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The Enigma of the Open Cluster M29 (NGC 6913) Solved

Determining the distance to the open cluster M29 (NGC 6913) has proven difficult, with distances determined by various authors differing by a factor of two or more. To solve this problem, we have initiated a new photometric investigation of the cluster in the Vilnius seven-color photometric system supplementing it with available data in the BV and JHK_s photometric systems, and spectra of the nine brightest stars of spectral classes O and B. Photometric spectral classes and luminosities of 260 stars in a 15 arcmin x 15 arcmin area down to V = 19 mag are used to investigate the interstellar extinction run with distance and to estimate the distance of the Great Cygnus Rift, 800 pc. The interstellar reddening law in the optical and near-infrared regions is found to be close to normal, with the ratio of extinction to color excess R(BV) = 2.87. The extinction A(V) of cluster members is between 2.5 to 3.8 mag, with a mean value of 2.97 mag or E(B-V) = 1.03. The average distance of eight stars of spectral types O9-B2 is 1.54 +- 0.15 kpc. Two stars from the seven brightest ones are field stars: HDE 229238 is a background B0.5 supergiant and HD 194378 is a foreground F star. In the intrinsic color-magnitude diagram, seven fainter stars of spectral classes B3-B8 are identified as possible members of the cluster. The 15 selected members of the cluster of spectral classes O9--B8 plotted on the log L/L(sun) vs. log T(eff) diagram, together with the isochrones from the Padova database, give the age of the cluster as 5 +- 1 Myr.