Physical Properties of Fullerene-containing Galactic Planetary Nebulae

Abstract

We searched the Spitzer Space Telescope data archive for Galactic planetary nebulae (PNe), that show the characteristic 17.4 and 18.9 µm features due to C 60 , also known as buckminsterfullerene. Out of 338 objects with Spitzer/IRS data, we found eleven C 60 -containing PNe, six of which (Hen2-68, IC2501, K3-62, M1-6, M1-9, and SaSt2-3) are new detections, not known to contain C 60 prior to this work. The strongest 17.4 and 18.9 µm C 60 features are seen in Tc 1 and SaSt 2-3, and these two sources also prominently show the C 60 resonances at 7.0 and 8.5 µm. In the other nine sources, the 7.0 and 8.5 µm features due to C 60 are much weaker. We analyzed the spectra, along with ancillary data, using the photo-ionization code CLOUDY to establish the atomic line fluxes, and determine the properties of the radiation field, as set by the effective temperature of the central star. In addition, we measured the infrared spectral features due to dust grains. We find that the Polycyclic Aromatic Hydrocarbon (PAH) profile over 6-9 µm in these C 60 -bearing carbon-rich PNe is of the more chemically-processed class A. The intensity ratio of 3.3 µm to 11.3 µm PAH indicates that the number of C-atoms per PAH in C 60 -containing PNe is small compared to that in non-C 60 PNe. The Spitzer spectra also show broad dust features around 11 and 30 µm. Analysis of the 30-µm feature shows that it is strongly correlated with the continuum, and we propose that a single carbon-based carrier is responsible for both the continuum and the feature. The strength of the 11-µm feature is correlated to the temperature of the dust, suggesting that it is at least partially due to a solid-state carrier. The chemical abundances of C 60 -containing PNe can be explained by AGB nucleosynthesis models for initially 1.5-2.5 M ⊙ stars with Z=0.004. We plotted the locations of C 60 -containing PNe on a face-on map of the Milky Way and we found that most of these PNe are outside the solar circle, consistent with low metallicity values. Their metallicity suggests that the progenitors are an older population.