Hierarchical star formation in the Milky Way disk
  Hierarchical star formation leads to a progressive decrease in the clustering
of star clusters both in terms of spatial scale and age. Consistently, the
statistical analysis of positions and ages of clusters in the Milky Way disk
strongly suggests that a correlation between the duration of star formation in
a region and its size does exist. The average age difference between pairs of
open clusters increases with their separation as the ~0.16 power. In contrast
and for the Large Magellanic Cloud, Efremov & Elmegreen (1998) found that the
age difference scales with the ~0.35 power of the region size. This discrepancy
may be tentatively interpreted as an argument in support of intrinsically
shorter (faster) star formation time-scales in smaller galaxies. However, if
both the effects of cluster dissolution and incompleteness are taken into
consideration, the average age difference between cluster pairs in the Galaxy
increases with their separation as the ~0.4 power. This result implies that the
characteristic time-scale for coherent, clustered-mode star formation is nearly
1 Myr. Therefore, the overall consequence of ignoring the effect of cluster
dissolution is to overestimate the star formation time-scale. On the other
hand, in the Galactic disk and for young clusters separated by less than three
times the characteristic cluster tidal radius (10 pc), the average age
difference is 16 Myr, which suggests common origin. (Abridged)
