C60_C70_in_space - Detection of C60 and C70 in a Young...

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/ www.sciencexpress.org / 22 July 2010 / Page 1 / 10.1126/science.1192035 In the last decades, a number of molecules and diverse dust features have been identified by astronomical observations in various environments. Most of the dust that determines the physical and chemical characteristics of the interstellar medium (ISM), is formed in the outflows of asymptotic giant branch (AGB) stars and is further processed when these objects become planetary nebulae (PNe). Here, we report on the environment of Tc 1, a peculiar PN whose infrared spectrum shows emission from cold and neutral C 60 and C 70 . The two molecules amount to a few percent of the available cosmic carbon in this region, showing that if the conditions are right, fullerenes can and do form efficiently in space. Interstellar dust makes up only a small fraction of the matter in our galaxy, but plays a crucial role in the physics and chemistry of the ISM and star forming regions ( 1 ). The bulk of this dust is created in the slow (5–20 km/s), but massive (10 ! 8 –10 ! 4 solar masses per year) outflows of old, low-mass AGB stars ( 2 4 ). Once most of the envelope is ejected, the AGB phase ends and the stellar core—a hot white dwarf— becomes gradually more exposed. When this white dwarf ionizes the stellar ejecta, they become visible as a PN. Chemical reactions and nucleation in the AGB outflows transform the atomic gas into molecules and dust grains. For carbon-rich AGB stars (or carbon stars), this results in a large variety of carbonaceous compounds—to date, more than 60 individual molecular species and a handful of dust minerals have been identified in these outflows ( 5 ), including benzene, polyynes and cyanopolyynes up to about 13 atoms in size ( 6 , 7 ). These environments are also thought to be the birthplace for large aromatic species such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes ( 8 , 9 ), a class of large carbonaceous molecules that were discovered in laboratory experiments aimed at understanding the chemistry in carbon stars ( 10 ). Fullerenes have unique physical and chemical properties and therefore the detection of fullerenes and the identification of their formation site are considered one of the priorities in the field of interstellar organic chemistry ( 11 ). However, astronomical searches for fullerenes in interstellar and circumstellar media have not resulted in conclusive evidence ( 12 14 ). The most promising case to date is the detection of two diffuse interstellar bands (DIBs) in the near- infrared ( 15 ) whose wavelengths are close to laboratory spectra of C + 60 in solid matrices ( 16 ) and awaits confirmation from comparison to a cold, gas-phase spectrum. Here we report on the detection of the fullerenes C
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This note was uploaded on 07/24/2011 for the course CHEM 200 taught by Professor Stephenbest during the Two '11 term at University of Melbourne.

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C60_C70_in_space - Detection of C60 and C70 in a Young...

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