2009ApJ...705...54Y

The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18 kpc, is known to have a very low metallicity, about one-tenth that of the solar neighborhood. We obtained the deep near-infrared (NIR) images of two very young (∼0.5 Myr) star-forming clusters that are two 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 ≤0.1 AU, is significantly lower than those in the solar neighborhood. Our results suggest that most of the stars forming in the low-metallicity environment experience disk dispersal at an earlier stage (<1 Myr) than those forming in the solar metallicity environment (as much as ∼5-6 Myr). Such a rapid disk dispersal may make the formation of planets difficult, and the shorter disk lifetime with a lower metallicity, could contribute to the strong metallicity dependence of the well-known "planet-metallicity correlation," which states that the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be the 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.