Kinematic Structure of H₂and [Feii] in the Bipolar Planetary Nebula M2-9

Abstract

We present new high-dispersion, long-slit, infrared (IR) spectra of the double-shell bipolar planetary nebula M2-9 in the emission lines [ Fe ii] k16435 and H 2 v ¼ 1 0 S(1) k21218. H 2 spectra reveal for the first time the kinematic structure of the outer shell in M2-9. Kinematics of the inner shell, traced by [Fe ii], resemble those of optical forbidden lines like [N ii] k6583, although we note subtle differences. [Fe ii] and H 2 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 M2-9. 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. H 2 and [Fe ii] emission is distributed more uniformly than [N ii], and IR lines are not dominated by the moving ionization pattern like visualwavelength lines. We suggest that this is because IR lines of [Fe ii] and H 2 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 M2-9 and the Homunculus of Car, despite the different central engines powering the two nebulae.

The Structure of the Homunculus. I. Shape and Latitude Dependence from H2and [Feii] Velocity Maps of η Carinae

“…Dust in this region attenuates UV light, eventually allowing H 2 to survive at larger radii (see, e.g., Ferland et al 2005). An almost identical double-shell structure of H 2 and [Fe ii] also exists in the famous planetary nebula M2-9 (Hora & Latter 1994;Smith et al 2005).…”

Numerical simulations of wind-driven protoplanetary nebulae – I. near-infrared emission

“…Position-velocity diagrams were presented from long-slit spectroscopy along the central axis of the PN M2-9 by Smith et al (2005). These display two components which correspond to the near and far sides of the expanding elliptical structure.…”

UKIRT Widefield Infrared Survey for Fe+

“…Among 41 they observed, 16 showed H2 2.122 µm emission but only 3 showed [Fe II] 1.644 µm emission. While the fraction of PNe with detected [Fe II] 1.644 µm emission was small, the emission has been useful in studying the nature of mass loss, as demonstrated in the case of M2-9 (Smith et al 2005).…”