[Fe II] bubbles in the young planetary nebula Hubble 12

We have obtained narrowband images of the young planetary nebula Hubble 12 showing [Fe II] line emission in bipolar bubbles near the core. Bright [Fe II] emission is strong evidence for shocks, suggesting the presence of a high-velocity wind emanating from the central star. We compare our data to previously published images of Hubble 12-finding similar structures in hydrogen recombination and free-free emission-and we propose three possible interpretations of the data: that the bubbles indicate the inner shock of the fast wind, that the [Fe II] emission is evidence of the outer shock of an episodic wind, or that the emission is cooling line radiation from a photodissociation region. We argue that the first two interpretations are more likely, since the [Fe II] emission must be shock excited.