Location of the asteroid forming region in terms of disk mass M_disk and semimajor axis r. This can also be thought of as a “disk age vs. r” phase space. The disk mass is measured in terms of the minimum-mass solar nebula (M_MMSN). The top plot shows the region where R = 4mm chondrule aggregates can form clumps. The solid and dashed lines are lines of constant particle Stokes number τ_f = 0.0032, 0.0056, 0.01, and 0.0178. To form clumps, these particles require a solid concentration of Z = 0.069, 0.038, 0.026, and 0.021 respectively (see Fig. 8). The bottom plot shows the same regions for R = 1mm chondrules. As the disk evolves, and the disk mass drops, the planetesimal forming region moves closer to the star.