期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 103, 期 33, 页码 12274-12279出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0602277103
关键词
asymptotic giant branch stars; astrochemistry; molecular line observations
Mass loss from evolved stars results in the formation of unusual chemical laboratories: circumstellar envelopes. Such envelopes are found around carbon- and oxygen-rich asymptotic giant branch stars and red supergiants. As the gaseous material of the envelope flows from the star, the resulting temperature and density gradients create a complex chemical environment involving hot, thermodynamically controlled synthesis, molecule freeze-out, shock-initiated reactions, and photochemistry governed by radical mechanisms. in the circumstellar envelope of the carbon-rich star IRC+10216, > 50 different chemical compounds have been identified, including such exotic species as C8H, C3S, SiC3, and AlNC. The chemistry here is dominated by molecules containing long carbon chains, silicon, and metals such as magnesium, sodium, and aluminum, which makes it quite distinct from that found in molecular clouds. The molecular composition of the oxygen-rich counterparts is not nearly as well explored, although recent studies of VY Canis Majoris have resulted in the identification of HCO+, SO2, and even NaCl in this object, suggesting chemical complexity here as well. As these envelopes evolve into planetary nebulae with a hot, exposed central star, synthesis of molecular ions becomes important, as indicated by studies of NGC 7027. Numerous species such as HCO+, HCN, and CCH are found in old planetary nebulae such as the Helix. This survivor molecular material may be linked to the variety of compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon-rich fragments originally formed in circumstellar shells.
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