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 JHKs 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 ' 15 ' 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 RBV = 2.87. The extinction AV of cluster members is between 2.5 and 3.8 mag, with a mean value of 2.97 mag, or EB 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 stars 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/LO versus log Teff diagram, together with the isochrones from the Padova database, give the age of the cluster as 5 1 Myr.
Published in the Astronomical Journal, v148 n89, Nov 2014. The original document contains color images. Prepared in collaboration with the Institute of Theoretical Physics and Astronomy, Vilnius University, Vilnius, Lithuania, the Vatican Observatory Research Group, Steward Observatory, Tucson, AZ, the INAF Astronomical Observatory of Padova, Asiago, Italy, the Space Telescope Science Institute, Baltimore, MD, and the University School Ignatianum, Cracow, Poland.