In this paper we calculate emission line ratios from a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields. The models cover the initial conditions relevant to stellar jets, and we show how to estimate the ionization fractions and shock velocities in jets directly from observations of the strong emission lines in these flows. The ionization fractions in the HH 34, HH 47, and HH 111 jets are ~ 2%, considerably smaller than previous estimates, and the shock velocities are ~ 30 km/s . For each jet the ionization fractions were found from five different line ratios, and the estimates agree to within a factor of ~ 2. The scatter in the estimates of the shock velocities is also small (+/- 4 km/s).

The low ionization fractions of stellar jets imply that the observed electron densities are much lower than the total densities, so the mass loss rates in these flows are correspondingly higher (>~ 2x10^{-7} Msun/yr). The mass loss rates in jets are a significant fraction (1% - 10%) of the disk accretion rates onto young stellar objects that drive the outflows. The momentum and energy supplied by the visible portion of a typical stellar jet are sufficient to drive a weak molecular outflow.

Magnetic fields in stellar jets are difficult to measure because the line ratios from a radiative shock with a magnetic field resemble those of a lower velocity shock without a field. The observed line fluxes can in principle indicate the strength of the field if the geometry of the geometry of the shocks in the jet is well-known.

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