The star formation and therefore the chemical history of the Magellanic Clouds are different from that which occurred in the Galaxy. Detailed abundance determinations for different objects is essential to constrain efficiently the chemical evolution models. With the present day equipment, only the brightest youngest populations can be observed at high resolution, namely, supergiant stars, H II regions, Planetary Nebulae (PN) and Supernovae Remnants (SNR).
The CNO elements are key-elements for a better understanding of nucleosynthesis history. Oxygen is a reliable tracer of Supernovae Type II (SN II), carbon plus nitrogen abundances reflect rather the enrichment by asymptotic giant branch winds, whereas carbon deficiencies and nitrogen enhancements indicate internal convective mixing effects in the stars. In the SMC, the carbon abundance has raised much interest since Dufour's (Dufour et al. 1982; Dufour 1984) very low carbon abundance determination for the H II regions, and the work by Rocca-Volmerange et al. (1981) showing that the extinction curve does not show the 2200 bump generally attributed to graphite grains.
Concerning the stars in the field of the SMC, the C, N, O elements are available only for a dozen objects (cf. Table 4), among which F supergiants (Luck & Lambert 1992, hereafter LL92; Russell & Bessell 1989, hereafter RB89; Spite et al. 1989a, hereafter SBS89) and B stars (Rolleston et al. 1993).
In the present work, we derive C, N and O abundances for a sample of six field cool (K) supergiant stars from high resolution échelle spectra The basic stellar parameters for this sample, effective temperature (), surface gravity (log g), metallicity ([Fe/H]) and microturbulent velocity () were taken from Hill et al. (1996, hereafter Paper I).
In Sect. 2 we describe the observations. The stellar parameters for the sample stars are presented in Sect. 3. CNO abundances are derived in Sect. 4. Concluding remarks are given in Sect. 5.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998