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.

A search for hydrogenated fullerenes in fullerene-containing planetary nebulae

“…However, it has been demonstrated that fullerenes are efficiently formed only in H-rich circumstellar environments (García-Hernández et al 2010, 2011b, challenging our previous understanding of the formation of fullerenes in space. In particular, the simultaneous detection of C 60 fullerenes and PAHs in several PNe with normal hydrogen abundances has been reported (García-Hernández et al 2010, 2011a, 2012Otsuka et al 2014). García-Hernández et al (2010) propose that both fullerenes and PAHs in H-rich circumstellar ejecta may form from the photochemical processing of hydrogenated amorphous carbon (HAC) in agreement with the experimental results of Scott et al (1997) 1 .…”

Planetary nebula progenitors that swallow binary systems

“…M 1-6 (PN G211.2-03.5). This PN has two lobes, and a smaller and denser inner region (Sahai et al 2011;Otsuka et al 2014). Its image is presented in Fig.…”

New Identifications of the CCH Radical in Planetary Nebulae: A Connection to C60?

“…In the former scheme, hollow structures containing aromatic clusters connected by aliphatic bridging groups are created, which are subsequently dehydrogenated by UV radiation, leading to large fullerene cages that shrink to C 60 and C 70 by releasing C 2 (Micelotta et al 2012). Indeed, broad 6-9 μm emission, which may be associated with HACs (García-Hernández et al 2010Otsuka et al 2014), was seen in many of the fullerene-containing Magellanic Cloud PNe observed by García-Hernández et al (2011), as well as in the three PNe studied by Bernard-Salas et al (2012). Model spectra of 3 nm HAC particles produced by Bernard-Salas et al (2012) show that this broad feature can be reproduced relatively successfully, adding credence to this hypothesis.…”

“…Alternatively, nondetection of CCH in these nebulae could result from the large distances to these PNe (3.9 kpc and 7.3 kpc, respectively; Otsuka et al 2014) and the lack of sensitivity rather than the absence of CCH in either source. Using the upper limits for the peak line temperatures of the N = 3 → 2 transitions and the line width for M1-12 from Zhang et al (2000), an upper limit column density of 4.4 × 10 15 cm −2 (considering clumping) was calculated for both sources.…”