Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752

Abstract

In order to study the small-scale structure of the hot interstellar gas, we obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar absorption spectra of 4 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 à and 1037.617 à . The close proximity of these stars permits us to probe the hot interstellar gas over angular scale of only $2\farcm2 - 8\farcm9$, corresponding to spatial scales $\la 2.5-10.1$ 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 \pm 0.02 (log <N_\perp(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 $Δθ\la 12\arcmin$, corresponding to a spatial scale of less than 10 pc, and 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.

Accepted for publication in the PASP (to appear in the October 2004 issue)