Dust in Planetary Nebulae

Although I am interested in the nebular physics of PNs, my main interest is the dust in and around PNs. We normally observe the dust either from the mid-infrared spectra of PNs (see below) or by direct imaging.

The image of PN NGC 7027 below, from HST observations by R. Ciardullo, shows a blue ring which is the ionized region, surrounded by dusty material which we see silhouetted against the ionized gas. The detailed study of this particular PN was instrumental in showing that PNs were previously dusty evolved stars, and that the hot star in the middle of the nebula is illuminating the shell of previously ejected material, rather than having the whole atmosphere of the star ejected in a sudden event to form the nebula. NGC 7027 is dustier than most PNs, and we can trace the dust extinction over the face of the nebula. For most PNs the dust is not obvious in optical images.

Image credit: NASA, ESA, and R. Ciardullo.

I study the dust in and around PNs to see how it compares to the dust in evolved stars and proto-planetary nebulae. I have obtained images of a number of PNs at 11.7 microns, where the dust is shining by thermal emission, for comparison with the optical images of the ionized gas emission. I also have a few ground-based spectra of some PNs which happen to not have satellite observations available.

Mid-Infrared Spectroscopy of PNs

The plot below shows a spectrum in N-band (around 10 microns wavelength) with the Michelle instrument at Gemini North of NGC 6881. It shows the dust continuum, the 11.3 micron UIR feature and three emission lines from the ionized gas. The lines are from Ne⁺, S⁺³, and Ar⁺², so there is a range of ionization within the PN. PNs are the only stars hot enough to produce S⁺³ lines. Even the hottest known O-type stars do not produce these lines in the 10 micron region.

This spectrum was taken with the slit oriented along the bright bar of the nebula (see the PN images page). The continuum between the emission lines is due to cool dust in the PN, with a typical temperature of roughly 150 K.

The above plot shows another ground-based spectrum taken at Gemini South. In this case a spectrum was taken by Spitzer which gives much better data.

The photometry from the T-ReCS observations agrees extremely well with the Spitzer spectrum when the sections of the Spitzer spectrum are scaled to match each other. The continuum fit is a blackbody of temperature 149 K normalized to the IRAS [25] band. There is a very weak feature in the spectrum at about 30 microns, which is probably the usual 30 micron feature seen in many dusty carbon-rich stars, but in this spectrum it is not very strong.

The T-ReCS images of this PN suggest that the dust producing the long wavelength continuum and the UIRs have very different distributions in the nebula, which if true is unusual (see the images page).

Ground-Based Mid-Infrared Images of PNs