solutions. In the first solution, the jet beam becomes fully ionized through the passage of an R-type ionizat ion front. In the second solution, the ionization front slows down enough t o become a D-type front (or is already a D-type front at the point in which the jet emerges into the photoionized nebula), forming a partially ionized jet beam, with an expanding photoionized region and a compressed neutral r egion. We explore these two types of solutions both analytically and numerically, and discuss the observational effects introduced by this jet photoionizatio n process, concentrating in a region of parameter space that straddles the parameters deduced for HH 444 (the jet from V 510 Orionis).