2004PASP..116..895L

In order to study the small-scale structure of hot interstellar gas, we obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI absorption spectra of four post-extreme horizontal branch stars in the globular cluster NGC 6752 ([l,b]=[336°.50,-25°.63], d=3.9 kpc, z=-1.7 kpc). The good-quality spectra of these stars allow us to measure both lines of the O VI doublet at 1031.926 and 1037.617 Å. The close proximity of these stars permits us to probe the hot interstellar gas over angular scales of only 2'.2-8'.9, corresponding to spatial scales of ≲2.5-10.1 pc. On these scales we detect no variations in the O VI column density and velocity distribution. The average column density is log<N(O VI)≥14.34±0.02 (log<N_⊥(O VI)≥13.98). The measured velocity dispersions of the O VI absorption are also indistinguishable. Including the earlier results of Howk et al., this study suggests that interstellar O VI is smooth on scales Δθ≲12', corresponding to a spatial scale of ≲10 pc, and is quite patchy at larger scales. Although such small scales are only probed in a few directions, this suggests a characteristic size scale for the regions producing collisionally ionized O VI in the Galaxy.