2017AJ....154...57H

We present new SDSS and Washington photometry of the young outer-halo stellar system Segue 3. Combined with archival VI-observations, our most consistent results yield Z=0.006±0.001, log(Age)=9.42±0.08, (m-M)₀=17.35±0.08, and E(B-V)=0.09±0.01, with a high binary fraction of 0.39 ± 0.05 derived using the Padova models. We confirm that mass-segregation has occurred, supporting the hypothesis that this cluster is being tidally disrupted. A three-parameter King model yields a cluster radius of r_cl=0.○^017±0.○^007, a core radius of r_c=0.○^003±0.○^001, and a tidal radius of r_t=0.○^04±0.○^02. A comparison of Padova and Dartmouth model-grids indicates that the cluster is not significantly α-enhanced, with a mean [Fe/H]=-0.55_–0.12^+0.15 dex, and a population age of only 2.6 ± 0.4 Gyr. We rule out a statistically significant age spread at the main-sequence turnoff because of a narrow subgiant branch, and discuss the role of stellar rotation and cluster age, using Dartmouth and Geneva models: approximately 70% of the Seg 3 stars at or below the main-sequence turnoff have enhanced rotation. Our results for Segue 3 indicate that it is younger and more metal-rich than all previous studies have reported to date. From colors involving Washington C and SDSS-u filters, we identify several giants and a possible blue straggler for future follow-up spectroscopic studies, and we produce spectral energy distributions of previously known members and potential Segue 3 sources with Washington (CT₁), Sloan (ugri), and VI-filters. Segue 3 shares the characteristics of unusual stellar systems that have likely been stripped from external dwarf galaxies as they are being accreted by the Milky Way, or that have been formed during such an event. Its youth, metallicity, and location are all inconsistent with Segue 3 being a cluster native to the Milky Way.