DIFFUSE INTERSTELLAR BANDS IN ORION - THE ENVIRONMENT DEPENDENCE OF DIB STRENGTH

Four Diffuse Interstellar Bands (DIB) have been measured in 22 lines o f sight towards Orion and GMC214-13. The paper reports on the environm ent dependence of DIB strength, using reddening, the total extinction in the visual, and column densities of various forms of hydrogen as in dicators of line of sight conditions. In lines of sight with small red dening, E(B-V) < 0.08 mag, a weakening of lambda 6284 and lambda 5785 is found relative to reddening (= all matter), but no weakening relati ve to atomic + molecular hydrogen (= all neutral matter). This probabl y implies that the DIB carriers are absent in the ionised HII medium. At higher reddening, where a significant part of hydrogen is in a mole cular form, all DIBs in Orion decrease with increasing E(B_V) in a sim ilar way as is known for the Taurus and Ophiuchus dark clouds. In Taur us, where even in low reddened lines of sight a significant part of hy drogen is in molecular form, lambda 6284 is weakened compared to diffu se medium values at lower reddening than lambda 6196. In lines of sigh t towards the Orion Nebula, where molecular hydrogen is mostly dissoci ated, lambda 6284 is some 70% stronger, while lambda 6196 is a factor of 5 weaker than expected from the above dependence. The results are d iscussed in the context that the carrier of the diffuse bands might be a large molecule in ionised form. The dissociation and ionisation sta te of hydrogen is used as an indicator of the UV field. The results ab ove indicate that the lambda 6284 carrier is both ionised and destroye d by higher energy photons than the lambda 6196 carrier, which is cons istent with a molecular carrier.