Kim 3: an ultra-faint start cluster in the constellation of Centaurus

Abstract

We report the discovery of an ultra-faint star cluster in the constellation of Centaurus. This new stellar system, Kim 3, features a half-light radius of ${r}_{h}={2.29}_{-0.52}^{+1.28}$ pc and a total luminosity of MV = +0.7 ± 0.3. Approximately 26 stars are identified as candidate member stars down to four magnitudes below the main-sequence turn-off, which makes Kim 3 the least luminous star cluster known to date. The compact physical size and extreme low luminosity place it close to faint star clusters in the size–luminosity plane. The stellar population of Kim 3 appears to be relatively young (${9.5}_{-1.7}^{+3.0}$ Gyr) and metal-poor ([Fe/H]$\quad =\quad -{1.6}_{-0.30}^{+0.45}$) at a heliocentric distance of ${15.14}_{-0.28}^{+1.00}$ kpc. The cluster lacks a well-defined center, and a small but prominent group of stars consistent with the Kim 3 isochrone is present approximately 9.7 pc in projection south of the cluster center. Both are signs of the cluster being in the final stage of tidal disruption.