FORBIDDEN LINE FORMATION IN HYDROMAGNETIC DISK WINDS .1. OBLIQUE SHOCKS

The observed broad, blueshifted, low-excitation metallic forbidden emi ssion lines, [O I] lambda6300, [S II] lambdalambda6730, 6716 and [N II ] lambda6548, associated with young stellar objects (YSOs), are probes of the low-density regions of energetic outflows on scales ranging fr om 10 to 70 AU from the central engine. We investigate the idea that s uch lines are generating in oblique hydromagnetic shock waves in recol limating disk winds from YSOs. We solve for the typical postshock phys ical conditions that arise in such shocks. We find that conditions app ropriate for forbidden line emission are produced in oblique shocks wi th modest fast magnetosonic Mach numbers n1 between 2 to 4. The highes t velocity and density gas in the shocks is concentrated most toward t he flow axis. Slower, lower density emitting material is farther from the flow axis. This situation preserves to some degree the conditions in the preshock disk wind. The postshock material expands away from th e jet axis at angles up to 32-degrees. Our results prepare the stage f or the calculation of line profiles of the shocked, emitting gas which we present in the forthcoming second part of this investigation.