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

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 ∆v =−2 and −1 sequences (red system) which are in the Kand the H-window regions, respectively, give the consistent nitrogen abundance for each star. The analyses of NH lines in the Lwindow region have been done for 5 late M giants for which CN lines have been also analyzed. Although the triplet structure ofNH 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 abundancewas found in lateMgiants 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 dredgeup. However, there is no obvious evidence of other processes such as the 3α-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.