ON THE ABUNDANCES OF NITROGEN AND SULFUR IN LATE-B THROUGH F SUPERGIANTS ATMOSPHERES

An extensive non-LTE analysis of NI and SI lines at the spectral regio n around lambda similar to 8700 Angstrom was performed for late-B thro ugh F supergiants in order to determine the photospheric abundances of N and S in such massive evolved stars. It was revealed that the abund ances of nitrogen in these stars show a rather large diversity (simila r to +/-0.4 dex) around the mean of [N/H] similar or equal to 0.3 rela tive to the Sun, being roughly divided into three groups: (i) markedly large overabundances amounting to 0.6-0.7 dex, (ii) mild N-excesses o f 0.3-0.4 dex, and (iii) near-solar (or slightly subsolar) [N/H]-value s. The abundances of sulfur for F-supergiants were found to be nearly homogeneous and not much different from the solar composition ([S/H] s imilar to -0.2), while those for the A-type generally show a tendency of S-excess (up to +0.8 dex) systematically increasing with T-eff. The re also appears a trend of mass-dependence as well as a possibility of mutual correlation regarding the values of [N/H] and [S/H] in those A -supergiants. It was confirmed that the extent of the N-excess in A-F supergiants tends to depend on that of sodium as [N/H] similar or equa l to [Na/H].