Joseph L. Hora 1 & Lynne K. Deutsch 2
1 Smithsonian Astrophysical Observatory
2 Boston University
The infrared (IR) emission from most planetary nebulae (PNe) is strongest in the far-IR, peaking in the 20 to 60 micron region corresponding to dust temperatures of 70 - 150 K. This cool dust component is thought to be due to a different population of grains than those responsible for the near- and mid-IR emission. Most of a PN=92s energy is emitted by this cool dust, so knowledge of the cool dust=92s properties, including its temperature and spatial distribution, is essential to understand the energetics of the nebula. Also, one can learn about the mass loss history of the star, and the process of dust return to the interstellar medium from the PN. Little is known about the properties of this cool dust component, however, because of the difficulty associated with observing at these wavelengths.
Observations performed prior to the ISO mission, e.g. from IRAS, found that the FWHM sizes of the PNe in the far-IR were comparable to the PN=92s optical size. However, these studies were limited by the low resolution of the IRAS scans.
We have observed a sample of eleven PNe using the PHT-C100 camera on ISO, obtaining maps at 60 and 100 microns. The PHT-32 mapping mode was used, resulting in images of the PNe and nearby sky background. A preliminary reduction of the data has confirmed that the radial extent of the cool dust in general is similar to the optical nebula, but is distributed differently than the ionized nebular emission. For example, in the best-resolved object in our sample (NGC 6853 - ``The Dumbbell''), there are two bright lobes of emission that are oriented at a position angle roughly 90 degrees apart from the angle of the brightest optical emission. We are currently re-reducing our data with the latest PIA software, and will present calibrated images at each wavelength for the objects in our sample, as well as temperature and opacity images and comparisons with optical and near-IR images. The temperature images will aid in determining the relative importance of Lyman-alpha versus stellar continuum heating mechanisms, and the opacity images will show the dust density variations in the PNe.