Bipolar Symbiotic Planetary Nebulae in the Thermal Infrared: M2-9, Mz 3, and He 2-104

We present thermal-infrared images of three extreme bipolar objects, M2-9, Mz 3, and He 2-104. They are bipolar planetary nebulae with bright central stars and are thought to be powered by symbiotic binary systems. The mid-infrared images spatially resolve the spectral energy distributions of the central engines from the surrounding nebulae. A warm dust component of several hundred degrees can account for the core emission, while a cooler component of ~100 K produces the more extended emission from the bipolar lobes. In every case, the dust mass for the unresolved core region is orders of magnitude less than that in the extended lobes, raising doubts that the hypothetical disks in the core could have been responsible for pinching the waists of the nebulae. We find total masses of roughly 0.5-1 M_solar in the nebulae of M2-9 and Mz 3, requiring that this material was donated by intermediate-mass progenitor stars. The mass of He 2-104's nebula is much lower, and any extended emission is too faint to detect in our images. Extended dust emission is detected around both M2-9 and Mz 3, in both cases resembling the distribution of ionized gas. Our images of Mz 3 have the highest signal-to-noise ratio in the extended polar lobes, and we show that the fairly uniform color temperature derived from our images can explain the 110 K dust component that dominates the far-infrared spectral energy distribution. In the case of Mz 3, most of the mass traced by dust is concentrated at high latitudes, and we note possible evidence for grain destruction in shocks indicated by an anticorrelation between [Fe II] and dust emission. Except for these regions with enhanced [Fe II] emission, the dust continuum closely resembles the distribution of ionized gas.