THE DIFFUSE INTERSTELLAR BANDS AND THE GALACTIC LATITUDE

Existing measurements of three of the diffuse interstellar bands are presented in a new way, in order to investigate how the relative strengths of different bands depend on the Galactic latitude of the stars in whose light they are seen. It is found that none of the three ratios of bandstrength amongst lambdalambda 4430, 5780 and 5797 angstrom is constant, but all three are correlated with the modulus of the latitude. The abundance of the carrier of lambda 4430 relative to the others is found to be greatest at low latitude, while that of the carrier of lambda 5797 is greatest at high latitude. It is supposed that this dependence reflects a more basic dependence on height above and below the Galactic plane, the carrier of lambda 4430 evidently preferring conditions near the plane where the gas density is high and the carrier of lambda 5 797 preferring the more tenuous gas further out. In terms of a recent theory in which the carriers are different hydrocarbon molecules and ions of the fullerane family, these results imply that, of the bands studied here, the carrier of lambda 4430 bears the most hydrogen atoms and that of lambda 5797 bears the fewest.