A far UV study of interstellar gas towards HD 34078:: High excitation H2 and small scale structure

To investigate the presence of small scale structure in the spatial distribution of H-2 molecules we have undertaken repeated FUSE UV observations of the runaway O9.5V star, HD 34078. In this paper we present five spectra obtained between January 2000 and October 2002. These observations reveal an unexpectedly large amount of highly excited H-2. Column densities for H-2 levels from (v = 0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levels are determined. These results are interpreted in the frame of a model involving essentially two components: i) a foreground cloud (unaffected by HD 34078) responsible for the H-2 (J = 0, 1), CI, CH, CH+ and CO absorptions; ii) a dense layer of gas (n similar or equal to 10(4) cm(-3)) close to the O star and strongly illuminated by its UV flux which accounts for the presence of highly excited H-2. Our model successfully reproduces the H-2 excitation, the CI fine-structure level populations as well as the CH, CH+ and CO column densities. We also examine the time variability of H-2 absorption lines tracing each of these two components. From the stability of the J = 0, 1 and 2 damped H-2 profiles we infer a 3sigma upper limit on column density variations DeltaN(H-2)/N(H-2) of 5% over scales ranging from 5 to 50 AU. This result clearly rules out any pronounced ubiquitous small scale density structure of the kind apparently seen in HI. The lines from highly excited gas are also quite stable (equivalent to DeltaN/N less than or equal to 30%) indicating i) that the ambient gas through which HD 34078 is moving is relatively uniform and ii) that the gas flow along the shocked layer is not subject to marked instabilities.