UBVIphotometry of the intermediate-age open cluster NGC 6939

Abstract

ABSTRACT We present CCD UBVI photometry of the nearby, intermediate age open clusterNGC 6939. Using the synthetic Colour - Magnitude Diagrams technique we estimatethe following parameters: age between 1.3 and 1.0 Gyr (depending on whether or notovershooting is considered), reddening 0.34 ≤ E(B−V) ≤ 0.38 and distance modulus11.3 ≤ (m−M) 0 ≤ 11.4.Key words: Hertzsprung-Russell (HR) diagram – open clusters and associations:general – open clusters and associations: individual: NGC 6939 1 INTRODUCTIONA vast amount of information on the formation and evolu-tion of our Galaxy can be gathered from the study of openclusters (OCs). In particular, old OCs may be useful to de-rive information not only on the present day situation, butalso on the time evolution of the disc, since their ages coverthe whole existence of the disc, reaching to about 10 Gyr(Friel 1995). In order to fully exploit the information pro-vided by OCs we must derive for them in a homogeneousway: accurate absolute ages (and a consistent age ranking),distances, reddenings, and metal content. With this aim weare studying a number of old OCs (see Bragaglia & Tosi2003 and Sandrelli et al. 1999 for references), and we nowadd another cluster to our sample.Given its relative proximity, NGC 6939 (C2030+604)has been the target of several studies in the past: the firstbibliographic entry is 80 years ago (Kustner 1923) but, sur-prisingly, the first CCD data appeared only in 2002 (Rosvick& Balam 2002, hereafter RB02). As usual, the cluster pa-rameters found in literature do not agree with each other,and we present in this paper new and improved determi-nations for this intermediate age open cluster located atRA(2000) = 20:31:32, DEC(2000) = +60:39:00, or l = 95.88,b = 12.30.Photometry has been previously presented by severalauthors, but old photographic photometry only reachedabout one magnitude below the main sequence Turn-Off.Mermilliod, Huestamendia, & del Rio (1994) took UBV pho-toelectric photometry of 37 member stars all in the redclump phase, with the intent of discriminating between dif-ferent evolutionary models (with or without overshooting)by comparison with theoretical isochrones. It turned outthat most of the bright stars lying in the cluster directionare indeed cluster members (their Table 1 lists only 4 non-members among 45 objects). The recent work by Rosvick &Balam (2002) has presented the first deep BVI CCD data.They used the 1.85m Dominion Astrophysical Observatorytelescope, covering the central part of NGC 6939 (more orless our field A, see later). Their CMDs show considerablescatter, which they do not attribute to contamination fromfield stars (even if they could not prune their diagrams onthe main sequence, since no proper motion survey on thiscluster has reached so deep), but to differential reddening[E(B−V) = 0.29 to 0.41], which also influences the distancederivation [(m−M)