WIDESPREAD MOLECULAR-EMISSION IN THE GALACTIC PLANE FROM SPECIES OTHER THAN (CO)-C-12

Most millimeter-wave (CO)-C-13 J = 1-0 emission features in the inner regions of the northern Galactic plane are also readily detected in the low-lying rotational lines of rarer species with higher dipole moments such as HCO+, CS, HCN, and C2H. Such lines are typically 10% as strong as those of (CO)-C-13 or 2% as strong as (CO)-C-12. Because they are so much harder to excite, these lines might be expected to trace mainly the regions of highest density and temperature. However, their ubiquity and low typical intensity suggest that this is not the case. If the abundance of HCO+ is typical of local clouds, the observed HCO+ emission arises in regions of relatively moderate pressure [at T-K similar or equal to 15 K, n(H-2) similar or equal to 500 H-2 cm(-3)].