Mid-Infrared Imaging of the Bipolar Planetary Nebula M2-9 from SOFIA
  We have imaged the bipolar planetary nebula M2-9 using SOFIA's FORCAST
instrument in six wavelength bands between 6.6 and 37.1 $\mu m$. A bright
central point source, unresolved with SOFIA's $\sim$ 4${''}$-to-5${''}$ beam,
is seen at each wavelength, and the extended bipolar lobes are clearly seen at
19.7 $\mu m$ and beyond. The photometry between 10 and 25 $\mu m$ is well fit
by the emission predicted from a stratified disk seen at large inclination, as
has been proposed for this source by Lykou et al and by Smith and Gehrz. The
principal new results in this paper relate to the distribution and properties
of the dust that emits the infrared radiation. In particular, a considerable
fraction of this material is spread uniformly through the lobes, although the
dust density does increase at the sharp outer edge seen in higher resolution
optical images of M2-9. The dust grain population in the lobes shows that small
($<$ 0.1 $\mu m$) and large ($>$ 1 $\mu m$) particles appear to be present in
roughly equal amounts by mass. We suggest that collisional processing within
the bipolar outflow plays an important role in establishing the particle size
distribution.
