The following parameters are determined for 63 Galactic supergiants in the solar neighbourhood: effective temperature T eff ; surface gravity log g; iron abundance log (Fe); microturbulent parameter V t ; mass M/M ; age t and distance d. A significant improvement in the accuracy of the determination of log g, and all parameters dependent on it, is obtained through application of van Leeuwen rereduction of the Hipparcos parallaxes. The typical error in the log g values is now ±0.06 dex for supergiants with distances d < 300 pc and ±0.12 dex for supergiants with d between 300 and 700 pc; the mean error in T eff for these stars is ±120 K. For supergiants with d > 700 pc, parallaxes are uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3 dex.A new T eff scale for A5-G5 stars of luminosity classes Ib-II is presented. Spectral subtypes and luminosity classes of several stars are corrected. Combining the T eff and log g with evolutionary tracks, stellar masses and ages are determined; a majority of the sample has masses between 4 and 15 M and, hence, their progenitors were early to middle B-type main-sequence stars.Using Fe II lines, which are insensitive to departures from local thermodynamic equilibrium, the microturbulent parameter V t and the iron abundance log (Fe) are determined from highresolution spectra. The parameter V t is correlated with gravity: V t increases with decreasing log g. The mean iron abundance for the 48 supergiants with distances d < 700 pc is log (Fe) = 7.48 ± 0.09, a value close to the solar value of 7.45 ± 0.05, and thus the local supergiants and the Sun have the same metallicity.

Using new accurate fundamental parameters of 30 Galactic A and F supergiants, namely their effective temperatures T eff and surface gravities log g, we implemented a non-LTE analysis of the nitrogen abundance in their atmospheres. It is shown that the non-LTE corrections to the N abundances increase with T eff . The nitrogen overabundance as a general feature of this type of stars is confirmed. A majority of the stars have a nitrogen excess [N/Fe] between 0.2 and 0.9 dex with the maximum position of the star's distribution on [N/Fe] between 0.4 and 0.7 dex. The N excesses are discussed in light of predictions for B-type main sequence (MS) stars with rotationally induced mixing and for their next evolutionary phase, i.e. A-and F-type supergiants that have experienced the first dredge-up. Rotationally induced mixing in the MS progenitors of the supergiants may be a significant cause of the nitrogen excesses. When comparing our results with predictions of the theory developed for stars with the mixing, we find that the bulk of the supergiants (28 of 30) show the N enrichment that can be expected (i) either after the MS phase for stars with the initial rotational velocities v 0 = 200-400 km s −1 (ii) or after the first dredge-up for stars with v 0 = 50-400 km s −1 . The latter possibility is preferred on account of the longer lifetime for stars on red-blue loops following the first dredge-up. Two supergiants without a discernible N enrichment, namely HR 825 and HR 7876, may be post-MS objects with the relatively low initial rotational velocity of about 100 km s −1 . The suggested range for v 0 is approximately consistent with inferences from the observed projected rotational velocities of B-type MS stars, progenitors of A and F supergiants.

Effective temperatures Teff, surface gravities log g and interstellar extinctions AV are found for 107 B stars. Distances d of the stars, which are based on the derived Teff, log g and AV values, show good agreement with those obtained from the Hipparcos parallaxes. Comparing the Teff and log g values with evolutionary computations, we infer masses, radii, luminosities, ages and relative ages of the stars. Empirical relations between the Teff and log g parameters, on the one hand, and the photometric indices Q, [c1] and β, on the other hand, are constructed; these relations give a fast method for the Teff and log g estimation of early and medium B stars. Inclusion of the infrared J, H and K colours into the Teff, log g and AV determination shows that (i) the Teff and log g parameters are altered only slightly; (ii) the AV value is rather sensitive to these colours, so an accuracy better than 0.05 mag in the JHK data is necessary for precise AV evaluation.

From high-resolution spectra a non-local thermodynamic equilibrium analysis of the Mg II 4481.2-Å feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line Al III 4479.9-Å is considered. The magnesium abundance is determined; it is found that log ε(Mg) = 7.67 ± 0.21 on average. It is shown that uncertainties in the microturbulent parameter V t are the main source of errors in log ε(Mg). When using 36 stars with the most reliable V t values derived from O II and N II lines, we obtain the mean abundance log ε(Mg) = 7.59 ± 0.15. The latter value is precisely confirmed for several hot B stars from an analysis of the Mg II 7877-Å weak line. The derived abundance log ε(Mg) = 7.59 ± 0.15 is in excellent agreement with the solar magnesium abundance log ε (Mg) = 7.55 ± 0.02, as well as with the proto-Sun abundance log ε ps (Mg) = 7.62 ± 0.02. Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity.

Based on our prior accurate determination of fundamental parameters for 36 Galactic A-, F-and G-type supergiants and bright giants (luminosity classes I and II), we undertook a non-LTE analysis of the carbon abundance in their atmospheres. It is shown that the non-LTE corrections to the C abundances derived from C I lines are negative and increase with the effective temperature T eff ; the corrections are especially significant for the infrared C I lines with wavelengths 9060-9660Å. The carbon underabundance as a general property of the stars in question is confirmed; a majority of the stars studied has the carbon deficiency [C/Fe] between -0.1 and -0.5 dex, with a minimum at -0.7 dex. When comparing the derived C deficiency with the N excess found by us for the same stars earlier, we obtain a pronounced N vs. C anti-correlation, which could be expected from predictions of the theory. We found that the ratio [N/C ] spans mostly the range from 0.3 to 1.7 dex. Both these enhanced [N/C ] values and the C and N anomalies themselves are an obvious evidence of the presence on a star's surface of mixed material from stellar interiors; so, a majority of programme stars passed through the deep mixing during the main sequence (MS) and/or the first dredge-up (FD) phase. Comparison with theoretical predictions including rotationally-induced mixing shows that the stars are either post-MS objects with the initial rotational velocities V 0 = 200-300 km/s or post-FD objects with V 0 = 0-300 km/s. The observed N vs. C anti-correlation reflects a dependence of the C and N anomalies on the V 0 value: on average the higher V 0 the greater the anomalies. It is shown that an absence of detectable lithium in the atmospheres of the stars, which is accompanied with the observed N excess and C deficiency, is quite explainable.