Baltic Astronomy, vol. 5, 1-82, 1996. CLASSIFICATION OF POPULATION II STARS IN THE VILNIUS PHOTOMETRIC SYSTEM. I. METHODS A. Bartkevicius1 and R. Lazauskaite1'2 1 Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania 2 Department of Theoretical Physics, Vilnius Pedagogical University, Studenty, 39, Vilnius 2340, Lithuania Received February 7, 1996. Abstract. The methods used for classification of Population II stars in the Vilnius photometric system are described. An exten- sive set of standards with known astrophysical parameters compiled from the literature sources is given. These standard stars are classi- fied in the Vilnius photometric system using the methods described. The accuracy of classification is evaluated by a comparison of the as- trophysical parameters derived from the Vilnius photometric system with those estimated from spectroscopic studies as well as from pho- tometric data in other systems. For dwarfs and subdwarfs, we find a satisfactory agreement between our reddenings and those estimated in the uvby/3 system. The standard deviation of [Fe/H] determined in the Vilnius system is about 0.2 dex. The absolute magnitude for dwarfs and subdwarfs is estimated with an accuracy of <0.5 mag. Key words: techniques: photometric - stars: fundamental para- meters (classification) - stars: Population II 1. INTRODUCTION The first attempt to classify Population II stars and to estimate their astrophysical parameters by using the Vilnius photometric sys- tem was made by Bartkevicius & Straizys (1970a, b, c). Later on, Straizys & Bartkevicius (1982) demonstrated a possibility to deter- mine, from photometry alone, the metallicities, temperatures and color excesses of metal-deficient giants. The most detailed calibration 2 A. Bartkevicius and R. Lazauskaite of two-color diagrams and ultraviolet excesses in terms of metallicity for Population II stars has been made by Bartkevicius & Speraus- kas (1983). In addition, for the classification of Population II stars Bartkevicius and Sperauskas suggested a method of comparison of standard and program stars. The effective temperatures for late metal-deficient giants can be estimated using the calibration of Tautvaisiene (1987). In the same paper, a preliminary surface gravity calibration is presented. How- ever, the most recent calibration of metal-deficient giants in terms of log g was published by Tautvaisiene & Lazauskaite (1993). Also, the Vilnius photometric system has been calibrated for some specific types of Population II stars. Sperauskas (1987) showed the possibility of identifying and estimating astrophysical parameters for blue horizontal branch stars, RR Lyrae-type stars and blue strag- glers. CH, barium and carbon stars were investigated by Sleivyte (1985, 1986). In this paper, methods used for classification of Population II stars will be described. The main attention will be paid to the method of comparison of standard and program stars which allows one to recognize Population II stars among other stars, as well as to estimate their spectral types, metallicities and absolute magnitudes. Also, additional calibrations used for the estimation of [Fe/H] and My for metal-deficient stars will be discussed. The methods were applied for the classification of 809 Population II and normal stars observed in the Vilnius system and included into the list of the stan- dard stars. By a comparison of these results with the values deter- mined from spectroscopy, as well as from other photometric systems, the accuracy of our methods will be evaluated. In the second paper of this series (Bartkevicius &; Lazauskaite 1996), about 900 stars which are suspected belonging to Popula- tion II will be classified in the Vilnius system, using the methods described here. 2. METHODS At present, for the classification of Population II stars in the Vilnius photometric system we use the following methods. 1. The method of comparison of standard and program stars (in this case, the standard stars are those with known parameters; the program stars are stars whose parameters must be estimated); Classification of Population II Stars 3 2. Calibration of two-color diagrams and ultraviolet excesses in terms of metallicity; 3. Calibration of intrinsic color indices in terms of absolute mag- nitude. In this paper, the first and third methods will be described in more detail. We have not made any changes in the second method described by Bartkevicius & Sperauskas (1983). 2.1. Method of comparison of standard and program stars 2.1.1. Set of standard stars The method of comparison of standard and program stars is based on the assumption that stars with similar photometric quan- tities such as color indices and reddening-free Q-parameters have nearly the same astrophysical quantities. Within the set of standard stars with known astrophysical parameters, we look for stars which, according to their photometric quantities, are as close as possible to a program star. Then the mean parameters of the standard stars, clos- est to the program star, are ascribed to the latter. It is evident that in this case the accuracy and possibilities of classification depend on the accuracy of parameters of the standard stars and on the complete- ness of the available set of standard stars. Therefore, we attempted to form a set of standards, which included as many as possible Pop- ulation II stars with known astrophysical parameters and observed in the Vilnius photometric system. The term Population II is used in this paper for the stars with metallicity [Fe/H]< — 0.5 and with total space velocities V >(60 - 100) km s~K Thus, we included into our set both halo and thick disk (intermediate Population II) stars. Our preliminary set of Population II standard stars was de- scribed by Bartkevicius & Lazauskaite (1993). Later on, these stan- dards were critically analyzed once more, and now the set contains a total of 809 stars of different types (827 records including variable stars in different phases). The groups of standard stars are listed in Table 1. To recognize normal stars in a sample of unknown stars, we included into our set not only Population II stars but also dwarfs, gi- ants, subgiants and supergiants with normal chemical composition. A Bootis-type stars are Population I stars, however, they were in- cluded into our set due to their metal deficiency. White dwarfs were 4 A. Bartkevicius and R. Lazauskaite also included with the purpose of separating them from sdO-B and sdF-G stars. The mean values of astrophysical parameters of standard stars, which were derived as described below, are presented in Table 6. It contains star name, intrinsic color index (Y—V)o, color excess EY-V> metallicity, absolute magnitude, spectral type, group membership and six intrinsic color indices. In the notes after the Table, informa- tion about variability, duplicity and membership in moving groups is presented. Abbreviations used in these notes are explained in Table 7. When compiling astrophysical parameters of standard stars, much attention was given to their absolute magnitudes. The My values were compiled from the literature or estimated by us as de- scribed by Bartkevicius & Lazauskaite (1993). The individual values of My were averaged with the weights inversely proportional to the square of their errors. For averaging, we used only those My values which were consistent within 2 magnitudes. Color excesses EB-V or EY-v for standard stars were obtained from different sources. For a part of stars, their reddenings were es- timated by Bartkevicius &; Sperauskas (1983), Bartkevicius & Taut- vaisiene (1987) and Bartasiute (1989). Color excesses of the remain- ing stars were obtained using estimations of interstellar reddening of different authors, among them, the maps of EG-y constructed by Burstein & Heiles (1982) from H I distribution and deep galaxy counts. In the final stage of this work, for calculation of reddening we used the program kindly supplied us by Dr. David Burstein. Stars with unknown reddening were classified in the Vilnius photometric system by the method of comparison of reddening-free Q-parameters of the standard and the program stars. Color exesses derived in dif- ferent ways were averaged with equal weights. If, however, one source gave a clearly discrepant value, it was eliminated from the average. In addition, it was assumed that stars within 50 pc are unreddened by interstellar matter. Since in most cases reddening of the standard stars is small (Fig. 1), the errors of EY-y values are not significant in determination of the intrinsic color indices. The metallicities were taken from the Catalogue of the mean [Fe/H], Teff and log g compiled by Bartkevicius &: Lazauskaite (1995). In the case of [Fe/H] derived from high-resolution spectra, the values in Table 6 are given to two decimal places. The remaining metal- licities are derived from the medium-resolution spectra or were esti- mated in the Vilnius photometric system. Classification of Population II Stars 5 Table 1. Groups of standard stars No. Group Abbreviation N 1. Dwarfs ([Fe/H]>-0.2; Mv>4.0) D 159 2. Dwarfs with moderate metal deficiency MDD 85 (—0.2>[Fe/H]>—0.6; Mv>4.0) 3. Subdwarfs (-0.6>[Fe/H]>-1.3; Mv>4.0) SD 98 4. Extreme metal-deficient subdwarfs SDEXTR 66 ([Fe/H]< —1.3; My >4.0) 5. Giants ([Fe/H]> —0.3; Mv<2.0) G 32 6. Metal-deficient giants MDG 56 (—0.3>[Fe/H]> —1.3; Mv<2.0) 7. Extreme metal-deficient giants MDGE 67 ([Fe/H]< —1.3; Mv<2.0) 8. Subgiants SG 23 ([Fe/H]> —0.3; 2.0<MV<4.0) 9. Metal-deficient subgiants MDSG 13 (-0.3>[Fe/H]>-1.3; 2.0<Mv<4.0) 10. Extreme metal-deficient subgiants MDSGE 6 ([Fe/H]< —1.3; 2.0<Aiv<4.0) 11. Asymptotic giant branch stars AGB 7 12. CH giants CH 36 13. Carbon variables of RV Tau-type CH-RV 6 14.