THE DIFFUSE INTERSTELLAR BANDS .9. CONSTRAINTS ON THE IDENTIFICATION

Equivalent widths of the diffuse interstellar bands (DIBs) lambdalambd a5780, 5797 and of the interstellar D1 and D2 lines have been measured on Lick and Canada-France-Hawaii Telescope coude spectrograms of 93 s tars. Correlations with column densities of H I, H-2, Na I, C I, K I, and Ti II have led to the following conclusions: (1) the carrier of th ese DIBs is not produced by the processes based upon H-2 that control the abundance of carbon diatomics in diffuse clouds: in fact, (2) DIB column density is instead dependent upon N(H I) and appears not to res pond to N(H-2) at all; (3) the carrier is not significantly influenced by the depletion of Ti, regarded as an indicator of whether the solid grains have been shocked or not; (4) the carrier is not affected by s hock heating of the gas, believed to be responsible for the appearance of strong CH+ lines in the Pleiades; (5) DIB strength does not depend upon the over- or underabundance of the small grains that dominate th e extinction at 1250 angstrom, or upon the value of R = A(V)/E(B-V); ( 6) DIB strength follows a power-law dependence upon N(Na I), N(C I), a nd N(K I) with exponents of 0.5-0.7, but in most lines of sight is dir ectly proportional to E(B-V); (7) despite this close correlation, the carrier of these DIBs in the Orion Nebula does not reside in the solid particles that produce the reddening. In general it behaves like a fr ee neutral species in the gas and responds to the ionization level of the gas as if its ionization/dissociation threshold is somewhat higher than 5 eV.