García-Hernández, D. A.; Villaver, E.; García-Lario, P.; Acosta-Pulido, J. A.; Manchado, A.; Stanghellini, L.; Shaw, R. A.; Cataldo, F.
Bibliographical reference
The Astrophysical Journal, Volume 760, Issue 2, article id. 107, 16pp, (2012).
Description
We present a study of 16 planetary nebulae (PNe) where fullerenes have
been detected in their Spitzer Space Telescope spectra. This large
sample of objects offers a unique opportunity to test conditions of
fullerene formation and survival under different metallicity
environments because we are analyzing five sources in our own Galaxy,
four in the Large Magellanic Cloud (LMC), and seven in the Small
Magellanic Cloud (SMC). Among the 16 PNe studied, we present the first
detection of C60 (and possibly also C70)
fullerenes in the PN M 1–60 as well as of the unusual ~6.6, 9.8,
and 20 μm features (attributed to possible planar C24) in
the PN K 3–54. Although selection effects in the original samples
of PNe observed with Spitzer may play a potentially significant role in
the statistics, we find that the detection rate of fullerenes in C-rich
PNe increases with decreasing metallicity (~5% in the Galaxy, ~20% in
the LMC, and ~44% in the SMC) and we interpret this as a possible
consequence of the limited dust processing occurring in Magellanic Cloud
(MC) PNe. CLOUDY photoionization modeling matches the observed IR fluxes
with central stars that display a rather narrow range in effective
temperature (~30,000-45,000 K), suggesting a common evolutionary status
of the objects and similar fullerene formation conditions. Furthermore,
the data suggest that fullerene PNe likely evolve from low-mass
progenitors and are usually of low excitation. We do not find a
metallicity dependence on the estimated fullerene abundances. The
observed C60 intensity ratios in the Galactic sources confirm
our previous finding in the MCs that the fullerene emission is not
excited by the UV radiation from the central star. CLOUDY models also
show that line- and wind-blanketed model atmospheres can explain many of
the observed [Ne III]/[Ne II] ratios using photoionization, suggesting
that possibly the UV radiation from the central star, and not shocks, is
triggering the decomposition of the circumstellar dust grains. With the
data at hand, we suggest that the most likely explanation for the
formation of fullerenes and graphene precursors in PNe is that these
molecular species are built from the photochemical processing of a
carbonaceous compound with a mixture of aromatic and aliphatic
structures similar to that of hydrogenated amorphous carbon dust.
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Low- to intermediate-mass (M < 8 solar masses, Ms) stars represent the majority of stars in the Cosmos. They finish their lives on the Asymptotic Giant Branch (AGB) - just before they form planetary nebulae (PNe) - where they experience complex nucleosynthetic and molecular processes. AGB stars are important contributors to the enrichment of the
Domingo Aníbal
García Hernández
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