Detection of Ks-excess stars in the 14Myr open cluster NGC4755

Abstract

We derive the structure, distribution of MS and PMS stars and dynamical state of the young open cluster NGC 4755. 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. The radial density profile follows King'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 $14\pm 2Myr$. Field-star decontamination reveals a low-MS limit at $\rm\approx1.4 \ms$. The core MF ($\chi =0.94\pm 0.16$) is flatter than the halo's ($\chi =1.58\pm 0.11$). NGC 4755 contains $\sim 285$ candidate PMS stars of age $\sim 1-15Myr$, and a few evolved 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 are cluster members. \ks-excess fractions in PMS and MS stars are $5.4\pm 2.1$ and $3.9\pm 1.5$ respectively, consistent with the cluster age. The core is deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halo but they are scarce in the core. Compared to open clusters in different dynamical states studied with similar methods, NGC 4755 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 $\sim 14Myr$. Star formation in NGC 4755 began $\approx 14Myr$ 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 $\sim 10^7\mathrm{yr}$, occurring both in some MS and PMS stars for the age and spread observed in NGC 4755.

10 figs. Astronomy & Astrophysics, in press

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