Open clusters in dense fields: the importance of field-star decontamination for NGC 5715, Lynga 4, Lynga 9, Trumpler 23, Trumpler 26 and Czernik 37

Abstract

Star clusters projected on to dense stellar fields in general require field-star (FS) decontam- ination to confirm their nature and derive intrinsic photometric and structural fundamental parameters. This work focuses on the open clusters (OCs) or candidates NGC 5715, Lynga4 , Lynga 9, Trumpler 23, Trumpler 26 and Czernik 37 which are projected within 317 ◦ � 2. ◦ 2 and |b| 2. ◦ 8, against crowded bulge and/or disc fields. To tackle these difficult objects we develop a colour-magnitude diagram FS-decontamination algorithm based on 2MASS J, (J − H) and (J − Ks) data, and respective uncertainties, to detect cluster star excesses over the background. On the other hand, colour-magnitude filters are used to build stellar radial density profiles and mass functions. The results convey compelling evidence that Lynga9 and Czernik 37 are intermediate-age OCs, and their fundamental parameters are measured for the first time. Trumpler 23 is a particularly challenging object, since besides high back- ground level, its field presents variable absorption in near-infrared bands. We confirm it to be an intermediate-age OC. Trumpler 26 is studied in more detail than in previous works, while NGC 5715 and Lynga 4 have fundamental parameters determined for the first time. These OCs are located 0.9-1.6 kpc within the solar circle, with ages similar to that of the Hyades. Structurally, they are well described by King profiles. In all cases, core and limiting radii are significantly smaller than those of nearby OCs outside the solar circle. We test the effect of background levels on cluster radii determinations by means of simulations. They indicate that for central clusters, radii may be underestimated by about 10-20 per cent, which suggests that the small sizes measured for the present sample reflect as well intrinsic properties related to dynamical evolution effects. The objects probably have been suffering important tidal ef- fects that may have accelerated dynamical evolution, especially in Czernik 37, the innermost object.