Detection of Ks-excess stars in the 14 Myr open cluster NGC 4755

Abstract

Aims. We derive the structure, distribution of MS and PMS stars and dynamical state of the young open cluster NGC4755. We explore the possibility that, at the cluster age, some MS and PMS stars still present infrared excesses related to dust envelopes and proto-planetary discs. Methods. J, H and Ks 2MASS photometry is used to build CMD and colour–colour diagrams, radial density profiles, luminosity and mass functions. Field-star decontamination is applied to uncover the cluster’s intrinsic CMD morphology and detect candidate PMS stars. Proper motions from UCAC2 are used to determine cluster membership. Results. The radial density profile follows King’s law with a core radius Rcore = 0.7 ± 0.1 pc and a limiting radius Rlim = 6.9 ± 0.1 pc. The cluster age derived from Padova isochrones is 14 ± 2 Myr. Field-star decontamination reveals a low-MS limit at ≈1.4 Mʘ. The core MF (χ = 0.94 ± 0.16) is flatter than the halo’s (χ = 1.58 ± 0.11). NGC 4755 contains ∼285 candidate PMS stars of age ∼1−15 Myr, and a few evolved stars. The mass locked up in PMS,MS and evolved stars amounts to ∼1150 M . Proper motions show that Ks-excess MS and PMS stars are cluster members. Ks-excess fractions in PMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5% respectively, consistent with the cluster age. The core is deficient in PMS stars, as compared with MS ones. NGC4755 hosts binaries in the halo but they are scarce in the core. Conclusions. Compared to open clusters in different dynamical states studied with similar methods, NGC4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass. On the other hand, the flatter core MF probably originates from primordial processes related to parent molecular cloud fragmentation and mass segregation over ∼14 Myr. Star formation in NGC4755 began ≈14 Myr ago and proceeded for about the same length of time. Detection of Ks-excess emission in member MS stars suggests that some circumstellar dust discs survived for ∼107 yr, occurring both in some MS and PMS stars for the age and spread observed in NGC4755.