VERY ACCURATE DISTANCES AND RADII OF OPEN CLUSTER CEPHEIDS FROM A NEAR-INFRARED SURFACE BRIGHTNESS TECHNIQUE

We have obtained the radii and distances of 16 galactic Cepheids suppo sed to be members in open clusters or associations using a new optical and two near-infrared calibrations of the surface brightness (Barnes- Evans) method. We find excellent agreement of the radii and distances produced by both infrared techniques, which use the V, V-K (K on the C arter system) and the K, J-K magnitude-color combinations, respectivel y, with typical random errors that are as little as similar to 2%. We discuss possible systematic errors in our infrared solutions in detail and conclude that the typical total uncertainty of the infrared dista nce and radius of a Cepheid is about 3% in both infrared solutions, pr ovided that the data are of excellent quality and that the amplitude o f the color curve used in the solution is larger than similar to 0.3 m ag. The optical V, V-R distance and radius of a given Cepheid can devi ate by as much as similar to 30% from the infrared value because of la rge systematic and random errors caused by microturbulence and gravity variations: these affect the optical but not the infrared colors. We find excellent agreement of our infrared radii with the infrared radii derived previously for these variables by Laney & Stobie from an appl ication of the maximum likelihood technique, which further increases o ur confidence that the total errors in our infrared solutions are not larger than similar to 3%. In an Appendix we discuss the relative adva ntages and disadvantages of our infrared surface brightness technique and the maximum likelihood technique. We compare the adopted infrared distances of the Cepheid variables to the zero-age main-sequence-fitti ng (ZAMS-fitting) distances of their supposed host clusters and associ ations (assuming a Pleiades distances modulus of 5.57) and find an unw eighted mean value of the distance ratio of 1.02 +/- 0.04. A detailed discussion of the individual Cepheids shows that the uncertainty of th e ZAMS-fitting distances varies considerably from cluster to cluster. We find clear evidence that four Cepheids are not cluster members (SZ Tau, T Mon, U Car, and SV Vul), while we confirm cluster membership fo r V Cen and BE Sgr, for which former evidence for cluster membership w as only weak. After rejection of nonmembers, we find a weighted mean d istance ratio of 0.969 +/- 0.014, with a standard deviation of 0.05, w hich demonstrates that both distance indicators are accurate to better than 5%, including systematic errors, and that there is excellent agr eement between both distance scales.