Baltic Astronomy, vol.1, 216-238, 1992. CLASSIFICATION OF POPULATION II SUPERGIANTS AND RELATED STARS IN THE VILNIUS SYSTEM A. Bartkevicius, R. Lazauskaite and G. Tautvaisiene Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania Received February 26, 1992 . Abstract. The results of photometric classification in the Vilnius system of 117 Population II supergiants, suspected supergiants and related stars are given. Their photometric spectral types, intrinsic colour indices (Y-V)o, colour excesses Ey-v, metallicities [Fe/H] and absolute magnitudes My are determined. It is shown that the system allows us to detect the UU Herculis-type supergiants pho- tometrically. The analyzed SRd star sample falls into two groups of metallicity and luminosity. Our photometric classification assigns luminosity classes from III to V to 37 F-K stars with |6| > 16° clas- sified by Bartaya (1979) from objective prism spectra as supergiants. Key words: supergiant stars - Population II stars - UU Herculis- type stars - photometric classification - Vilnius photometric system 1. Introduction A part of Population II stars evolve into supergiants when they reach the tip of the asymptotic gi;ant branch (AGB) and the post- AGB stage. Space density of such stars, because of their rapid evo- lution, is not as high as for other Population II stars (e.g. subdwarfs, metal-deficient giants, RR Lyrae-ty pe stars, horizontal branch stars). This is also confirmed by the H-R diagrams of globular clusters. How- ever, high luminosity and location sd high galactic latitudes where in- terstellar extinction is low allows us to observe a rather large number Send offprint requests to: A. Bartbevicius Classification of Population II supergiants and related stars 217 of these stars even among bright stars. Similarly to other Population II stars, this group of stars can be distinguished by low metallicity, high space velocity or location at high galactic latitudes. Some types of variable stars such as SRd, W Virginis and RV Tauri also belong to Population II supergiants. The description of these variables is pre- sented in the monograph "Metal-deficient stars" by Straizys (1982) and in the catalogue of Population II supergiants of A-F spectral classes (Bartkevicius, 1992). Population II supergiants were first detected by Bidelman (1951). Later on there followed discussion concerning their chem- ical composition, formation and evolution (Abt, 1960; Searle et al., 1963; Fernie, 1981; Fernie and Garrison, 1984). The discussion be- came especially active after the works of Sasselov (1984, 1986), Bond (1970), Luck et al. (1983) and Luck and Bond (1984a,b). At first the UU Herculis-type supergiants were considered of normal metal- licity. Later on Bond and his group showed that a part of Popula- tion II supergiants exhibit mild and intermediate metal-deficiency. Five F supergiants located at high galactic latitudes were attributed by Sasselov to the UU Herculis-type group. The group includes BD+38°2803 (UU Her, F2 Ib-cF8), HD 112374 (BS 4912, LN Hya, F3 la), HD 161796 (V 814 Her, F3 lb), HD 163506 (89 Her, BS 6685, V 441 Her, F2 Ibe) and HD 177300 (BL Tel, F5 I + M). Sasselov (1984) also has suspected that a large number of UU Herculis-type stars may be among F-supergiants at high galactic latitudes clas- sified by Bartaya (1979) from the objective prism spectra of 166 A/mm dispersion and has presented an additional list of 33 suspected UU Herculis-type stars selected from this catalogue. The mentioned works encouraged us to analyse the properties of Population II su- pergiants using the Vilnius photometric system. 2. Observational data and method of classification The Vilnius system colour indices of 117 Population II super- giants and related stars have been taken from Lazauskaite and Taut- vaisiene (1990) and Straizys et al. (1989) and after dereddening are presented in Table 1 along with their equatorial coordinates, V mag- nitudes and the number of observations. The sample of stars under investigation consists of supergiants with \Z\ > 400 pc, a few well-known UU Herculis-type stars, su- pergiants of spectral classes F, G and K with |6| > 16° classified by Bartaya (1979) and variables of SRd and RV Tauri types. Some 218 A. Bartkevicius, R. Lazauskaité and G. 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