In quiescent clouds far from any luminous stars, it is difficult to excite the rotational states of OH. Referring again to Figure 5.18, the energy gap between the ground and first excited level of the 2n3/2 ladder has an equivalent temperature of 120 K, while the jump to the ground state of the 2nV2 ladder is 180 K. We may therefore restrict our attention to the four ground-state hyperfine transitions depicted in the figure. The Einstein A-coefficient for the strongest line at 1667 MHz (F = 2+ ^ 2-) is quite low at 7.2 x 10-11 s-1. Hence, the critical density for

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