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Open clusters in dense fields: the importance of field-star decontamination for NGC 5715, Lyngå 4, Lyngå 9, Trumpler 23, Trumpler 26 and Czernik 37
Star clusters projected on to dense stellar fields in general requirefield-star (FS) decontamination to confirm their nature and deriveintrinsic photometric and structural fundamental parameters. This workfocuses on the open clusters (OCs) or candidates NGC5715, Lyngå4,Lyngå9, Trumpler23, Trumpler26 and Czernik37 which are projectedwithin and , against crowded bulge and/or disc fields. To tackle thesedifficult objects we develop a colour-magnitude diagramFS-decontamination algorithm based on 2MASS J, (J - H) and (J -Ks) data, and respective uncertainties, to detect clusterstar excesses over the background. On the other hand, colour-magnitudefilters are used to build stellar radial density profiles and massfunctions. The results convey compelling evidence that Lyngå9 andCzernik37 are intermediate-age OCs, and their fundamental parameters aremeasured for the first time. Trumpler23 is a particularly challengingobject, since besides high background level, its field presents variableabsorption in near-infrared bands. We confirm it to be anintermediate-age OC. Trumpler26 is studied in more detail than inprevious works, while NGC5715 and Lyngå4 have fundamentalparameters determined for the first time. These OCs are located0.9-1.6kpc within the solar circle, with ages similar to that of theHyades. Structurally, they are well described by King profiles. In allcases, core and limiting radii are significantly smaller than those ofnearby OCs outside the solar circle. We test the effect of backgroundlevels on cluster radii determinations by means of simulations. Theyindicate that for central clusters, radii may be underestimated by about10-20 per cent, which suggests that the small sizes measured for thepresent sample reflect as well intrinsic properties related to dynamicalevolution effects. The objects probably have been suffering importanttidal effects that may have accelerated dynamical evolution, especiallyin Czernik37, the innermost object.
| Uniform survey of clusters in the southern Milky Way.
Abstract image available at:http://adsabs.harvard.edu/abs/1975AJ.....80...11V
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