Baltic Astronomy, vol.3, 348-360, 1994• SEVEN-COLOR PHOTOELECTRIC PHOTOMETRY OF STARS IN THE ALPHA PERSEI OPEN CLUSTER U. Dzervltis1, 0. Paupers1 and V. Vansevicius2 1 Radioastrophysical Observatory, Latvian Academy of Sciences, Turgeqeva 19, Riga LV 1527, Latvia 2 Institute of Physics, Gostauto 12, Vilnius 2600, Lithuania Received Received December 15, 1994. Abstract. Photoelectric observations in the Vilnius seven-color sys- tem of 78 stars with V < 11.5 mag in the Alpha Persei open cluster are presented. The photometric quantification of stars in terms of spectral class and absolute magnitude has been made and their in- dividual reddening values determined. The mean reddening value of the cluster members is Εγ-ν — 0.06 ± 0.03 mag (equivalent to Eß-V — 0.07 ± 0.04 mag). By fitting the zero-age main se- quences of the Alpha Persei and the Hyades clusters and taking into account the difference in their metallicities, the true distance modulus Vo ~ My — 6-24 ± 0.06 mag (177 ± 5 pc) has been determined. The average value of individual distance moduli of 69 cluster members is found to be 6.28 ± 0.04 mag, which corresponds to 180 ± 3 pc distance. The absolute magnitude of the F5Ib supergiant a Per is found to be My = -4.7 mag. Key words: methods: observational - techniques: photometric: Vilnius photometric system - open clusters: Alpha Persei 1. Introduction The open cluster (I = 147°.0, b = -7°.l, φ ~ 5°), named after its brightest member, the supergiant α Per, contains about a dozen B-type stars visible to the naked eye, which gives evidence of its relative proximity. Therefore, this richly populated cluster, together with other nearby clusters, plays a crucial role in finding the location of the zero-age main sequence (ZAMS) in the color vs. magnitude Seven-color photometry of stars in the Alpha Persei cluster 349 diagram. For this reason, the Alpha Persei cluster was investigated in the past in various aspects. Photoelectric photometry in the UBV system of stars in the cluster area, selected as members in the proper motion survey of Heckmann et al. (1956), was made by Mitchell (1960). Observations of the cluster stars in the Geneva system were analyzed and the cluster distance was determined by Nicolet (1981). Photometry in the uvbyß system was published by Crawford and Barnes (1974); they also estimated the interstellar reddening of the cluster. Radial velocity measurements in the Alpha Persei cluster were carried out by Petrie and Heard (1970) and more recently by Stauffer et al. (1985) and Prosser (1992). Also, in these latest investigations numerous faint members of the cluster were found. Our main goal in undertaking the photoelectric photometry of the Alpha Persei cluster in the seven-color Vilnius system, was to locate more exactly the position of the ZAMS on the magnitude vs. color index diagrams of the Vilnius system for the stars of A and F spectral classes as well as to determine the cluster distance using the individual stars. This is impossible to determine reddenings, extinc- tions and distances of individual stars by a broad-band photometry without spectral observations. 2. Results of photometry The observations were made in 1986-1988 with the 1 meter reflector of the Institute of Theoretical Physics and Astronomy (Vilnius, Lithuania) at the Maidanak Observatory (Uzbekistan). The standard glass filter set of the Vilnius photometric system was used in combination with a photomultiplier FEU-79 (S-20 cathode). The atmospheric extinction was determined by the Nikonov method from repeated observations of two auxiliary stars separated by approximately 4 hours in the hour angle. To ensure reduction of the observed color indices and V magnitude from the instrumen- tal to the standard system, 33 standard stars of the Vilnius system in the Cygnus region were observed. All reductions of the observa- tional data were made according to the usual techniques (Straizys 1977, 1992). The mean square errors of one observation, calculated from observations of the standard stars, are ±0.02 mag for the color indices U-V and P-V, ±0.01 mag for other indices and ±0.015 mag for the V magnitude. The results of our observations of 78 stars are presented in Table 1. The column η contains the number of 350 U. Dzermtis, Ο. Paupers and V. Vansevicius Table 1. Results of photometry No* V U-V P-V X-V Y-V Z-V V-S η Remark 29 10.63 2.44 1.92 1.31 0.56 0.22 0.52 2 nm 33 11.13 2.57 2.11 1.46 0.64 0.23 0.61 1 nm 94 10.36 2.52 2.01 1.40 0.59 0.22 0.59 1 135 9.70 2.15 1.74 1.14 0.50 0.19 0.46 2 151 8.96 - 2.24 1.64 0.92 0.37 0.13 0.34 1 175 5.91 3.58 2.98 2.01 0.75 0.30 0.71 1 nm 212 7.15 1.74 1.19 0.49 0.21 0.08 0.19 1 SB 220 9.11 2.34 1.70 0.94 0.38 0.14 0.35 1 228 9.93 2.50 1.84 1.11 0.49 0.19 0.43 1 nm 270 10.12 2.29 1.77 1.19 0.52 0.20 0.48 1 299 11.13 2.50 1.97 1.35 0.60 0.21 0.56 2 309 9.99 2.23 1.73 1.16 0.49 0.18 0.47 2 334 10.37 2.34 1.85 1.26 0.52 0.20 0.52 2 340 11.50 2.56 2.09 1.44 0.61 0.24 0.60 2 SB1? 350 11.11 2.61 2.10 1.47 0.62 0.24 0.62 1 361 9.67 2.22 1.71 1.10 0.46 0.16 0.42 2 365 9.98 2.26 1.75 1.17 0.51 0.20 0.48 2 386 7.93 2.08 1.48 0.64 0.26 0.09 0.18 2 387 10.29 2.44 1.92 1.28 0.58 0.20 0.53 2 SB2? 401 5.02 1.08 0.77 0.37 0.18 0.07 0.14 2 421 9.20 2.34 1.77 1.09 0.46 0.17 0.44 2 423 7.61 1.94 1.33 0.56 0.23 0.08 0.17 2 SB1 490 9.57 2.25 1.72 1.10 0.46 0.17 0.43 2 501 9.13 2.29 1.69 0.96 0.39 0.15 0.35 2 520 11.64 2.75 2.21 1.59 0.65 0.27 0.66 2 var RV 557 5.30 1.07 0.76 0.36 0.17 0.07 0.13 1 574 8.97 4.27 3.57 2.42 0.93 0.38 0.88 1 nm 581 6.95 1.64 1.14 0.47 0.21 0.07 0.14 1 588 9.96 2.37 1.86 1.23 0.55 0.21 0.50 2 601 11.46 2.68 2.20 1.55 0.61 0.24 0.63 2 nm 606 8.95 2.27 1.70 0.94 0.38 0.14 0.34 4 609 9.19 2.45 1.83 1.06 0.46 0.18 0.42 2 612 7.84 1.98 1.41 0.59 0.22 0.08 0.15 3 621 9.86 2.23 1.73 1.12 0.48 0.16 0.46 1 622 11.66 2.79 2.30 1.58 0.65 0.20 0.65 1 625 7.59 1.98 1.40 0.62 0.27 0.09 0.20 2 635 8.90 2.25 1.68 0.95 0.39 0.16 0.35 2 639 8.11 2.08 1.52 0.66 0.25 0.09 0.18 2 647 10.36 2.60 1.89 1.06 0.44 0.17 0.37 1 nm 651 8.38 2.18 1.57 0.73 0.28 0.10 0.23 2 658 9.21 2.55 1.90 1.05 0.43 0.16 0.36 2 nm 660 10.07 2.46 1.91 1.26 0.56 0.20 0.52 2 var RV? 675 6.05 1.13 0.82 0.36 0.16 0.06 0.12 2 S even-color photometry of stars in the Alpha Persei cluster 351 Table 1 (continued) No.* V U-V P-V X-V Y-V Z-V V-S η Remarks 676 11.39 2.59 2.06 1.42 0.64 0.25 0.52 2 nm 679 8.95 2.48 1.76 0.86 0.38 0.16 0.35 2 nm 684 10.59 2.34 1.86 1.28 0.55 0.20 0.51 2 692 7.46 1.83 1.27 0.51 0.21 0.07 0.14 2 694 8.45 2.20 1.60 0.76 0.29 0.10 0.22 2 nm 696 11.56 2.73 2.26 1.60 0.67 0.26 0.59 1 699 11.14 2.62 2.14 1.48 0.65 0.28 0.61 1 709 10.94 2.54 2.04 1.41 0.62 0.23 0.59 2 715 9.68 2.23 1.74 1.13 0.50 0.17 0.46 2 SB2? 727 10.24 2.33 1.87 1.26 0.56 0.19 0.52 2 var RV 729 7.67 1.97 1.39 0.60 0.25 0.09 0.19 2 nm? 733 9.85 2.29 1.80 1.18 0.52 0.18 0.47 2 735 6.80 1.58 1.07 0.44 0.20 0.07 0.15 2 750 10.51 2.37 1.90 1.30 0.56 0.20 0.53 2 756 7.91 2.08 1.50 0.65 0.25 0.09 0.17 2 SB? 767 10.67 2.44 1.96 1.37 0.58 0.22 0.54 2 771 11.10 2.57 2.03 1.39 0.63 0.21 0.60 1 nm 775 7.22 1.65 1.16 0.50 0.22 0.07 0.18 2 SB1 780 8.08 2.13 1.55 0.69 0.29 0.10 0.23 2 SB 794 10.11: • 2.48: 1.95: 1.30 0.62 0.22 0.49 2 var RV 802 8.40 2.14 1.55 0.70 0.28 0.10 0.19 1 SB? 810 5.54 1.20 0.85 0.39 0.19 0.07 0.14 1 831 7.33 1.67 1.17 0.48 0.20 0.07 0.14 2 833 10.01 2.27 1.78 1.17 0.51 0.17 0.49 2 841 10.25 2.34 1.83 1.25 0.55 0.21 0.49 1 875 7.62 2.01 1.41 0.61 0.25 0.08 0.2Ó 2 nm? 885 8.77 2.27 1.68 0.88 0.34 0.12 0.27 1 906 8.73 2.33 1.71 0.86 0.35 0.12 0.28 2 917 10.55 3.41 2.96 2.00 0.70 0.40 0.75 2 0.44 0.13 955 6.73 1.51 1.06 0.05 0.18 2 SB1? 0.42 0.20 965 6.59 1.42 0.99 0.06 0.15 1 SB1? 968 10.45 2.38 1.88 1.29 0.55 0.20 0.53 2 970 8.17 2.17 1.56 0.85 0.49 0.09 0.18 2 1082 7.21 1.72 1.20 0.51 0.22 0.07 0.16 2 1153 6.86 1.40 1.01 0.43 0.19 0.06 0.14 1 * Number is from Heckman et al.