The properties of OB supergiants in the Large Magellanic Cloud. II - Spectral types and intrinsic colors

We have isolated a sample of about 90 of the most lightly reddened "normal" O and B supergiants in the LMC for a study of their spectroscopic and photometric properties. New MK spectral classifications are presented for 83 of the stars, about half of which have no previously reported slit spectral types. Intrinsic color relations are derived for the LMC supergiants using new observations in the Stromgren uvby system and published data in the Johnson UBV system. The shapes of the intrinsic UBV and uvby relations for the LMC stars are similar to published Galactic calibrations for Ia supergiants. In the Stromgren system, the derived spectral type versus intrinsic color calibration for the B supergiants also agrees well with Galactic calibrations. In the Johnson system, the LMC calibration is systematically redder, particularly in (U-B)0, than reported for Galactic stars. Because the LMC calibrations in the two systems have been derived in a mutally consistent manner using the same sample of stars, this suggests that the Galactic calibrations are not consistent. In other words, the Stromgren and Johnson colors attributed to a given Galactic B supergiant subtype cannot be produced by the same star. This inconsistency probably results from the small numbers and the different samples of Galactic supergiants being used in the various calibration studies and from uncertainties introduced by the typically much higher reddening of the Galactic stars. Comparison of the LMC intrinsic color calibrations with synthetic photometry of line-blanketed, plane-parallel, LTE model atmospheres shows that the B-type supergiant colors can be reproduced by this class of models.