The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud Planetary Nebulae
D. A. Garcia-Hernandez, S. Iglesias-Groth, J. A. Acosta-Pulido, A. Manchado, P. Garcia-Lario, L. Stanghellini, E. Villaver, Richard A. Shaw, F. Cataldo
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Accepted for publication in The Astrophysical Journal Letters (16 pages, 2 Tables and 3 figures)
We present ten new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C60 and C70 abundances in space. Also, we report evidence for the possible detection of planar C24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ~6.6, 9.8, and 20 um. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), PAH clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C24). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young PNe. This is supported by the fact that many of our fullerene-detected PNe show altered [NeIII]/[NeII] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.
我们提出了一个研究16 PNe富勒烯已被发现在他们的斯皮策光谱。这个大样本的对象提供了一个独特的机会来测试条件下的富勒烯形成和生存在不同的金属环境，因为我们正在分析五个来源在我们自己的银河系，四个在LMC，和七个在SMC。在研究中的16个PNe中，我们首次检测到PN M 1-60中的C60（也可能是C70）富勒烯，以及PN K 3-54中不寻常的6.6，9.8和20 um特征（可能是平面C24）。虽然选择效应在原始样品中观察到的PNE与斯皮策可能会发挥潜在的重要作用的统计数据，我们发现，富勒烯的检测率在C丰富的PNE的金属含量降低（5%的银河系，20%的LMC，和44%的SMC）增加。云光电离模型将观测到的红外通量与中心恒星相匹配，这些恒星的有效温度范围相当窄（30，000 - 45，000 K），表明这些物体的共同进化状态和类似的富勒烯形成条件。在银河系源中观测到的C60强度比证实了我们先前在MC中的发现，即富勒烯发射不是由来自中心恒星的紫外辐射激发的。云模型还表明，线和风覆盖的模型大气可以通过光电离解释许多观测到的[NeIII]/[NeII]比率，这表明可能是来自中央恒星的紫外线辐射，而不是冲击，引发了星周尘埃颗粒的分解。手头的数据，我们建议，最有可能的解释形成的富勒烯和石墨烯前体在PNe是，这些分子物种是建立从光化学处理的含碳化合物的芳香族和脂肪族结构的混合物类似的HAC灰尘…
Formation and delivery of complex organic molecules to the Solar System and early Earth
现在已知，从渐进巨分支星到行星状星云的恒星演化的后期阶段是分子合成的活跃阶段。超过80个气相分子已被检测到通过在mm/submm区域的旋转跃迁。红外光谱也检测到无机矿物，富勒烯和有机固体。这些分子和固体的合成发生在非常低的密度（$<10^6$ cm $^{-3}$）和短的（$\sim10^3$ yr）时间尺度上。复杂的有机物被观察到具有混合的芳香族/脂肪族结构，并且可能与在陨石、彗星、行星际尘埃颗粒和行星卫星中发现的复杂有机物有关。恒星和太阳系有机物之间的可能联系进行了讨论。
