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By Chikako Yasui, Naoto Kobayashi, Alan T. Tokunaga, Masao Saito and Chihiro Tokoku


The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18kpc, is known to have very low metallicity, about one-tenth that of the solar neighborhood. We obtained deep near-infrared (NIR) images of two very young (∼0.5Myr) star-forming clusters that are one of the most distant embedded clusters in the EOG. We find that in both clusters the fraction of stars with NIR excess, which originates from the circumstellar dust disk at radii of ≤0.1AU, is significantly lower than those in the solar neighborhood. Our results suggest that most stars forming in the low-metallicity environment experience disk dispersal at an earlier stage (<1Myr) than those forming in the solar metallicity environment (as much as ∼5–6Myr). Such rapid disk dispersal may make the – 2 – formation of planets difficult, and the shorter disk lifetime with lower metallicity could contribute to the strong metallicity dependence of the well-known “planetmetallicity correlation”, which states the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be increase of the mass accretion rate and/or the effective far-ultraviolet photoevaporation due to the low extinction; however, another unknown mechanism for the EOG environment could be contributing significantly. Subject headings: infrared: stars – planetary systems: protoplanetary disks – stars: pre-main-sequence – open clusters and associations: individual (Digel Cloud 2) – stars: formation 1

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