Physical Properties of Fullerene-containing Galactic Planetary Nebulae
  We searched the Spitzer Space Telescope data archive for Galactic planetary
nebulae (PNe), that show the characteristic 17.4 and um features due to C60,
also known as buckminsterfullerene. Out of 338 objects with Spitzer/IRS data,
we found eleven C60-containing PNe, six of which (Hen2-68, IC2501, K3-62, M1-6,
M1-9, and SaSt2-3) are new detections. 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 um in these C60-bearing carbon-rich
PNe is of the more chemically-processed class A. The intensity ratio of 3.3 um
to 11.3 um PAH indicates that the number of C-atoms per PAH in C60-containing
PNe is small compared to that in non-C60 PNe. The Spitzer spectra also show
broad dust features around 11 and 30 um. Analysis of the 30-um 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-um 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 C60-containing PNe can be explained by AGB
nucleosynthesis models for initially 1.5-2.5 Msun stars with Z=0.004. We
plotted the locations of C60-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.
