Critical Velocity of Vortex Nucleation in Rotating Superfluid 3 He- A

We have measured the critical velocity {upsilon}{sub c} at which {sup 3}He{minus}A in a rotating cylinder becomes unstable against the formation of quantized vortex lines with continuous (singularity-free) core structure. We find that {upsilon}{sub c} is distributed between a maximum and a minimum limit, which we ascribe to a dependence on the texture of the orbital angular momentun axis {bold {cflx l}}({bold r}) in the cylinder. Slow cooldown through T{sub c} in rotation yields {bold {cflx l}}({bold r}) textures for which the measured {upsilon}{sub c} {close_quote}s are in good agreement with the calculated instability of the expected {bold {cflx l}} texture. {copyright} {ital 1997} {ital The American Physical Society}