Kim 3: an Ultra-faint Star 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 $M_V=+0.7\pm 0.3$. 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]$=-1.6_{-0.30}^{+0.45}$) 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.

7 pages, 7 figures, 1 table. Accepted for publication in the ApJ (w.r.t v1: Figure 1 and the last paragraph of Section 4 updated, minor corrections throughout the text)

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