The (j,k) = (3,3) inversion line of NH3 - a possible interstellar chronometer

Calculations show that ortho-H2 and para-H2 exhibit different propensity rules in collisions with NH3. In particular, para-H2 tends to preferentially excite the upper (3 3-) component of the (j,k)=(3,3) inversion doublet. Assuming collisional excitation, the existence of a maser in this transition within a molecular cloud implies an upper limit to the ratio of densities, n(o-H2)/n(p-H2). As this density ratio evolves (chemically) with time, to its upper limit there corresponds a lower limit to the age of the cloud. Calculations of the time-dependence of n(o-H2)/n(p-H2) enable this interstellar chronometer to be calibrated. (author)