Kinematic Structure of H2 and [Fe II] in the Bipolar Planetary Nebula M 2-9

We present high-dispersion long-slit IR spectra of the double-shell bipolar planetary nebula M 2-9 in the emission lines [Fe II] 16435 and H2 v=1–0 S(1) 21218. H2 spectra reveal for the first time the kinematic structure of the outer shell in M 2-9. Kinematics of the inner shell, traced by [Fe II], resemble those of optical lines like [N II]. [Fe II] and H2 shells have expansion speeds roughly proportional to distance from the star (Hubble'' flows) and share the same dynamical age of 1200--2000 yr, depending on the distance to M 2-9. Thus, the inner ionized lobes and outer molecular lobes, as well as the molecular torus and outer loops’’ measured by other observers, were all formed around the same time. Consequently, their nested structure likely arises from an excitation gradient rather than independent ejections. H2 and [Fe II] emission is distributed more uniformly than [N II], and IR lines are not dominated by the moving ionization pattern like visual-wavelength lines. We suggest that this is because IR lines of [Fe II] and H2 are excited by relatively isotropic far-UV radiation (Balmer continuum), whereas optical lines respond to a directed rotating beam of Lyman continuum. Finally, we highlight intriguing similarities between M 2-9 and the Homunculus of eta Carinae, despite the different central engines powering the two nebulae.