Abundances of selected gas-phase species (top row) and grain-surface species (labeled with asterisks; bottom row) as functions of medium density in the L1544 model. The abundances are displayed at three distinct time steps, ranging from 10⁴ yr (left) to 10⁶ yr (right). The different line styles correspond to model M1 (solid lines), model M3 (dashed lines), and model M4 (dotted lines). The ice abundances represent sums over all populations in models M3 and M4.