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We derive the structure, distribution of MS and PMS stars and dynamical stateof the young open cluster NGC 4755. We explore the possibility that, at thecluster age, some MS and PMS stars still present infrared excesses related todust envelopes and proto-planetary discs. The radial density profile followsKing's law with a core radius $\rm\rc=0.7\pm0.1 pc$ and a limiting radius$\rm\rl=6.9\pm0.1 pc$; the cluster age is $\rm14\pm2 Myr$. Field-stardecontamination reveals a low-MS limit at $\rm\approx1.4 \ms$. The core MF($\chi=0.94\pm0.16$) is flatter than the halo's ($\chi=1.58\pm0.11$). NGC 4755contains $\rm\sim285$ candidate PMS stars of age $\rm\sim1 - 15 Myr$, and a fewevolved stars. The mass locked up in PMS, MS and evolved stars amounts to$\rm\sim1150 \ms$. Proper motions show that \ks-excess MS and PMS stars arecluster members. \ks-excess fractions in PMS and MS stars are $\rm5.4\pm2.1%$and $\rm3.9\pm1.5%$ respectively, consistent with the cluster age. The core isdeficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries inthe halo but they are scarce in the core. Compared to open clusters indifferent dynamical states studied with similar methods, NGC 4755 fitsrelations involving structural and dynamical parameters in the expected locusfor its age and mass. On the other hand, the flatter core MF probablyoriginates from primordial processes related to parent molecular cloudfragmentation and mass segregation over $\rm\sim14 Myr$. Star formation in NGC4755 began $\rm\approx14 Myr$ ago and proceeded for about the same length oftime. Detection of \ks-excess emission in member MS stars suggests that somecircumstellar dust discs survived for $\rm\sim10^7 yr$, occurring both in someMS and PMS stars for the age and spread observed in NGC 4755.Comment: 10 figs. Astronomy & Astrophysics, in pres

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

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