The average oxygen deficiency of =-0.18 dex for the SMC supergiants is similar to what is found in the LMC (-0.15 dex) and for Galactic supergiants and other young objects (-0.2 to -0.3 dex). This result is of great importance in understanding the chemical evolution of the Magellanic Clouds, and could mean that the Clouds and the Galaxy have achieved the same level of chemical evolution, although the Clouds would have had a star formation rate (per unit gas mass) lower than in the galaxy. However, to constrain models, the oxygen abundance in older objects such as 12 Gyr globular clusters and intermediate age objects ( 4Gyr) are badly needed, in particular, to discriminate between star formation occurring in bursts or continuously.
Carbon deficiencies ( =-0.3 dex) and nitrogen enhancements ( =+0.22 dex), together with the low 12 C/13 C of 10-20 are indicative of a mild convective mixing.
Lithium abundances are found within the range (Li)=0.0 to 0.6 dex, which are compatible with the values found for similar stars in the Galaxy, thus indicating that mixing has occured in the Magellanic K supergiants, in agreement with the measured 12 C/13 C and C/N ratios.
Carbon plus nitrogen ([C+N/Fe]) abundances are mildly deficient in our sample of K supergiants, whereas it appears overabundant in F supergiants in the SMC and the LMC. Since the lines used are different, there could be systematic effects in the abundance determinations. In the H II regions, it appears even more drastically deficient than in our K supergiants. In order to resolve this puzzle, it would be important to analyse larger samples of F, G, K supergiants, and to have a better understanding of the HII regions.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998