INFRARED-SPECTRA OF MASSIVE STARS IN TRANSITION - WNL, OF, OF WN, BE,B[E], AND LUMINOUS BLUE VARIABLE-STARS/

We present an overview of the spectroscopic properties of several lumi nous stars belonging to Of supergiant, Ofpe/WN9, late-type nitrogen se quence Wolf-Rayet (WNL), B[e], Be supergiant, and luminous blue variab le (LBV) classes at 2 mu m and 1.6 mu m, using recently published and new spectra of moderate to high resolution (500-1600). These objects a re ''transitional'' in their optical. classification and may be relate d in their evolution. The 2 mu m spectrum of the LBV AC Car has change d from its 1984-1985 appearance as an Of/WN star, now appearing as a B e or B[e] star by comparison to new spectra of HD 72754 (B2Iape) and G G Car (B[e], whose spectrum has varied since 1984-1985). Further suppo rt for a link between B[e] stars and LBVs is seen in the case of the S MC B[e] star S18, which has changed its 1987-1989 spectrum from one of Br gamma emission only to one in which emission lines of Fe II, Mg II , Na I, (CO)-C-12 overtone, He I 2.112-3 mu m, and He I 2.058 mu m (st rong) are present. As the earlier observations were not of a high sign al-to-noise ratio, we confirm only the He I and (CO)-C-12 as new emiss ion, where the latter was previously expected on the basis of TiO emis sion but was undetected. The overall morphology of the atomic spectrum of S18, including the He I 2.112-3 mu m emission, are shared by only one other B star, namely, the quiescent LBV P Cyg (B1 Ia(+)), but is s hared also by the LBV and Ofpe/WN9 star HDE 269582. Only AG Car is obs erved to have also varied in the 2.112-3 mu m line. We thus consider S 18 a strong candidate LBV. Our new high-resolution 2 mu m spectra also include HD 5980, a WN binary recently observed to undergo an LBV-like outburst in short-term brightness and spectroscopic variations. We pr ovide a detailed K-band line identification of the probable LBV He 3-5 91 (WRA 751), which is rich in lines of permitted and forbidden iron ( mainly Fe II) and includes new identification of [Ni II] at 2.308 mu m and 2.369 mu m. A significant degree of overlap in spectral morpholog y exists between the groups, where at least one example from each grou p may be classified as a member of another from its 2 mu m spectrum. T his has serious consequences for observations of hot, luminous objects in visually obscured regions. The overlapping infrared spectral morph ology reinforces the notion that the objects in this study are interre lated in their evolution. We propose that ''transitional'' massive sta rs with hydrogen present at their surfaces (including the least extrem e WN types) may not yet be in the stage of core-helium burning but rat her are in a previous phase in which the stellar atmosphere/wind is so metimes dynamically unstable.