Mid-Infrared Observations of Planetary Nebulae

Abstract

We have obtained mid-infrared images of 21 planetary nebulae chosen based on estimated N-band surface brightness. In most cases the PNs were imaged using three filters, one of which was a medium-band 11.7 μm filter that only includes dust continuum emission. Most of the images presented here are from this filter. Two objects, Vo1 and Hb12, were unresolved although they are brighter than many of the resolved objects, indicating distinctly different dust properties than normal for these nebulae. Of the other objects only NGC 6210 shows distinct morphologies between the 11.7μm filter and the optical emission line images, as well as images in the N′ filter which includes the 10.5 and 12.8μm lines. Almost all objects have very similar optical and mid-infrared morphologies, indicating that the dust is generally uniformly mixed through the ionized region, possibly in the form of globules. This is particularly interesting for objects such as Pe1-7, NGC 6881, and Fg3 which show faint lobes as well as a bright central region in both optical emission line and mid-infrared dust emission images. The large differences in surface brightness for these objects may reflect changes in the grain size or in optical depth. Three objects were of unknown dust composition due to a lack of previous mid-infrared spectroscopy. Fg3 and H1-35 are shown to have silicate grains and NGC 6881 is shown to have carbon-based grains from our observations. We also have resolved the protoplanetary nebula IRAS 19500−1709, which is about two times smaller than expected based on dust radiative transfer models of its Infrared Space Observatory (ISO) spectrum. This probably indicates that the dust grains are much larger than expected, of order 2 μm in radius. Figure 1 below shows our images. All are given in Jy/square arc-second based upon direct calibration of the imaging observations. Where ISO or IRAS spectra are available, the in-band fluxes we derive are usually very close to what is expected from the space observations, but for a few objects there are significant differences at about the 40% level. We intend to derive dust colour temperature and optical depth maps from these images, to seek indications of whether optical depth or dust size plays a role in the wide range of surface brightnesses in these PNs.