Optical spectrum of the bipolar nebula AFGL 2688 |
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Authors: | V G Klochkova R Szczerba V E Panchuk |
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Institution: | (1) Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, Karachaevo-Cherkessiya, 369167, Russia;(2) N. Copernicus Astronomical Center, Torun, Poland |
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Abstract: | Based on echelle spectra obtained at the prime focus of the 6-m telescope, we have determined for the first time the detailed chemical composition of one of the components of the bipolar nebula identified with the intense infrared source AFGL 2688 by the model-atmosphere method. The iron abundance Fe/H]=?0.59 dex derived for AFGL 2688 suggests that the object probably belongs to an intermediate population of the Galaxy. The stellar atmosphere exhibits high carbon and nitrogen abundances, C/Fe]=+0.73 and N/Fe]=+2.00, and C/O>1, confirming that the object is at the post-AGB stage. However, the detected overabundance of s-process elements (yttrium and barium) relative to iron is modest: X/Fe]=+0.55. The lanthanides are even less enhanced: for La, Ce, Pr, and Nd, the mean abundance relative to iron is la/Fe]=+0.26. This behavior of the heavy metals is consistent with the low intensity of the 21-μm band in the infrared spectrum of AFGL 2688; the intensity of this emission band is great in the spectra of all the studied PPN with large overabundances of s-process elements. An analysis of the radial velocities measured from spectral features originating in the atmosphere and in the circumstellar shell has revealed a high-velocity (~60 km s?1) component of the stellar wind from AFGL 2688. |
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Keywords: | bipolar nebula abundance analysis |
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