The differential carbon abundances with respect to S285-6, deduced from the two C II lines at 6578 and 6582 , are summarised in Table 2, together with the parameters used in the model stellar atmosphere calculations. These show significant variations covering a range of approximately 0.6 dex. The re-reduction of the data for the cluster S289 confirms the large discrepancy between the carbon abundances in the stars S289-2 and S289-4, which was also observed for nitrogen (Smartt et al. 1996b).
Table 2. Atmospheric parameters together with absolute and differential C II abundances with respect to S285-6 for the programme and previously observed stars. The abundance in the stars marked * was determined from the equivalent width of the 6578 line only. For reference the differential abundances of oxygen and nitrogen, taken from Smartt et al. (1996a), are shown
Carbon differential abundances are plotted against those of nitrogen in Fig. 2a. Despite typical error bars of dex they clearly show a positive correlation. An unweighted linear least squares fit to the data generates a correlation coefficient of +0.76. Indeed if the point due to RLWT-41 (a star believed to have formed between the Local and Perseus spiral arms in a region of inefficient star formation and possibly abnormal metal abundances - see Smartt et al. 1996b) is ignored then this coefficient rises to +0.94. Also plotted in Fig. 2a is the locus representing equal changes in the carbon and nitrogen abundances. Most of the stars cluster around this locus implying that abundance changes for these two elements are similar in magnitude. For comparison, the lack of correlation between carbon and oxygen and nitrogen and oxygen is shown in Figs. 2b and c respectively.
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998