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Astron. Astrophys. 328, 175-186 (1997)

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High resolution infrared spectroscopy of CN and NH lines: nitrogen abundance in oxygen-rich giants through K to late M *

W. Aoki 1, 2 and T. Tsuji 1

1 Institute of Astronomy, The University of Tokyo, Osawa, Mitaka, Tokyo 181, Japan
2 Department of Astronomy, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Received 2 January 1997 / Accepted 7 April 1997

Abstract

The analyses of high resolution infrared spectra have been done for CN lines in oxygen-rich cool evolved stars including 2 K giants, 20 M giants and 1 S-type star. Since CN lines analyzed in the present work are weak and resolved well, they are appropriate for quantitative analyses. CN lines of [FORMULA] and -1 sequences (red system) which are in the K - and the H -window regions, respectively, give the consistent nitrogen abundance for each star. The analyses of NH lines in the L -window region have been done for 5 late M giants for which CN lines have been also analyzed. Although the triplet structure of NH lines cannot be fully resolved, they are preferable because determination of nitrogen abundance is almost independent of other elemental abundances while nitrogen abundance based on CN depends on carbon abundance. The nitrogen abundances derived from NH for late M giants agree well with those from CN for which we adopt 7.75 eV as the dissociation energy in the analysis.

The results show that the nitrogen abundances in late M giants are larger than those in early M giants while decrease of the carbon abundance was found in late M giants by our previous work (Tsuji 1991). These variations of abundances can not be explained by the first dredge-up model but require additional processing by the CN cycle and mixing after the first dredge-up. However, there is no obvious evidence of other processes such as the [FORMULA] -process and subsequent hot bottom burning in our program stars. Such variation of the carbon and nitrogen abundances is not well understood by the present evolutionary models of low-mass and intermediate-mass stars.

Key words: stars: abundances – stars: atmospheres – stars: late-type – stars: AGB and post AGB

* Table 10 is only available in electronic form at the CDS (Strasbourg) via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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