COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2688 Konkoly Observatory Budapest 20 March 1985 HU ISSN 0374 - 0676 VERIFICATION OF CEPHEIDS IN OPEN CLUSTERS AND ASSOCIATIONS AS DISTANCE INDICATORS The Cepheids, members of open clusters and associations, play important role as objects used for distance and absolute magnitude calibrations. Therefore it is interesting to note that four papers dealing with these stars: Fernie and McGonegal (1983, henceforth FM), Caldwell (1983), Stothers (1983) and Cester and Marsi (1984) differ from each other not only in methods of reduction but even in the selection of stars. The list of 29 stars given by FM is presented in Table I with the addition of four Cepheids: GU Nor, SX Vel, Y Sct, and X Cyg. These latter stars were included to the calibration Cepheids by Cester and Marsi but not by FM. To provide an independent verification of these stars as distance indicators, the moduli Mod_FM = (m-M)_0 taken from FM paper have been compared with the similar values calculated from the new P-L-C relation for galactic field Cepheids, see Ciurla and Opolski (1984): M_ = 5.40_0 - 5.20 log P - 3.44 (1) +-.81 +-.84 +-.60 s.d. = 0.29 Assuming the ratio of total to selective absorption R = 3.20 we have: E_B-V = - (2) _0 = - 3.20 ( - _0) (3) and Mod_O = _0 - M or Mod_O = - 5.40 + 5.20 log P + 2.20 E_B-V + 3.44 (4) For the present study the values of and have been taken from the catalogue of Schaltenbrand and Tammann (1971) and supplemented for five stars according to the references given by FM. These values are quoted in Table I with two decimal figures. The E_B-V values are given according to the papers by Dean et al. (1978) and Pel (1978) after reducing to the Dean's et al. system. In four cases when these quantities were missing, the Dean's et al. (1978) P-C relation has been applied to get _0 and E_B-V. These values are given in brackets. For four additional stars the reddenings E_B-V Table I. Cepheids in open clusters and associations ----------------------------------------------------------------------------------------- Star log P E_B-V Mod_O Mod_FM DeltaMod reliable distance indicators, |DeltaMod| < 0.45 ----------------------------------------------------------------------------------------- SU Cas 0.290 5.969 0.725 0.24 7.54 7.72 +0.18 EV Sct .490 10.128 1.157 0.64 11.28 11.15 -0.13 GU Nor .538 10.406 1.297 0.68 11.14 (11.11) -0.03 CE Cas b .651 10.988 1.140 (0.54) 12.85 12.85 0.00 CF Cas .688 11.110 1.227 0.55 12.72 12.85 +0.13 CE Cas a .711 10.919 1.213 (0.58) 12.79 12.85 +0.06 UY Per .730 11.306 1.591 0.91 11.96 12.05 +0.09 CV Mon .731 10.295 1.369 0.75 11.80 11.50 -0.30 VY Per .743 11.193 1.644 1.00 11.82 12.05 +0.23 U Sgr .829 6.714 1.142 0.43 9.25 9.21 -0.04 DL Cas .903 8.942 1.216 0.51 11.64 11.35 -0.29 S Nor 0.989 6.414 0.969 0.22 10.26 9.95 -0.31 Y Sct 1.015 9.631 1.587 0.75 11.43 (11.28) -0.15 TW Nor .033 11.68 2.03 1.20 12.17 11.77 -0.40 VX Per .037 9.300 1.242 0.49 12.51 12.06 -0.41 X Cyg .214 6.395 1.213 0.23 10.10 (10.29) +0.19 VY Car .278 7.446 1.193 0.24 11.63 11.86 +0.23 RU Sct .294 9.500 1.779 1.02 12.32 11.89 -0.43 RZ Vel .310 7.114 1.209 0.32 11.55 11.52 -0.03 WZ Sgr .039 8.018 1.455 0.43 11.52 11.26 -0.26 SW Vel .371 8.126 1.252 0.38 12.78 12.38 -0.40 T Mon .432 6.137 1.252 0.18 10.67 11.10 +0.43 RS Pup .617 7.006 1.489 0.52 11.97 11.55 -0.42 SV Vul .654 7.221 1.534 0.41 11.89 12.10 +0.21 GY Sge .708 10.23 2.25 1.11 12.85 12.99 +0.14 S Vul 1.826 9.00 1.92 0.77 13.27 13.51 +0.24 questionable distance indicators, |DeltaMod| > 0.60 V Cen 0.740 6.818 0.914 0.32 9.88 9.24 -0.64 CS Vel 0.771 11.70 1.37 0.68 13.26 11.95 -1.31 V367 Sct 0.799 11.578 1.700 (1.03) 12.26 11.49 -0.77 SX Vel 0.980 8.263 0.904 0.28 12.53 (11.60) -0.93 SZ Cas 1.134 9.826 1.505 0.85 12.91 12.05 -0.86 KQ Sco 1.458 9.349 1.980 (0.99) 11.90 12.60 +0.70 V810 Cen 2.115 5.06 0.81 0.26 15.70 12.44 -3.26 ------------------------------------------------------------------------------------------- according to Cester and Marsi (1984) have been adopted. Table I contains also the moduli Mod_O resulting from the formula (4) and FM moduli, Mod_FM. For four stars taken additionally from the paper by Cester and Marsi we have assumed as Mod_FM the values of (m-M)_o given by these authors increased by 0.30, because there is a systematic difference about 0.30 between the moduli published in these two papers for the 23 common stars. In Table I these Mod_FM values are given in brackets. The differences DeltaMod = Mod_FM - Mod_O are presented in the last column of Table I. From these values the following conclusion may be drawn: 1. In Table I there are 33 Cepheids proposed as distance indicators, but only for 26 ones,listed in the upper part of the Table the values |DeltaMod| are smaller than 0.45. They can be explained as the result of uncertainties in cluster distances and as due to the limited accuracy of formula (4). Therefore these 26 stars should be recommended as reliable distance indicators. These stars determine the P - L relation given by FM and discussed by Ciurla and Opolski (1984). 2. The remaining seven stars in the lower part of Table I have |DeltaMod| greater than 0.6, which makes them questionable as distance indicators. Two of them, CS Vel and V 810 Cen, have been already omitted by FM in the investigation of the P - L relation. V Cen has been rejected by Stothers (1983) and SZ Cas by Caldwell (1983). The double mode Cepheid, V 367 Sct, has been discussed a number of times with conflicting conclusions. Also the membership of KQ Sco in the association may be questioned. 3. The distances of U Sgr and V Cen resulting from the moduli Mod_O, Mod_FM and Mod_C, the latter from the paper by Caldwell (1983) can be compared with the distances published recently by Gieren (1984). His values were obtained by the surface brightness method: distances in pc ------------------------- U Sgr V Cen ------------------------- from Mod_O 709 947 from Mod_FM 695 705 from Mod_C 673 689 Gieren 709 897 ------------------------- The consistency in the distance of U Sgr is confirmed in contrast with the differences in V Cen data, in accordance with the foregoing suggestion that the latter star belongs to the questionable distance indicators. A. OPOLSKI Wroclaw University Observatory ul. Kopernika 11, 51-622 Wroclaw Poland References: Caldwell, J.A.R., 1983, The Observatory, 103, No. 1056, 244. [BIBCODE 1983Obs...103..244C ] Cester, B., and Marsi, C., 1984, Astrophys. Space Sci., 107, 167. [BIBCODE 1984Ap&SS.107..167C ] Ciurla, T., and Opolski, A., 1984, Inf. Bull. Var. Stars, No. 2629. Dean, J.F., Warren, P.R., and Cousins, A.W.J., 1978, M.N.R.A.S., 183, 569. [BIBCODE 1978MNRAS.183..569D ] Fernie, J.D., and McGonegal, R., 1983, Astrophys. J., 275, 732. [BIBCODE 1983ApJ...275..732F ] Gieren, W., 1984, Astrophys. J., 282, 650. [BIBCODE 1984ApJ...282..650G ] Pel, J.W., 1978, Astron. Astrophys., 62, 75. [BIBCODE 1978A&A....62...75P ] Schaltenbrand, R., and Tammann, G.A., 1971, Astron. Astrophys. Suppl.Ser., 4, 265. [BIBCODE 1971A&AS....4..265S ] Stothers, R.E., 1983, Astrophys. J., 274, 20. [BIBCODE 1983ApJ...274...20S ]