Magnetohydrodynamic Models of the Bipolar Knotty Jet in Henize 2-90

Abstract

A remarkably linear, bipolar, knotty jet was recently discovered in Hen 2-90, an object classified as a young planetary nebula. Using two-dimensional, magnetohydrodynamic simulations, we investigate periodic variations in jet density and velocity as the mechanism for producing the jet and its knotty structures. From a detailed comparison between the Hα emission derived from our models and the observations, we find that a nonmagnetized jet with density or velocity variations does not reproduce in detail the observed structure of the Hen 2-90 jet-a magnetized jet with periodic velocity variations is required. This jet has a radius of 125 AU, an average velocity of 300 km s^-1 with periodic variations (period = 43 yr) in the jet velocity of amplitude +/-15 km s^-1, and a toroidal magnetic field with a characteristic strength of 0.6 mG. The average mass-loss rate in the jet has decreased by about a factor of 3 in 600 yr (i.e., from 4.7×10^-7 M_solar yr^-1 at 20" to 1.7×10^-7 M_solar yr^-1 at 1" from the center along the jet axis). The progenitor asymptotic giant branch wind is assumed to have a mass-loss rate of 1.5×10^-6 M_solar yr^-1 (derived from the round, tenuous, Hα halo surrounding the central source) and a typical expansion velocity of 10 km s^-1. We find a fairly detailed similarity in the physical properties of the jet in Hen 2-90 with that in the young stellar object HH 34. This similarity suggests that the jets in both objects may be launched in a similar manner, namely, from an accretion disk, despite the fact that these objects are at very different evolutionary stages.