Complex Organic and Inorganic Compounds in Shells of Lithium-rich K Giant Stars

Abstract

Hydrocarbon organic material, as found in the interstellar medium, exists in complex mixtures of aromatic and aliphatic forms. It is considered to be originated from carbon enriched giant stars during their final stages of evolution, when very strong mass loss occurs in a few thousand years on their way to become planetary nebulae. We show here that the same organic compounds appear to be formed in previous stages of the evolution of giant stars. More specifically, during the first ascending giant branch K-type stars. According to our model this happens only when these stars are being abruptly enriched with lithium together with the formation of a circumstellar shell with a strong mass loss during just a few thousand years. This sudden mass loss is, on an average, a thousand times larger than that of normal Li-poor K giant stars. This shell would later be detached, specially when the star stops its Li enrichment and a rapid photospheric Li depletion occurs. In order to gain extra carbon-based material to form the organic hydrocarbons, and also to explain the presence of complex inorganic compounds in these stars, we propose an interaction of these strong winds with remaining asteroidal/cometary disks that already existed around these stars since they were dwarf A-type stars. The mechanism of interaction presented here is successful to explain the presence of inorganic compounds, however it is unable to produce new carbon free atoms to form the organic hydrocarbon compounds. Finally, we discuss some suggestions and speculations that can eventually help solving the long-standing puzzle of Li-rich giants.

arXiv commentAccepted for publication in ApJ, 19 pages, 5 figures, 1 table

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