Interstellar absorptions towards the LMC: Small-scale density variations
  in Milky Way disc gas
  Observations show that the ISM contains sub-structure on scales less than 1
pc, detected in the form of spatial and temporal variations in column densities
or optical depth. Despite the number of detections, the nature and ubiquity of
the small-scale structure in the ISM is not yet fully understood. We use UV
absorption data mainly from FUSE and partly from STIS of six LMC stars
(Sk-67{\deg}111, LH54-425, Sk-67{\deg}107, Sk-67{\deg}106, Sk-67{\deg}104, and
Sk-67{\deg}101), all located within 5 arcmin of each other, and analyse the
physical properties of the Galactic disc gas in front of the LMC on sub-pc
scales. We analyse absorption lines of a number of ions within the UV spectral
range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and
fine-structure levels of neutral carbon have been used in order to study
changes in the density and the physical properties of the Galactic disc gas
over small angular scales. While most species do not show any significant
variation in their column densities, we find an enhancement of almost 2 dex for
H_2 from Sk-67{\deg}111 to Sk-67{\deg}101, accompanied by only a small
variation in the OI column density. Based on the formation-dissociation
equilibrium, we trace these variations to the actual density variations in the
molecular gas. On the smallest spatial scale of < 0.08 pc, between
Sk-67{\deg}107 and LH54-425, we find a gas density variation of a factor of
1.8. The line of sight towards LH54-425 does not follow the relatively smooth
change seen from Sk-67{\deg}101 to Sk-67{\deg}111, suggesting that
sub-structure might exist on a smaller spatial scale than the linear extent of
our sight-lines. Our observations suggest that the detected H_2 in these six
lines of sight is not necessarily physically connected, but that we are
sampling molecular cloudlets with pathlengths < 0.1-1.8 pc and possibly
different densities.
