Short Lifetime of Protoplanetary Disks in Low-metallicity Environments

In order to understand protoplanetary disk evolution and planet formation in a wider range of environments than previously studied, we explored the lifetime of protoplanetary disks in the outermost Galaxy (Rg> ~ 15 kpc), which are known to have a very different metallicity from the solar neighborhood, [M/H] ~ −1 dex. We obtained the deep near-infrared (NIR) images of six young clusters that are located in the outer Galaxy with known metallicity of [O/H] −0.7 dex using Subaru 8.2 m telescope[1]. We derived disk fraction for each cluster (Figure 1) and estimated disk lifetime in the environments (Figure 2). As a result, we found that disk fraction of the low-metallicity clusters declines rapidly in < 1 Myr, which is much faster than the ~ 5–7 Myr observed for the solar-metallicity clusters, suggesting that disk lifetime shortens with decreasing metallicity possibly with an ~10Z dependence[2]. Since the shorter disk lifetime reduces the time available for planet formation, this could be one of the major reasons for 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 and/or X-ray photoevaporation due to the low extinction; however, another unknown mechanism for the outer Galaxy environment could be contributing significantly. Although more quantitative observational and theoretical assessments are necessary, our results present the first direct observational evidence that can contribute to explaining the planet-metallicity correlation